Java Interview Questions

Q:What is the difference between an Interface and an Abstract class?
A: An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

Q:What is the purpose of garbage collection in Java, and when is it used?
A: The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Q:Describe synchronization in respect to multithreading.
A: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

Q: Explain different way of using thread?
A: The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.

Q: What are pass by reference and passby value?
A: Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.

Q: What is HashMap and Map?
A: Map is Interface and Hashmap is class that implements that.

Q: Difference between HashMap and HashTable?
A: The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

Q: Difference between Vector and ArrayList?
A: Vector is synchronized whereas arraylist is not.

Q: Difference between Swing and Awt?
A: AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.

Q: What is the difference between a constructor and a method?
A: A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

Q: What is an Iterator?
A: Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

Q: State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
A: public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.

Q: What is an abstract class?
A: Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.
A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

Q: What is static in java?
A: Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

Q: What is final?
A: A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

Q: What if the main method is declared as private?
A: The program compiles properly but at runtime it will give "Main method not public." message.

Q: What if the static modifier is removed from the signature of the main method?
A: Program compiles. But at runtime throws an error "NoSuchMethodError".


Q: What if I write static public void instead of public static void?
A: Program compiles and runs properly.


Q: What if I do not provide the String array as the argument to the method?
A: Program compiles but throws a runtime error "NoSuchMethodError".


Q: What is the first argument of the String array in main method?
A: The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.


Q: If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
A: It is empty. But not null.


Q: How can one prove that the array is not null but empty using one line of code?
A: Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.


Q: What environment variables do I need to set on my machine in order to be able to run Java programs?
A: CLASSPATH and PATH are the two variables.


Q: Can an application have multiple classes having main method?
A: Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.


Q: Can I have multiple main methods in the same class?
A: No the program fails to compile. The compiler says that the main method is already defined in the class.


Q: Do I need to import java.lang package any time? Why ?
A: No. It is by default loaded internally by the JVM.


Q: Can I import same package/class twice? Will the JVM load the package twice at runtime?
A: One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.


Q: What are Checked and UnChecked Exception?
A: A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.
Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method·
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the
exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.


Q: What is Overriding?
A: When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.


Q: What are different types of inner classes?
A: Nested -level classes, Member classes, Local classes, Anonymous classes

Nested -level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other -level class.
Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. -level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested -level variety.

Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested -level class. The primary difference between member classes and nested -level classes is that member classes have access to the specific instance of the enclosing class.

Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a
more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.

Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;

Q:Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.

Q: What is the difference between declaring a variable and defining a variable?
A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.

Q: What is the default value of an object reference declared as an instance variable?
A: null unless we define it explicitly.

Q: Can a level class be private or protected?
A: No. A level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a level class can not be private. Same is the case with protected.


Q: What type of parameter passing does Java support?
A: In Java the arguments are always passed by value .

Q: Primitive data types are passed by reference or pass by value?
A: Primitive data types are passed by value.

Q: Objects are passed by value or by reference?
A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .

Q: What is serialization?
A: Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

Q: How do I serialize an object to a file?
A: The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.

Q: Which methods of Serializable interface should I implement?
A: The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.

Q: How can I customize the seralization process? i.e. how can one have a control over the serialization process?
A: Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.

Q: What is the common usage of serialization?
A: Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.

Q: What is Externalizable interface?
A: Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

Q: When you serialize an object, what happens to the object references included in the object?
A: The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.

Q: What one should take care of while serializing the object?
A: One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.

Q: What happens to the static fields of a class during serialization?
A: There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.

Q: Does Java provide any construct to find out the size of an object?
A: No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.

Q: Give a simplest way to find out the time a method takes for execution without using any profiling tool?
A: Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.

To put it in code...

long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();

System.out.println ("Time taken for execution is " + (end - start));

Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing.

Q: What are wrapper classes?
A: Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.

Q: Why do we need wrapper classes?
A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.

Q: What are checked exceptions?
A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.

Q: What are runtime exceptions?
A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

Q: What is the difference between error and an exception?
A: An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).

Q: How to create custom exceptions?
A: Your class should extend class Exception, or some more specific type thereof.

Q: If I want an object of my class to be thrown as an exception object, what should I do?
A: The class should extend from Exception class. Or you can extend your class from some more precise exception type also.

Q: If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

Q: How does an exception permeate through the code?
A: An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

Q: What are the different ways to handle exceptions?
A: There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.

Q: What is the basic difference between the 2 approaches to exception handling.
1> try catch block and
2> specifying the candidate exceptions in the throws clause?
When should you use which approach?
A: In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

Q: Is it necessary that each try block must be followed by a catch block?
A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

Q: If I write return at the end of the try block, will the finally block still execute?
A: Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.

Q: If I write System.exit (0); at the end of the try block, will the finally block still execute?
A: No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.

Q: How are Observer and Observable used?
A: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

Q: What is synchronization and why is it important?
A: With respect to multithreading, synchronization is the capability to control
the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.

Q: How does Java handle integer overflows and underflows?
A: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.

Q: Does garbage collection guarantee that a program will not run out of memory?
A: Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.

Q: What is the difference between preemptive scheduling and time slicing?
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Q: When a thread is created and started, what is its initial state?
A: A thread is in the ready state after it has been created and started.

Q: What is the purpose of finalization?
A: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

Q: What is the Locale class?
A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.

Q: What is the difference between a while statement and a do statement?
A: A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

Q: What is the difference between static and non-static variables?
A: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

Q: How are this() and super() used with constructors?
A: This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

Q: What are synchronized methods and synchronized statements?
A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q: What is daemon thread and which method is used to create the daemon thread?
A: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.

Q: Can applets communicate with each other?
A: At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable.

An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members.

It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.

Q: What are the steps in the JDBC connection?
A: While making a JDBC connection we go through the following steps :

Step 1 : Register the database driver by using :

Class.forName(\" driver classs for that specific database\" );

Step 2 : Now create a database connection using :

Connection con = DriverManager.getConnection(url,username,password);

Step 3: Now Create a query using :

Statement stmt = Connection.Statement(\"select * from TABLE NAME\");

Step 4 : Exceute the query :

stmt.exceuteUpdate();


Q: How does a try statement determine which catch clause should be used to handle an exception?
A: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.

Q: Can an unreachable object become reachable again?
A: An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

Q: What method must be implemented by all threads?
A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

Q: What are synchronized methods and synchronized statements?
A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q: What is Externalizable?
A: Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

Q: What modifiers are allowed for methods in an Interface?
A: Only public and abstract modifiers are allowed for methods in interfaces.


Q: What are some alternatives to inheritance?
A: Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).

Q: What does it mean that a method or field is "static"?
A: Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.

Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.

Q: What is the difference between preemptive scheduling and time slicing?
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Q: What is the catch or declare rule for method declarations?
A: If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.


Q: Is Empty .java file a valid source file?
A: Yes, an empty .java file is a perfectly valid source file.

Q: Can a .java file contain more than one java classes?
A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.


Q: Is String a primitive data type in Java?
A: No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.


Q: Is main a keyword in Java?
A: No, main is not a keyword in Java.


Q: Is next a keyword in Java?
A: No, next is not a keyword.

Q: Is delete a keyword in Java?
A: No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.

Q: Is exit a keyword in Java?
A: No. To exit a program explicitly you use exit method in System object.

Q: What happens if you dont initialize an instance variable of any of the primitive types in Java?
A: Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.

Q: What will be the initial value of an object reference which is defined as an instance variable?
A: The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.

Q: What are the different scopes for Java variables?
A: The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.

Q: What is the default value of the local variables?
A: The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..

Q: How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.

Q: Can a public class MyClass be defined in a source file named YourClass.java?
A: No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.


Q: Can main method be declared final?
A: Yes, the main method can be declared final, in addition to being public static.

Q: What will be the output of the following statement?
System.out.println ("1" + 3);
A: It will print 13.

Q: What will be the default values of all the elements of an array defined as an instance variable?
A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.

Interview Questions On Java Servlets

What is a servlet? Explain its lifecyle.

The Servlets are server side java programs, which are used to generate dynamic web content for a web clients. They reside inside a servlet container on a web server or an application server. The servlet container provides them a runtime environment.

If an instance of servlet is non existent then web container loads the servlet class and creates an instance of the servlet.Once the servlet instantiates, web container calls init() method on it to initialize the servlet.This process of initialization can be customized to allow servlet to read persistent configuration data,initialize resources like database connections etc. by overriding init() method of Servlet interface.If initialization of a servelet fails it throws UnavailableException.

Once initialization is done, web container invokes the service method, passing a request and response object depending upon incoming request.

If the container needs to remove the servlet(e.g.when web container is shutting down), it finalizes the servlet by calling the servlet's destroy method.

The javax.servlet.Servlet interface defines the three life-cycle method:-

public void init(ServletConfig config) throws ServletException

public void service( ServletRequest req, ServletResponse res) throws ServletException, IOException

public void destroy()

What is the difference between CGI and servlets?

In traditional CGI, a new process is started with each client request and this will correspond to initiating a heavy OS level process each time when a client request comes. While in case of servlets JVM handles client requests at a web server end and each client request correspond to thread which consumes less resources as compared with CGI process, thus making CGI inefficient. In Java servlet, there is only a single instance, which answers all requests concurrently. This saves memory and allows a Servlet to easily manage persistent data.Java servlets resides in Servlet engine and are executed within sandbox making it more secure and robust.

What is a middleware and what is the functionality of Webserver?

A middleware in a 3-tier architecture sits in the middle of a client's machine and a database server. A middleware is where all the business related logic resides and it is also known as Web Application Server. (e.g. WebLogic from BEA, WebSphere from IBM, Oracle 9iAS from Oracle etc.)In distributed programming divide and rule is the name of the game and in this paradigm a middleware has a good share of responsibilities viz. load balancing, database connection pooling, security, transaction management related services, web and enterprise component deployment services, content management related services etc.

Can there be more than one instance of a servlet at one time ?

It is important to note that there can be more than one instance of a given Servlet class in the servlet container. For example, this can occur where there was more than one servlet definition that utilized a specific servlet class with different initialization parameters. This can also occur when a servlet implements the SingleThreadModel interface and the container creates a pool of servlet instances to use.

Why there are no constructors in servlets?

A servlet is just like an applet in the respect that it has an init() method that acts as a constrcutor. Since the servlet environment takes care of instantiating the servlet, an explicit constructor is not needed. Any initialization code you need to run should be placed in the init() method since it gets called when the servlet is first loaded by the servlet container.

What is a Servlet Context?

A ServletContext interface empowers a servlet to view its environment. A servlet can use this interface to get following informations:

- Initial Web Application Parameters
- Application Scope for binding objects
- Virtual Directory Translation
- A common mechanism for Logging information

Each vendor provides specific ServletContext object but they all provide the same functionality defined by the ServletContext interface.

What is meant by Session tell me something about HttpSession?

A web client makes a request to a web server over HTTP. As long as a client interacts with the server through a browser on his or her machine,this interaction is called as session. HTTP is a stateless protocol. A client's each request is treated as a fresh one with no info of client to the server and the moment browser is closed, session is also closed. In an online shopping or web cart kind of application where session related information is critical for successive purchases made by a client, there have been suggested several techniques for session tracking in Java Servlets as mentioned below:

-Hidden form fields
-URL Rewriting
-Cookies
-HttpSession object

HttpSession is an interface, which belongs to javax.servlet.http. * package This provides a facility to identify a user across the several pages' requests by looking up the HttpSession object associated with the current request.
This is done by calling the getSession method of HttpServletRequest. If this returns null, you can create a new session, but this is so commonly done that there is an option to automatically create a new session if there isn't one already. Just pass true to getSession. Thus, your first step usually looks like this:

HttpSession session = request.getSession (true);

To ensure the session is properly maintained, this method must be called at least once before any output is written to the response.

You can add data to an HttpSession object with the putValue() method:

public void HttpSession.putValue(String name, Object value)

This method binds the specified object value under the specified name. Any existing binding with the same name is replaced.

To retrieve an object from a session, use getValue():

public Object HttpSession.getValue(String name)

This methods returns the object bound under the specified name or null if there is no binding.

You can also get the names of all of the objects bound to a session with getValueNames():

public String[] HttpSession.getValueNames()

This method returns an array that contains the names of all objects bound to this session or an empty (zero length) array if there are no bindings.

Finally, you can remove an object from a session with removeValue():

public void HttpSession.removeValue(String name)

This method removes the object bound to the specified name or does nothing if there is no binding. Each of these methods can throw a java.lang.IllegalStateException if the session being accessed is invalid.

What is the difference between GenericServlet and HTTPServlet?

GenericServlet is an abstract class that defines a generic, protocol-independent servlet.Any protocol dependent servlet has to extend this class in order to provide a specific implementaion and override service method.e.g . HTTPServlet class extends GenericServlet class.

GenericServlet has a service(ServletRequest req, ServletResponse res) method which is called by the servlet container to allow the servlet to respond to a request.

HttpServlet extends GenericServlet and adds support for doGet(), doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(), doDelete(), doTrace() methods (HTTP 1.1). Both these classes are abstract.

What is the difference between doGet and doPost methods of HttpServlet?

A GET or POST request is sent to servlet in order to expedite the request by calling corresponding doGet() and doPost() methods.

doGet is called in response to an HTTP GET request. This happens when users click on a link, or enter a URL into the browser's address bar. It also happens with some HTML FORMs (those with METHOD="GET" specified in the FORM tag).

doPost is called in response to an HTTP POST request. This happens with some HTML FORMs (those with METHOD="POST" specified in the FORM tag).

Both methods are called by the default (superclass) implementation of service in the HttpServlet base class. You should override one or both to perform your servlet's actions. You probably shouldn't override service().
There is a restriction of numbers of parameters to be sent through doGet method around 2k of data can be sent and moreover whole of URL is to be mentioned in case of doGet as mentioned below:
http://www.xyz.com/servlet?param1=value1&param2=value2&...&paramN=valueN

So it is always better to use doPost() when sending parameters from a FORM as it doesn't show off information related with password on the network etc.

Why do GenericServlet and HttpServlet class implement Serializable interface?

In order to support HTTP session states, all servlet session data must be serializable.Moreover, servlets need to communicate over the network to Java objects(e.g. in Applet-Servlet communication),in such scenarios it becomes necessary to enable serialization of objects' states through implementation of Serializable interface.This is the way GenericServlet and HttpServlet classes have been designed by their designers to make them capable of serialization.

How will you pass values from HTML to the servlet?

The values from an HTML page are passed to a servlet either through a GET or POST method. In a servlet, request.getParameter ("strParam") method is called in order to retrieve values from HTML form to a servlet end where String strParam represents the name of the input type OR values can be passed concatenated with URL itself which request the servlets receive.

Can you use System.exit in your servlet end code?

All the best...just kidding.The answer is big 'No'.At best, you'll get a security exception.At worst, you'll make the servlet engine, or maybe the entire web server, quit.

If my browser does not support Cookie and my server sends a Cookie instance what will happen?

In such scenario, browser will not accept incoming cookie. Whenever the browser sends the new request,it will not send the session id to server.As client has not joined to server session through cookie,server will not have any information about client in such a way. However,session tracking could still be preformed by using URL rewriting or hidden form fields.

What is the difference in between encodeRedirectURL and encodeURL

encodeURL is used for all URLs in a servlet's output. It helps session ids to be encoded with the URL. Moreover it rewrites URLs relative to the current document.if the url is not starting with 'http//.' and encodes any parameters added with the request.addQueryParameter or request.addURLParameter methods.As per Servlet API the encodeRedirectURL must ALWAYS be used within response.sendRedirect() commands.It is used for encoding session ids with URL but only while redirecting.

How do you make servlet thread-safe?

SingleThreadModel interface once implemented makes a servlet thread safe,though not in complete sense.It ensures that servlet handles only one request at a time. In this case only one thread will be able to execute service() method at a time.Though it affects the performance of the servlet to a great extent. In STM container creates multiple instances of servlet.This interface is deprecated from Servlet API version 2.4.
It does not resolve all thread safety related issues of a servlet, as most of session attributes and static variables can still be shared across multiple threads at the same time.The other ways to handle such situations can be avoid creating instance variables or synchronizing the block of code accessing those resources.

How do you communicate between applet and servlet?

The communication between an Applet and Servlet can occur through 'Http Tunneling' with Object serialization.It is technique through which an applet can send data to servlet through object serialization bypassing firewall over HTTP.A socket connection is established from client to server and in the process if there is any firewall at client side then that can also be bypassed but does not work in case if there is a firewall on remote host server side.

Q: Explain the life cycle methods of a Servlet.
A: The javax.servlet.Servlet interface defines the three methods known as life-cycle method.
public void init(ServletConfig config) throws ServletException
public void service( ServletRequest req, ServletResponse res) throws ServletException, IOException
public void destroy()
First the servlet is constructed, then initialized wih the init() method.
Any request from client are handled initially by the service() method before delegating to the doXxx() methods in the case of HttpServlet.

The servlet is removed from service, destroyed with the destroy() methid, then garbaged collected and finalized.


Q: What is the difference between the getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface and javax.servlet.ServletContext interface?
A: The getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface accepts parameter the path to the resource to be included or forwarded to, which can be relative to the request of the calling servlet. If the path begins with a "/" it is interpreted as relative to the current context root.

The getRequestDispatcher(String path) method of javax.servlet.ServletContext interface cannot accepts relative paths. All path must sart with a "/" and are interpreted as relative to curent context root.

Q: Explain the directory structure of a web application.
A: The directory structure of a web application consists of two parts.
A private directory called WEB-INF
A public resource directory which contains public resource folder.

WEB-INF folder consists of
1. web.xml
2. classes directory
3. lib directory


Q: What are the common mechanisms used for session tracking?
A: Cookies
SSL sessions
URL- rewriting

Q: Explain ServletContext.
A: ServletContext interface is a window for a servlet to view it's environment. A servlet can use this interface to get information such as initialization parameters for the web applicationor servlet container's version. Every web application has one and only one ServletContext and is accessible to all active resource of that application.

Q: What is preinitialization of a servlet?
A: A container doesnot initialize the servlets ass soon as it starts up, it initializes a servlet when it receives a request for that servlet first time. This is called lazy loading. The servlet specification defines the element, which can be specified in the deployment descriptor to make the servlet container load and initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called preloading or preinitializing a servlet.


Q: What is the difference between Difference between doGet() and doPost()?
A: A doGet() method is limited with 2k of data to be sent, and doPost() method doesn't have this limitation. A request string for doGet() looks like the following:
http://www.allapplabs.com/svt1?p1=v1&p2=v2&...&pN=vN
doPost() method call doesn't need a long text tail after a servlet name in a request. All parameters are stored in a request itself, not in a request string, and it's impossible to guess the data transmitted to a servlet only looking at a request string.

Q: What is the difference between HttpServlet and GenericServlet?
A: A GenericServlet has a service() method aimed to handle requests. HttpServlet extends GenericServlet and adds support for doGet(), doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(), doDelete(), doTrace() methods (HTTP 1.1).
Both these classes are abstract.

Q: What is the difference between ServletContext and ServletConfig?
A: ServletContext: Defines a set of methods that a servlet uses to communicate with its servlet container, for example, to get the MIME type of a file, dispatch requests, or write to a log file.The ServletContext object is contained within the ServletConfig object, which the Web server provides the servlet when the servlet is initialized

ServletConfig: The object created after a servlet is instantiated and its default constructor is read. It is created to pass initialization information to the servlet.

http://interviewjava.blogspot.com/2007/04/session-tracking-in-servlets.html

http://interviewjava.blogspot.com/2007/04/more-on-applet-servlet-communication.html

Interview Questions on JSP

What is the difference between Java Servlets and Java ServerPages (JSP)?

A JSP is a Servlet with a Web page flavor.There are more similarities than differences between the two.JSP uses its own tags which are ultimately using tag libraries implemented in Java.
The source code of a servlet with HTML embedded in out. print (...) statements, gives dynamic features to webpages but that makes a cumbersome process to maintain servelts from web design perspective.If someone has implemented GUI related code inside servlet then each time a small GUI change will result in recompilation of the whole code.Hence business logic centric development should be done inside servlet and JSP gives a choice of combining both in much targeted way.JSP was devised to differentiate the tasks of web designers,responsible for look and feel of the webpages, from web developers who deal in business logic development.So both entities have focused interest met through JSP.In fact, a JSP gets compiled into a servlet.

What is the difference between ' include file = ' and 'jsp:include page = ...'

<@ include file = ...>: The content of included file is textually embedded in the page that has <@ include file=".."> directive.It occurs at compile time. If included file changes, the changed content will not be included in the output. This approach is used in template creations where code from one jsp file is included in multiple jsp files.
<jsp:include page="..">: It is like a function call from one jsp to another jsp. The inclusion of one jsp content to another happens at execution time. This approach is handy for web application modularization.If included file changes then the new contents will be included in the output.

What is the difference between 'jsp:forward' and HttpResponse.sendRedirect()?

jsp:forward is processed at the server end for forwarding a client request to a specified URL while HttpResponse.sendRedirect() method is used at the client end for request redirection.

What are implicit objects in JSP?

The objects which are created by web container and are available in a JSP page for various information extractions from there objects.The different implicit objects in JSPs are(click on image to enlarge):

Explain different constituents of JSP like comments,expressions,declarations,scriptlets.

JSP Comments:
The JSP supports two type of comments :-
-Output
-Hidden
JSP Output Comments are those ones which can be viewed on the HTML file, while Hidden Comments are never shown.

JSP Expressions:
The JSP expression tag is inserts Java values directly into the output. The syntax of the Expression tag is:
<%= expression %>

This kind of tag contains is evaluated, converted to a String, and inserted where the expression appears in the JSP file.

JSP Declarations:
JSP declarations are used to declare variables.To add a declaration, the <%! and %> sequences is used to enclose declarations,and ends in semi-colon. These variables can be used within functions and expressions.It is not a good practice to declare variables in global space as such variable are shared by multiple threads and which may change it's values in an inconsistent manner.To avoid such a scenario one must make these variables as synchronized but then it hampers the performance of the JSP .Usually variables are declared within local scope of Java scriptlets.

JSP Scriptlets:
JSP allows to write block of Java code within scriptlet tags which are <% some Java code goes here %>.It does not contribute anything to HTML page and Java code is evaluated each time when JSP loads.

Here is an example which shows use of JSP Comments,Expression,Declaration and Scriptlet:

Explain the life-cycle methods of JSP

JSP has following three life cycle methods:

a) jspInit(): It is the very first method which is called by JSP container to initialize the servlet instance.All JSPs implement the javax.servlet.jsp.JspPage interface that has two methods: jspInit and jspDestroy. It is important to implement when one has to load some database driver for example.

b)_jspService(): The JSP container calls the _jspservice() for each request and it passes the request and the response objects.The _jspService() method corresponds to the body of the JSP page. This method is defined automatically by the JSP container and should never be defined by the JSP page author. That is why _jspService() method can not be overridden.


c) jspDestroy(): When an instance of JSP is going to be destroyed then JSP container calls this method .It is used for cleaning up Both jspInit() and jspDestroy() methods can be overridden within a JSP page.This method helps in reclaiming resources like network and database connections when a JSP instance is destroyed and JSP no longer services any client request.

package javax.servlet.jsp;

public interface JspPage extends javax.servlet.Servlet {

public void jspInit();
public void jspDestroy();
public void _jspService(ServletRequestSubtype request,
ServletResponseSubtype response)
throws ServletException, IOException;
// _jspService - depends on the specific protocol used and
// cannot be expressed in a generic way in Java.
}

Does JSP support XML pages creation?

The answer is Yes. There are two ways you could use XML data in a JSP page:
• Convert the XML elements into server-side objects and then extract the object properties.
• Invoke a transformation on the XML data.

The static XML tags may be included as static template portions of the JSP page.The dynamic generation of XML tags occur through bean components or custom tags that generate XML output.The other mechanism of dynamic XML data use in JSP could be by XML transformation.This technique uses XSLT(a W3C standard for transforming an XML to XML/HTML/PDF) which could be used either for formatting data in a desired fashion or simply extracting the data from an XML.

How can I show various kind of documents to a web client from JSP's and/or servlets?

You can show MS Word, Excel or PDF or any other type of document through your JSP or servlet by using :

response.setContentType("mime/type"); and

in jsp directive

<%@ page contentType="mime/type" %>.

In case of MS Word documents contentType is "application/msword"
In case of Excel it is "application/vnd.ms-excel"
In case of PDF it is "application/pdf"

How will you handle runtime exceptions in your JSPs?

JSP runtime exceptions are handled through the implicit object named "exception". This exception object can be used in a special type of JSP page called an error page, where you display the exception's name and class, its stack trace, and an informative message for your user.
In any JSP which is made eligible for catching exceptions then it must have a mention to error page in @page directive,something like:

<%@ page errorPage="ErrorHandling.jsp" %>

The error page, in this case ' ErrorHandling.jsp', has 'isErrorPage' attribute in page directive which should be set to true. When a JSP page has been declared an errorPage, it is made available an object with name of "exception" of type java.lang.Throwable.

How can you prevent caching in JSP?

Execute the following scriptlet at the beginning of your JSP pages to prevent JSPs from being cached by browsers.You need both the statements to take care of some of the older browser versions.

<%
response.setHeader("Cache-Control","no-cache"); //HTTP 1.1
response.setHeader("Pragma","no-cache"); //HTTP 1.0
response.setDateHeader ("Expires", 0); //prevents caching at the proxy server
%>

If the above fails, try changing the first line to

response.setHeader("Cache-Control","no-store"); //HTTP 1.1



Q: What is a output comment?
A: A comment that is sent to the client in the viewable page source.The JSP engine handles an output comment as uninterpreted HTML text, returning the comment in the HTML output sent to the client. You can see the comment by viewing the page source from your Web browser.

JSP Syntax
<!-- comment [ <%= expression%> ] -->

Example 1
<!-- This is a commnet sent to client on
<%= (new java.util.Date()).toLocaleString()%>
-->

Displays in the page source:
<!-- This is a commnet sent to client on January 24, 2004 -->


Q: What is a Hidden Comment?
A: A comments that documents the JSP page but is not sent to the client. The JSP engine ignores a hidden comment, and does not process any code within hidden comment tags. A hidden comment is not sent to the client, either in the displayed JSP page or the HTML page source. The hidden comment is useful when you want to hide or "comment out" part of your JSP page.

You can use any characters in the body of the comment except the closing --%> combination. If you need to use --%> in your comment, you can escape it by typing --%\>.
JSP Syntax
<%-- comment --%>

Examples
<%@ page language="java"%>
<html>
<head><title>A Hidden Comment </title></head>
<body>
<%-- This comment will not be visible to the colent in the page source --%>
</body>
</html>


Q: What is a Expression?
A: An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. Because the value of an expression is converted to a String, you can use an expression within text in a JSP file. Like
<%= someexpression%>
<%= (new java.util.Date()).toLocaleString()%>
You cannot use a semicolon to end an expression


Q: What is a Declaration?
A: A declaration declares one or more variables or methods for use later in the JSP source file.

A declaration must contain at least one complete declarative statement. You can declare any number of variables or methods within one declaration tag, as long as they are separated by semicolons. The declaration must be valid in the scripting language used in the JSP file.

<%! somedeclarations%>
<%! int i = 0;%>
<%! int a, b, c;%>


Q: What is a Scriptlet?
A: A scriptlet can contain any number of language statements, variable or method declarations, or expressions that are valid in the page scripting language.Within scriptlet tags, you can

1.Declare variables or methods to use later in the file (see also Declaration).

2.Write expressions valid in the page scripting language (see also Expression).

3.Use any of the JSP implicit objects or any object declared with a tag.
You must write plain text, HTML-encoded text, or other JSP tags outside the scriptlet.

Scriptlets are executed at request time, when the JSP engine processes the client request. If the scriptlet produces output, the output is stored in the out object, from which you can display it.


Q: What are implicit objects? List them?
A: Certain objects that are available for the use in JSP documents without being declared first. These objects are parsed by the JSP engine and inserted into the generated servlet. The implicit objects re listed below

* request
* response
* pageContext
* session
* application
* out
* config
* page
* exception


Q: Difference between forward and sendRedirect?
A: When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completly with in the web container. When a sendRedirtect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completly new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward.


Q: What are the different scope valiues for the ?
A: The different scope values for are

1. page
2. request
3.session
4.application


Q: Explain the life-cycle mehtods in JSP?
A: THe generated servlet class for a JSP page implements the HttpJspPage interface of the javax.servlet.jsp package. Hte HttpJspPage interface extends the JspPage interface which inturn extends the Servlet interface of the javax.servlet package. the generated servlet class thus implements all the methods of the these three interfaces. The JspPage interface declares only two mehtods - jspInit() and jspDestroy() that must be implemented by all JSP pages regardless of the client-server protocol. However the JSP specification has provided the HttpJspPage interfaec specifically for the JSp pages serving HTTP requests. This interface declares one method _jspService().
The jspInit()- The container calls the jspInit() to initialize te servlet instance.It is called before any other method, and is called only once for a servlet instance.
The _jspservice()- The container calls the _jspservice() for each request, passing it the request and the response objects.
The jspDestroy()- The container calls this when it decides take the instance out of service. It is the last method called n the servlet instance.


Q: How do I prevent the output of my JSP or Servlet pages from being cached by the browser?
A: You will need to set the appropriate HTTP header attributes to prevent the dynamic content output by the JSP page from being cached by the browser. Just execute the following scriptlet at the beginning of your JSP pages to prevent them from being cached at the browser. You need both the statements to take care of some of the older browser versions.

<%
response.setHeader("Cache-Control","no-store"); //HTTP 1.1
response.setHeader("Pragma\","no-cache"); //HTTP 1.0
response.setDateHeader ("Expires", 0); //prevents caching at the proxy server
%>


Q: How does JSP handle run-time exceptions?
A: You can use the errorPage attribute of the page directive to have uncaught run-time exceptions automatically forwarded to an error processing page. For example:
<%@ page errorPage=\"error.jsp\"%> redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during request processing. Within error.jsp, if you indicate that it is an error-processing page, via the directive: <%@ page isErrorPage=\"true\"%> Throwable object describing the exception may be accessed within the error page via the exception implicit object. Note: You must always use a relative URL as the value for the errorPage attribute.


Q: How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it?
A: You can make your JSPs thread-safe by having them implement the SingleThreadModel interface. This is done by adding the directive <%@ page isThreadSafe="false"%> within your JSP page. With this, instead of a single instance of the servlet generated for your JSP page loaded in memory, you will have N instances of the servlet loaded and initialized, with the service method of each instance effectively synchronized. You can typically control the number of instances (N) that are instantiated for all servlets implementing SingleThreadModel through the admin screen for your JSP engine. More importantly, avoid using the tag for variables. If you do use this tag, then you should set isThreadSafe to true, as mentioned above. Otherwise, all requests to that page will access those variables, causing a nasty race condition. SingleThreadModel is not recommended for normal use. There are many pitfalls, including the example above of not being able to use <%!%>. You should try really hard to make them thread-safe the old fashioned way: by making them thread-safe .


Q: How do I use a scriptlet to initialize a newly instantiated bean?
A: A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone.

The following example shows the “today” property of the Foo bean initialized to the current date when it is instantiated. Note that here, we make use of a JSP expression within the jsp:setProperty action.

<jsp:usebean id="foo" class="com.Bar.Foo">

<jsp:setProperty name="foo" property="today"
value="<%=java.text.DateFormat.getDateInstance().format(new java.util.Date())%>" / >

<%-- scriptlets calling bean setter methods go here --%>

</jsp:useBean >


Q: How can I prevent the word "null" from appearing in my HTML input text fields when I populate them with a resultset that has null values?
A: You could make a simple wrapper function, like

<%!
String blanknull(String s) {
return (s == null) ? \"\" : s;
}
%>

then use it inside your JSP form, like

<input type="text" name="lastName" value="<%=blanknull(lastName)% >" >

Q: What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization?
A: Although the SingleThreadModel technique is easy to use, and works well for low volume sites, it does not scale well. If you anticipate your users to increase in the future, you may be better off implementing explicit synchronization for your shared data. The key however, is to effectively minimize the amount of code that is synchronzied so that you take maximum advantage of multithreading.

Also, note that SingleThreadModel is pretty resource intensive from the server\'s perspective. The most serious issue however is when the number of concurrent requests exhaust the servlet instance pool. In that case, all the unserviced requests are queued until something becomes free - which results in poor performance. Since the usage is non-deterministic, it may not help much even if you did add more memory and increased the size of the instance pool.


Q: How can I enable session tracking for JSP pages if the browser has disabled cookies?
A: We know that session tracking uses cookies by default to associate a session identifier with a unique user. If the browser does not support cookies, or if cookies are disabled, you can still enable session tracking using URL rewriting. URL rewriting essentially includes the session ID within the link itself as a name/value pair. However, for this to be effective, you need to append the session ID for each and every link that is part of your servlet response. Adding the session ID to a link is greatly simplified by means of of a couple of methods: response.encodeURL() associates a session ID with a given URL, and if you are using redirection, response.encodeRedirectURL() can be used by giving the redirected URL as input. Both encodeURL() and encodeRedirectedURL() first determine whether cookies are supported by the browser; if so, the input URL is returned unchanged since the session ID will be persisted as a cookie.

Consider the following example, in which two JSP files, say hello1.jsp and hello2.jsp, interact with each other. Basically, we create a new session within hello1.jsp and place an object within this session. The user can then traverse to hello2.jsp by clicking on the link present within the page. Within hello2.jsp, we simply extract the object that was earlier placed in the session and display its contents. Notice that we invoke the encodeURL() within hello1.jsp on the link used to invoke hello2.jsp; if cookies are disabled, the session ID is automatically appended to the URL, allowing hello2.jsp to still retrieve the session object. Try this example first with cookies enabled. Then disable cookie support, restart the brower, and try again. Each time you should see the maintenance of the session across pages. Do note that to get this example to work with cookies disabled at the browser, your JSP engine has to support URL rewriting.

hello1.jsp
<%@ page session=\"true\"%>
<%
Integer num = new Integer(100);
session.putValue("num",num);
String url =response.encodeURL("hello2.jsp");
%>
< a href="\'<%="url%">\' > hello2.jsp</a>

hello2.jsp
<%@ page session="true"%>
<%
Integer i= (Integer )session.getValue("num");
out.println("Num value in session is " + i.intValue());
%>

Q: What is the difference b/w variable declared inside a declaration part and variable declared in scriplet part?
A: Variable declared inside declaration part is treated as a global variable.that means after convertion jsp file into servlet that variable will be in outside of service method or it will be declared as instance variable.And the scope is available to complete jsp and to complete in the converted servlet class.where as if u declare a variable inside a scriplet that variable will be declared inside a service method and the scope is with in the service method.


Q: Is there a way to execute a JSP from the comandline or from my own application?
A: There is a little tool called JSPExecutor that allows you to do just that. The developers (Hendrik Schreiber & Peter Rossbach ) aim was not to write a full blown servlet engine, but to provide means to use JSP for generating source code or reports. Therefore most HTTP-specific features (headers, sessions, etc) are not implemented, i.e. no reponseline or header is generated. Nevertheless you can use it to precompile JSP for your website.

Interview Questions On JMS

What is messaging and how is it different from RMI?

Messaging occurs between applications or software components which may or may not be distributive in nature.In enterprise applications where the possibility of distributed components across the geographies are too high, messaging comes as an obvious choice to make them communicate with each other.

In Java , middleware(which acts as an infrastructural support for messaging) level messaging is supported with Java Message Service(JMS) APIs.JMS is a specification (java.sun.com/products/jms ) that describes the properties and behavior of an information pipe for Java software. It also describes how Java client applications interact with the information pipe.

JMS helps business applications asynchronously send and receive critical business data and events.It supports both message queueing and publish-subscribe styles of messaging.

The constituents of JMS are producers,consumers and messages themselves through interfaces' abstraction.The beauty lies here in loose coupling and infact message broker systems need not go in details of message-contents, they just have to deliver them between systems reliably.There is not dependency on APIs of systems involved in message communications.

In case of RMI, the message passing is tightly coupled with APIs of remote application or component.That is what makes JMS as an obvious choice in enterprise applications for reliable messaging.

When is JMS needed?

JMS can be used in following scenarios:

- When distributed components,applications within an enterprise need to communicate with one another without creating any sort of dependencies(loose coupling support by JMS).This communication could either be synchronous or asynchronous.

- When an application has to communicate with a provider without worrying its availability,.i.e. in an asynchronous way.A synchronous communication occurs when message requested is consumed immediately that means both producer and consumer are up and running at the same time while in case of asynchronous communication it is not necessary to have both entities available at the same time. An application may like to send a message to other without waiting for response and keep on doing its job.

How Does the JMS API Work with the Java EE Platform?

The JMS API in the Java EE platform has the following features.

Application clients, Enterprise JavaBeans (EJB) components, and web components can send or synchronously receive a JMS message. Application clients can in addition receive JMS messages asynchronously. (Applets, however, are not required to support the JMS API.)
Message-driven beans, which are a kind of enterprise bean, enable the asynchronous consumption of messages. A JMS provider can optionally implement concurrent processing of messages by message-driven beans.
Message send and receive operations can participate in distributed transactions, which allow JMS operations and database accesses to take place within a single transaction.

The JMS API enhances the Java EE platform by

-allowing loosely coupled, reliable, asynchronous interactions among Java EE components and legacy systems capable of messaging.

-supporting distributed transactions and allowing for the concurrent consumption of messages. For more information, see the Enterprise JavaBeans specification, v3.0.

Moreover, a JMS provider can be integrated with the application server using the Java EE Connector architecture. You access the JMS provider through a resource adapter. This capability allows vendors to create JMS providers that can be plugged in to multiple application servers, and it allows application servers to support multiple JMS providers.

Explain JMS API Architecture.

A JMS application is consisted of the following parts as shown in the figure at the end of this post:

-JMS Provider is a messaging system that implements the JMS interfaces and provides administrative and control features.
-JMS Clients are any Java EE application component except Applets.
-Messages are objects that exchange information between JMS clients.
-Administrative objects are preconfigured JMS objects created by an administrator for the use of clients.

Administrative tools bind destinations and connection factories into a JNDI namespace. A JMS client can then use resource injection to access the administered objects in the namespace and then establish a logical connection to the same objects through the JMS provider.

Image Source:java.sun.com

Explain Point-to-Point Messaging Domain

The constituents of a point-to-point (PTP) product or application are message queues, senders, and receivers. Each message is addressed to a specific queue, and receiving clients extract messages from specific queue(s) . Queues retain all messages sent to them until the messages are consumed or until the messages expire.

So in PTP messaging domain:-

Each message has only one consumer.

There are no timing dependencies on a sender and a receiver of a message . The receiver can fetch the message oblivious of its availability when the client sent the message.

The receiver acknowledges the successful processing of a message.

PTP messaging is used when every message sent must be processed successfully by one consumer.

PTP Messaging Model(Image Source:java.sun.com)

Explain Publish/Subscribe Messaging Domain.

In a publish/subscribe (pub/sub) product or application, clients(publishers as well as subscribers) address messages to a topic, which functions similar to a bulletin board. Both publishers and subscribers are generally anonymous and can dynamically publish or subscribe to the content hierarchy. The system takes care of distributing the messages arriving from a topic's multiple publishers to its multiple subscribers. Topics retain messages only as long as it takes to distribute them to current subscribers.

Pub/sub messaging has the following characteristics.

Each message can have multiple consumers.
Publishers and subscribers have a timing dependency. A client that subscribes to a topic can consume only messages published after the client has created a subscription, and the subscriber must continue to be active in order for it to consume messages.

Pub-Sub Messaging Domain(Image Source:java.sun.com)

Q: What is JMS?

A: JMS is an acronym used for Java Messaging Service. It is Java's answer to creating software using asynchronous messaging. It is one of the official specifications of the J2EE technologies and is a key technology.

Q: How JMS is different from RPC?

A: In RPC the method invoker waits for the method to finish execution and return the control back to the invoker. Thus it is completely synchronous in nature. While in JMS the message sender just sends the message to the destination and continues it's own processing. The sender does not wait for the receiver to respond. This is asynchronous behavior.

Q: What are the advantages of JMS?

A: JMS is asynchronous in nature. Thus not all the pieces need to be up all the time for the application to function as a whole. Even if the receiver is down the MOM will store the messages on it's behalf and will send them once it comes back up. Thus at least a part of application can still function as there is no blocking.

Q: Are you aware of any major JMS products available in the market?

A: IBM's MQ Series is one of the most popular product used as Message Oriented Middleware. Some of the other products are SonicMQ, iBus etc.All the J2EE compliant application servers come built with thier own implementation of JMS.

Q: What are the different types of messages available in the JMS API?

A: Message, TextMessage, BytesMessage, StreamMessage, ObjectMessage, MapMessage are the different messages available in the JMS API.

Q: What are the different messaging paradigms JMS supports?

A: Publish and Subscribe i.e. pub/suc and Point to Point i.e. p2p.

Q: What is the difference between ic and queue?

A: A ic is typically used for one to many messaging i.e. it supports publish subscribe model of messaging. While queue is used for one-to-one messaging i.e. it supports Point to Point Messaging.

Q: What is the role of JMS in enterprise solution development?

A: JMS is typically used in the following scenarios
1. Enterprise Application Integration: - Where a legacy application is integrated with a new application via messaging.
2. B2B or Business to Business: - Businesses can interact with each other via messaging because JMS allows organizations to cooperate without tightly coupling their business systems.
3. Geographically dispersed units: - JMS can ensure safe exchange of data amongst the geographically dispersed units of an organization.
4. One to many applications: - The applications that need to push data in packet to huge number of clients in a one-to-many fashion are good candidates for the use JMS. Typical such applications are Auction Sites, Stock Quote Services etc.

Q: What is the use of Message object?

A: Message is a light weight message having only header and properties and no payload. Thus if theIf the receivers are to be notified abt an event, and no data needs to be exchanged then using Message can be very efficient.

Q: What is the basic difference between Publish Subscribe model and P2P model?

A: Publish Subscribe model is typically used in one-to-many situation. It is unreliable but very fast. P2P model is used in one-to-one situation. It is highly reliable.

Q: What is the use of BytesMessage?

A: BytesMessage contains an array of primitive bytes in it's payload. Thus it can be used for transfer of data between two applications in their native format which may not be compatible with other Message types. It is also useful where JMS is used purely as a transport between two systems and the message payload is opaque to the JMS client. Whenever you store any primitive type, it is converted into it's byte representation and then stored in the payload. There is no boundary line between the different data types stored. Thus you can even read a long as short. This would result in erroneous data and hence it is advisable that the payload be read in the same order and using the same type in which it was created by the sender.

Q: What is the use of StreamMessage?

A: StreamMessage carries a stream of Java primitive types as it's payload. It contains some conveient methods for reading the data stored in the payload. However StreamMessage prevents reading a long value as short, something that is allwed in case of BytesMessage. This is so because the StreamMessage also writes the type information alonwgith the value of the primitive type and enforces a set of strict conversion rules which actually prevents reading of one primitive type as another.

Q: What is the use of TextMessage?

A: TextMessage contains instance of java.lang.String as it's payload. Thus it is very useful for exchanging textual data. It can also be used for exchanging complex character data such as an XML document.

Q: What is the use of ObjectMessage?

A: ObjectMessage contains a Serializable java object as it's payload. Thus it allows exchange of Java objects between applications. This in itself mandates that both the applications be Java applications. The consumer of the message must typecast the object received to it's appropriate type. Thus the consumer should before hand know the actual type of the object sent by the sender. Wrong type casting would result in ClassCastException. Moreover the class definition of the object set in the payload should be available on both the machine, the sender as well as the consumer. If the class definition is not available in the consumer machine, an attempt to type cast would result in ClassNotFoundException. Some of the MOMs might support dynamic loading of the desired class over the network, but the JMS specification does not mandate this behavior and would be a value added service if provided by your vendor. And relying on any such vendor specific functionality would hamper the portability of your application. Most of the time the class need to be put in the classpath of both, the sender and the consumer, manually by the developer.

Q: What is the use of MapMessage?

A: A MapMessage carries name-value pair as it's payload. Thus it's payload is similar to the java.util.Properties object of Java. The values can be Java primitives or their wrappers.

Q: What is the difference between BytesMessage and StreamMessage?

A: BytesMessage stores the primitive data types by converting them to their byte representation. Thus the message is one contiguous stream of bytes. While the StreamMessage maintains a boundary between the different data types stored because it also stores the type information along with the value of the primitive being stored. BytesMessage allows data to be read using any type. Thus even if your payload contains a long value, you can invoke a method to read a short and it will return you something. It will not give you a semantically correct data but the call will succeed in reading the first two bytes of data. This is strictly prohibited in the StreamMessage. It maintains the type information of the data being stored and enforces strict conversion rules on the data being read.