EJB 3.0 Tutorial An application server is an application program that accepts connections in order to service requests, by sending back responses.

Application Server

An application server is an application program that accepts connections in order to service requests, by sending back responses. An application server can run remotely (connected to client through a computer network) or can exist on the same computer where the client application is running. Examples include file server, database server, backup server, print server, mail server, web server, FTP server, application server, VPN server, DHCP server, DNS server, WINS server, logon server, security server, domain controller, backup domain controller, proxy server, firewall, etc.

Application server are developed to support the quick development of the enterprise applications. They provide security and state maintenance with the data base and persistence.

An application server may be a part of a three tier architecture model. A three tier architecture includes the Client Tier, Middle Tier and the EIS (Enterprise Information System) Tier. It may consists of Presentation Tier (as the GUI interface), Middle Tier as the collection of business logic application and the EIS (Enterprise Information System). The view tier is nothing but the web based graphical user interface to interact with the clients, Middle tier is the combination of web containers and the EJB containers. EIS contains persistence and the database management systems to support the applications. JEE Platform requires database to store the business data. This database is accessible by means of JDBC, JDO or the SQLJ APIs. We can also access the database through enterprise beans, web components and the application client components.

EJB Container or EJB ServerAn EJB container is nothing but the program that runs on the server and implements the EJB specifications.EJB Container or EJB Server

An EJB container is nothing but the program that runs on the server and implements the EJB specifications. EJB container provides special type of the environment suitable for running the enterprise components. Enterprise beans are used in distributed applications that typically contains the business logic. The container performs the various tasks few of them are illustrated below:

Transaction Management:EJB container allows you for transaction management that enables the transaction services, a low level implementation of transaction management and coordination. Container uses the Java Transaction APIs to expose the transaction services. JTA, a high level interface is used to control transactions.

Security:JSEmainly focuses on how to become the environment more secure. Enterprise beans add this feature to provide transparent security so that access to the beans can be made secure just by applying the security attributes rather than coding against the security API.

Resource and Life Cycle Management:EJB container manages the resources like database connections, threads and socket on behalf of enterprise beans. Container creates, destroys, registers the objects and also activates and passivates them. The container is also capable of reusing them whenever required.

Remote Accessibility:A client on the remote machine containing JVM can invoke an enterprise bean running on the host machine. To support the remote accessibility the container uses the remote procedure call technology. When the software is developed by using the OOPs concept then Remote procedure call (RPC) may be referred to as Remote Method Invocation (RMI).

Concurrency Control:Concurrency control is necessary to know the basics of collisions and type of collisions that can occur. If you are not interested to occur them then you can bypass them later they will cause to create problems. So try to detect and resolve them. To do so the EJBContainer supports for various types of concurrency controls. First we will concentrate on collision and then techniques to resolve these collisions.Collision:A collision can occur when two or more transactions tries to change the entities within a system of records. There are three typesthrough which twoor more activities may interfere:

Dirty read

Non Repeatable read

Phantom read

Control mechanism:Mainly two mechanism are used to control the concurrency.

Optimistic Locking

Pessimistic Locking

Clustering and load-balancing:Clustering is the process of combining the multiple peripherals, computers and other resources into a single unit. A clustered system then works as load balanced system. In a distributed system when a request is send to the server, an algorithm running on the server decides which server has less load and sends the request to that server. EJB container encapsulates these features to provide smooth and efficient service.

Enterprise beans are the Java EE server side components that run inside the ejb container and encapsulates the business logic of an enterprise application.

Enterprise Beans

Enterprise beans are the Java EE server side components that run inside the ejb container and encapsulates the business logic of an enterprise application. Enterprise applications are the software applications developed intended to use at large scale. These applications involves large number of data accessing concurrently by many users. Enterprise beans are used to perform various types of task like interacting with the client, maintaining session for the clients retrieving and holding data from the database and communicating with the server.

Benefits of enterprise beans:Enterprise beans are widely used for developing large and distributed applications. EJB container provides the system-level services (like transaction management and security authorization) to the enterprise beans. These services simplifies the development process of large and distributed applications and the developer just have to concentrate on solving the problem occurred in it.

The client developer can focus only on the representation style of the client as all the business logic of the application is contained in the enterprise beans. The client developer does not bother about the code that implements business logic or the logic to access the databases. This means that the clients are thinner. It benefits the client to be thin because the client runs on the small devices.

Developing new applications from the existing beans is much easier because the enterprise beans are portable components. That means applications developed by using the enterprise components can run on any complaint J2EE server.

When to use the enterprise beans:Some of the points are illustrated below that signifies the use of enterprise beans.

Applications developed by using the enterprise beans deal with a variety of clients, simply by writing few lines of code, so that the client can locate the enterprise beans. The client locating the enterprise bean may be various, numerous and thin.

Enterprise beans support transaction to ensure the integrity of the database. Transaction is the mechanism of managing the concurrent access of shared objects.

Managing the fast growing number of users requires distributed application components across multiple machines means that the application must be scalable.

Types of enterprise beans:EJB 3.0 defines two types of enterprise beans. These are:

1. Session bean:These types of beans directly interact with the client and contains business logic of the business application.

2. Message driven bean:It works like a listener or a consumer for a particular messaging service such as Java Message API or JPA for short.

Features of EJB 3.0

Now its time to look over the new features of EJB 3.0 that provides some simplification over the previous EJB API. There are various simplification made in EJB 3.0 like:

No need of home and object interface.

No need of any component interface.

Made use of java annotations.

Simplifies APIs to make flexible for bean's environment.

Now we will discuss all the above aspects of EJB3.0 that makes the EJB programming model simple and more efficient.

Elimination of Home and Remote Interfaces:Deprecation of home and remote interfaces simplifies the development. The new session beans contain all the business methods inside the business interface. The bean provider designates the business interface as local business interface or the remote business interface or both according to the client whether it is local or remote. Business methods on remote interface can throw arbitrary application exceptions, while they are not allowed to throw java.rmi.RemoteException. While in case of EJB 2.1 all the methods of home and object interface throws the java.rmi.RemoteException. Package javax.ejb.EJBException encapsulates exceptions such as protocols, system level problems, or otherwise that the container returns to the client. Since EJBException is the subclass of the java.rmi.RemoteException therefore we did not include it in throws clause of business methods.

A message driven bean does not need to include the business interface as there is no direct interaction of the client with the message driven bean. Whenever a MDB has an unexpected problem then the container logs the error and communicate it with the help of javax.ejb.EJBException to the corresponding resource adapter rather than the client.

Elimination of Component Interface:Component interface in EJB2.1 or in earlier versions are used to provide a way through which the container notifies the bean instances of various life cycles they are affecting it. The previous versions of the component interface are used to stay in the events in its lifecycle. These component interfaces includes the various life cycles methods implemented by the bean class. The container used to call the appropriate method of the component interface to handle the bean's instance life cycle events according to the way it wants.For example, the container notifies the message driven bean instance that it is about to destroy, simply by invoking ejbDestroy() method on the corresponding class of the message driven bean. Bean class can close the JDBC database connection within the ejbDestroy() method to free up the resources. Similarly the container that is going to associate a client in case of stateful notifies the bean instance by calling the ejbCreate() method on the bean class and the bean class instantiates the bean instance. Consider the situation, when the bean does not receive the notification from the container about its life cycle's methods then the bean has to implement the component interfaces regardless whether it is needed or not. In case of session bean, the bean class does not need to implement the javax.ejb.SessionBean or javax.ejb.MessageDrivenBean in case of message driven bean.

Now the next question that arises is that how a bean class get notified by the container if it is interested? The solution is that there are two ways to do so, the first one is, the bean provider can implement a separate bean class that consists of all the callback notification methods that inform the container to treat it as a listener class. The second way is that, a bean provider can implement the notification method inside the bean class and designate this method to handle the corresponding events In both the cases bean class uses annotations. Annotations are the additional key features of EJB 3.0 specifications. To know about the annotation just click on the linkannotations.

Simplified Access to Environment:Almost all the EJBs are required to access the environment to gain access to external resources, enterprise beans and other entries like properties. To get hold of these entries EJB mainly relies on JNDI API. EJB 3.0 also includes the features like lookup method on the EJBContest and dependency injection to access the bean's dependencies.

Dependency Injection:Dependency injection is the mechanism through which the container injects the requested environmental entry to make available to the bean instance before any bean instance is invoked on that particular instance. Then the container injects these entries into bean variables or methods. It is bean provider's duty to tell the container that which method or variables should be injected at runtime. The bean provider can do this by using the deployment descriptor or annotations. Bean methods used for dependency injection should follow the java naming convention for properties in that they should follow the setXXX() convention.Consider the situations like dependency injection fails due to some reasons, the container can not make available the environmental entries due to which the bean is functioning properly, in such situations the container discards the bean instances and creates new instances.

EJB Context:Bean must know about its environment at runtime such as security principle, transaction context in which its method is invoked and so on. javax.ejb.EJBContext API works like a window for the bean to the outside world through which it is interacting to the container. EJBContext is further categorized into SessionContext and MessageDrivenContext for the session beans and message driven beans respectively. Bean instances may use the dependency injection to access EJBContext instance. Another way through which a JNDI accesses the environment variables is the lookup() method of the EJBContext interface. Bean must use the JNDI API to access the environmental dependencies.

Enhanced Lifecycle Methods and Callback Listener Classes:EJB 3.0 does not enforce to implement all unnecessary callback methods but can designate any other method to receive the notification for life cycle events. We can also use the callback listener class instead of callback methods defined in the same bean class.

Interceptors:An intercept is a method used to intercept a business method invocation. Stateless session beans, Stateful session beans and message driven beans may includes the interceptors. We can also define an interceptor class instead of defining the interceptor methods in the bean class.

Simple JNDI lookup of EJB:Lookup of the EJB has been simplified so that the client can directly invoke methods on EJB rather than creating the bean instance simply by invoking create method on EJB.

Sun Microsystem added the features like annotation to make the development easier and more efficient in jdk 5. The main objective to develop the annotations is to make the development easier.

Annotations

Sun Microsystem added the features like annotation to make the development easier and more efficient in jdk 5. The main objective to develop the annotations is to make the development easier. Annotations behaves like the meta. The literal meaning of meta data is data about data. Java also signifies this meaning. Annotations are like meta data, means you are free to add your code and can also apply them to variables, parameters, fields type declarations, methods and constructors. Metadata is also used to create the documentation to perform rudimentary compile time checking and even for tracking down the dependencies in code. XDoclet contains all these features and is widely used. Annotations provide a means of indicating about methods, classes, dependencies, incompleteness and also about the references on other methods and classes respectively. Quoting from Sun's official site, "It (annotation-based development) lets us avoid writing boilerplate code under many circumstances by enabling tools to generate it from annotations in the source code. This leads to a declarative programming style where the programmer says what should be done and tools emit the code to do it."

Annotation is the way of associating the program elements with the meta tags so that the compiler can extract program behavior to support the annotated elements to generate interdependent code when necessary.

Fundamentals of annotationsWhile going through the annotations you should consider two things. The first one is the"annotation"itself and second one is the"annotations types". An annotation is the meta tag, used to give some life to the code you are using. While annotation type is used to define annotations so that you can use them while creating your own custom annotations.

An annotation type definition appends an "at" @ sign at the start of the interface keyword with the annotation name. On the other hand, an annotation includes the "at" @ sign followed by the annotation type. You can also add the data within the parenthesis after the annotation name. Lets illustrate the concept more clearly by using some examples.

Defining an annotation (Annotation type)public @interface Example {String showSomething();}

Annotating the code (Annotation)Example (showSomething="Hi! How r you")public void anymethod() {....}

Annotation Types:Three types of annotations types are there in java.

Marker:Like the marker interface, marker annotations does not contain any elements except the name itself. The example given below clarifies the concept of marker interface.

Example: public @interface Example{ }

Usage: @Example public void anymethod() { ------------ }

Single-value:This type of elements provide only single value. It means that these can be represented with the data and value pair or we can use the shortcut syntax (just by using the value only within the parenthesis).

Example: public @interface Example{

String showSomething();

}

Usage: @Example ("Hi ! How r you") public void anymethod(){

--------

}

Multi-value or Full-value:These types of annotations can have multiple data members. Therefore use full value annotations to pass the values to all the data members.

Example:public @interface Example{

String showSomething();int num; String name; }

Usage: @Example (showSomething = "Hi! How r you", num=5, name="zulfiqar" ) public void anymethod{

// code here

}

Rules defining the Annotation type:Here are some rules that one should follow while defining and using annotations types

Start the annotation declaration starting with the symbol "at" @ following the interface keyword that should follow the annotation name.

Method declaration should not throw any exception.

Method declaration should not contain any parameter.

Method using annotations should return a value, one of the types given below:

String

primitive

enum

Class

array of the above types

Annotations:JDK 5 (Tiger) contains two types of annotations:

Simple annotations:These types of annotations are used to annotate the code only. We can not use these types of annotations for creating the custom annotation type.

Meta annotations:Also known as annotations of annotations are used to annotate the annotation-type declaration.

Simple annotations:JDK 5 includes three types of simple annotations.

Override

Depricated

Suppresswarnings

JDK 5 (also known as Tiger) does not include many built-in annotations but it facilitates to core java to support annotation features. Now will discuss in brief each of the above simple annotation types along with examples.

Override annotation:The override annotation ensures that the annotated method is used to override the method in the super class. If the method containing this type of annotation does not override the method in the super class then the compiler will generate a compile time error.

Lets take an example and demonstrate what will happen if the annotated method does not override the method in the super class.

Example 1:public class Override_method{@Overridepublic String toString(){return super.toString() + "Will generate an compile time error.";}}

Suppose there is spell mistake in the method name such as the name is changed from toString to toStrimg. Then on compiling the code will generate the message like this:

Compiling 1 source file to D:tempNew Folder (2)TestJavaApplication1buildclassesD:tempNew Folder (2)TestJavaApplication1srctestmyannotationTest_Override.java:24: method does not overridea method from its superclass@Override1 errorBUILD FAILED (total time: 0 seconds)

Deprecated annotation:These types of annotations ensure that the compiler warns you when you use the deprecated element of the program. The example given below illustrates this concept.

Example:Lets first create the class containing the deprecated method.

public class Deprecated_method{@Deprecatedpublic void showSomething() {System.out.println("Method has been depricated'");}}

Now lets try to invoke this method from inside the other class:

public class Test_Deprication {public static void main(String arg[]) throws Exception {new Test_Deprication();}public Test_Deprication() {Deprecated_method d = new Deprecated_method();d.showSomething();}

The method showSomething() in the above example is declared as the deprecated method. That means we can't further use this method any more. On compiling the classDepricated_method does not generate any error. While compiling the class Test_Deprication generates the message like this:

Compiling 1 source file to D:tempNew Folder(2)TestJavaApplication1buildclassesD:tempNew Folder(2)TestJavaApplication1srctestmyannotationTest_Deprication.java:27:warning: [deprecation] showSomething() intest.myannotation.Deprecated_method has been deprecatedd.showSomething();1 warning

The Suppresswarning annotation:These types of annotations ensure that the compiler will shield the warning message in the annotated elements and also in all of its sub-elements. Lets take an example:

Suppose you annotate a class to suppress a warning and one of its method to suppress another warning, then both the warning will be suppressed at the method level only. Lets demonstrate it by an example:

public class Test_Depricated {public static void main(String arg[]) throws Exception {new TestDepricated().showSomething();}@SuppressWarnings({"deprecation"})public void showSomething() {Deprecation_method d = new Deprecation_method();d.showSomething();}}

This example is suppressing the deprecation warnings that means we can't see the warnings any more.

Note:Applying annotation at most deeply nested elements is a good idea. It is better to apply annotations at the method level rather than the classto annotate a particular method.

Meta-Annotations (Annotation Types):There are four types ofm Meta annotations (or annotations of annotations) defined by the JDK 5. These are as follows:

Target

Retention

Documented

Inherited

Target annotation:Target annotation specifies the elements of a class to which annotation is to be applied. Here is the listing of the elements of the enumerated types as its value:

@Target(ElementType.TYPE)?applicable to any element of a class.

@Target(ElementType.FIELD)?applicable to field or property.

@Target(ElementType.PARAMETER)?applicable to the parameters of a method.

@Target(ElementType.LOCAL_VARIABLE)?applicable to local variables.

@Target(ElementType.METHOD)?applicable to method level annotation.

@Target(ElementType.CONSTRUCTOR)?applicable to constructors.

@Target(ElementType.ANNOTATION_TYPE)?specifies that the declared type itself is an annotation type.

Here is an example that demonstrates the target annotation:

Example:@Target(ElementType.METHOD)public @interface Test_Element {public String doTestElement();}

Now lets create a class that use the Test_Element annotation:

public class Test_Annotations {public static void main(String arg[]) {new Test_Annotations().doTestElement();}@Test_Target(doTestElement="Hi ! How r you")public void doTestElement() {System.out.printf("Testing Target Element annotation");}}

The @Target(ElementType.METHOD) specifies that this type of annotation can be applied only at method level. Compiling and running the above program will work properly. Lets try to apply this type of annotation to annotate an element:

public class Test_Annotations {@Test_Target(doTestElement="Hi! How r you")private String str;public static void main(String arg[]) {new Test_Annotations().doTestElement();}public void doTestElement() {System.out.printf("Testing Target Element annotation");}}

Here we are trying to apply@Target(ElementType.METHOD)at the field level by declaring the elementprivate String str;after the @Test_Target(doTestElement="Hi ! How r you") statement. On compiling this code will generate an error like this:

"Test_Annotations.java":D:R_AND_DTest_AnnotationsrctestmyannotationTest_Annotations.java:16:annotation type not applicable to this kind of declaration at line16, column 0@Test_Target(doTestElement="Hi ! How r you")^Error in javac compilation

Retention annotation:These types of annotation specify where and how long annotation with this types are to be retained. There are three type of Retention annotations are of three types.

RetentionPolicy.SOURCE:This type of annotation will be retained only at source level and the compiler will ignore them.

RetentionPolicy.CLASS:This type of annotation will be retained at the compile time the virtual machine (VM) will ignore them.

RetentionPolicy.RUNTIME:Virtual machine will retained the annotation of this type and they can be read only at run-time.

Lets demonstrate that how this type of annotations are applied by taking an example using RetentionPolicy.RUNTIME.

Example:@Retention(RetentionPolicy.RUNTIME)public @interface Retention_Demo {String doRetentionDemo();}

This example uses the annotation type @Retention(RetentionPolicy.RUNTIME) that indicates the VM will retained your Retention_Demo annotation so that it can be read effectively at run-time.

Documented annotation:This type of annotation should be documented by the javadoc tool. javadoc does not include the annotation by default. Include the annotation type information by using@Documentedin the generated document. In this type of annotation all the processing is done by javadoc-like tool.

The given example demonstrates the use of the@Documentedannotations.

Example:@Documentedpublic @interface Documented_Demo {String doTestDocumentedDemo();}

Next, make changes in Test_Annotations class as follows:

public class Test_Annotations {public static void main(String arg[]) {new Test_Annotations().doTestRetentionDemo();new Test_Annotations().doTestDocumentedDemo();}@Retention_Demo (doTestRetentionDemo="Hello retention annotation")public void doTestRetentionDemo() {System.out.printf("Testing 'Retention' annotation");}@Documented_Demo (doTestDocumentedDemo="Hello Test documentation")public void doTestDocumentedDemo() {System.out.printf("Testing 'Documented' annotation");}}

Inherited Annotation:This annotation is little bit complex. It inherits the annotated class automatically. If you specify@Inheritedtag before defining a class then apply the annotation at your class and finally extend the class then the child class inherits the properties of the parent class automatically. Lets demonstrate the benefits of using the@Inheritedtag by an example:

Example:Lets first, define the annotation:

@Inheritedpublic @interface ParentObjectDemo {boolean isInherited() default true;String showSomething() default "Show anything?";}

Now, annotate the class with our annotation:

@ParentObjectDemopublic Class ChildObjectDemo {}

The above example shows that you do not need to define the interface methods inside the implemented class. The @Inherited tag automatically inherits the methods for you. Suppose you define the implementing class in the old-fashioned-java-style then let us see the effect of doing this:

public class ChildObjectDemo implements ParentObjectDemo {public boolean isInherited() {return false;}public String showSomething() {return "";}public boolean equals(Object obj) {return false;}public int hashCode() {return 0;}public String toString() {return "";}public Class annotationType() {return null;}}

Have you seen the difference? You have to implement all the methods of the parent interface. You will have to implement the equals(), toString(), and the hashCode() methods of the Object class and also the annotation type method of the java.lang.annotation.Annotation class. You will also have to include all these methods in your class regardless of whether you are implementing all these methods or not.

A session bean is the enterprise bean that directly interact with the user and contains the business logic of the enterprise application.

Session Beans

What is a Session beanA session bean is the enterprise bean that directly interact with the user and contains the business logic of the enterprise application. A session bean represents a single client accessing the enterprise application deployed on the server by invoking its method. An application may contain multiple sessions depending upon the number of users accessing to the application. A session bean makes an interactive session only for a single client and shields that client from complexities just by executing the business task on server side.For example, whenever a client wants to perform any of these actions such as making a reservation or validating a credit card, a session bean should be used. The session bean decides what data is to be modified. Typically, the session bean uses an entity bean to access or modify data. They implement business logic, business rules, algorithms, and work flows. Session beans are relatively short-lived components. TheEJBcontainer may destroy a session bean if its client times out.

A session bean can neither be shared nor can persist (means its value can not be saved to the database) its value. A session bean can have only one client. As long as the client terminates, session bean associated with this client is also terminated and the data associated with this bean is also destroyed.

The above figure shows how Session Bean interacts with the clients as well as with the Entity Beans.

Session beans are divided into two parts.

Stateless Session Beans:A stateless session bean does not maintain a conversational state with the client. When a client invokes the methods of a stateless bean, the instance ofbean variables may contain a state specific to that client only for the duration of a method invocation. Once the method is finished, the client-specific state should not be retained i.e. theEJBcontainer destroys a stateless session bean.These types of session beans do not use the class variables (instance variables). So they do not persist data across method invocation and therefore there is no need to passivates the bean's instance. Because stateless session beans can support multiple clients, they provide the better scalability for applications that require large numbers of clients.

StatefulSessionBeans:These types of beans use the instance variables that allows the data persistent across method invocation because the instance variables allow persistence of data across method invocation. The client sets the data to these variables which he wants to persist. A stateful session bean retains its state across multiple method invocations made by the same client. If the stateful session bean's state is changed during a method invocation, then that state will be available to the same client on the following invocation. The state of a client bean is retained for the duration of the client-bean session. Once the client removes the bean or terminates, the session ends and the state disappears. Because the client interacts with its bean, this state is oftencalledtheconversationalstate.

For example, consider a customer using a debit card at anATMmachine. The ATM could perform various operations like checking an account balance, transferring funds, or making a withdrawal. These operations could be performed one by one, by the same customer. So the bean needs to keep track its state for each of these operations to the same client.Thus Stateful session beans has the extra overhead for the server to maintain the state than the stateless session bean.

The user interface calls methods of session beans if the user wants to use the functionality of the session bean. Session beans can call to other session beans and entity beans.

When to use session beans:

Generally session beans are used in the following circumstances:

When there is only one client is accessing the beans instance at a given time.

When the bean is not persistent that means the bean is going to exist no longer.

The bean is implementing the web services.Statefulsession beans are useful in the following circumstances:

What the bean wants to holds information about the client across method invocation.

When the bean works as the mediator between the client and the other component of the application.

When the bean have to manage the work flow of several other enterprise beans.

Statelesssession beans are appropriate in the circumstances illustrated below:

If the bean does not contain the data for a specific client.

If there is only one method invocation among all the clients to perform the generic task.

Life Cycle of a Stateless Session Bean:Since the Stateless session bean does not passivates across method calls therefore a stateless session bean includes only two stages. Whether it does not exist or ready for method invocation. A stateless session bean starts its life cycle when the client first obtains the reference of the session bean. For this, the container performs the dependency injection before invoking the annotated@PreConstructmethod if any exists. After invoking the annotated @PreConstruct method the bean will be ready to invoke its method by the client.

The above figure demonstrates how the Stateless Session Beans are created and destroyed.

The container calls the annotated@PreDestroymethod while ending the life cycle of the session bean. After this, the bean is ready for garbage collection.

Life Cycle of a Stateful Session Bean:A Stateful session bean starts its life cycle when the client first gets the reference of a stateful session bean. Before invoking the method annotated@PostConstructthe container performs any dependency injection after this the bean is ready. The container may deactivate a bean while in ready state (Generally the container uses the least recently use algorithm to passivates a bean). In the passivate mechanism the bean moves from memory to secondary memory. The container invokes the annotated@PrePassivatemethod before passivating the bean. If a client invokes a business method on the passivated bean then the container invokes the annotated@PostActivatemethod to let come the bean in the ready state.

The above image shows the various states of the Stateful Session BeansWhile ending the life cycle of the bean, the client calls the annotated @Remove method after this the container calls the annotated @PreDestroy method which results in the bean to be ready for the garbage collection.

In this part of Enterprise Session Beans, you will learn how to develop, deploy, and run a simple Java EE application named example using stateless session bean.

Stateless Session Bean Example

In this part of Enterprise Session Beans, you will learn how to develop, deploy, and run a simple Java EE application named exampleusingstatelesssession bean. The purpose ofexampleis to performs the mathematical operations such as Addition, Subtraction, Multiplication, and Division.Theexampleapplication consists of an enterprise bean, which performs the calculations, and two types of clients: anapplication clientand aweb client.

There are following steps that you have to follow to develop a example JEE application.

1. Create the enterprise bean:CalculatorBean2. Create the web client:WebClient3. Deployexampleonto the server.

4. Using a browser, run the web client.

I. Creating the enterprise bean:

The enterprise bean in our example is a stateless session bean calledCalculatorBean. The source code forCalculatorBeanis in ?net/roseindia/ejb3/stateless? directory.

CreatingCalculatorBeanrequires these steps:

(i) Coding the bean?s Remote business interface and Enterprise bean class.(ii) Compiling the source code with the Ant tool.

(i) Coding the Business Interface

The business interface defines the business methods that a client can call remotely. The business methods are implemented in the enterprise bean class. The source code for theCalculatorRemotebusiness interface is given below.packagenet.roseindia.ejb3.stateless;importjava.math.*;importjavax.ejb.Remote;importjava.lang.annotation.*;@RemotepublicinterfaceCalculatorRemote{publicfloatadd(floatx,floaty);publicfloatsubtract(floatx,floaty);publicfloatmultiply(floatx,floaty);publicfloatdivision(floatx,floaty);}

Note that, the@Remoteannotation decorating the interface definition. This lets the container know thatCalculatorBeanwill be accessed by remote clients.

II. Coding the Enterprise Bean Class

The enterprise bean class for this example is calledCalculatorBean. This class implements the four business methods (add, subtract, multiply, division) that are defined in theCalculatorRemote business interface. The source code for theCalculatorBeanclass is given below.

packagenet.roseindia.ejb3.stateless;

importjava.math.*;importjavax.ejb.Stateless;importjavax.ejb.Remote;

@Stateless(name="CalculatorBean")@Remote(CalculatorRemote.class)publicclassCalculatorBeanimplementsCalculatorRemote{publicfloatadd(floatx,floaty){returnx+y;}

publicfloatsubtract(floatx,floaty){returnx-y;}publicfloatmultiply(floatx,floaty){returnx*y;}publicfloatdivision(floatx,floaty){returnx/y;}}

Note that, the@Statelessannotation decorating the enterprise bean class. This lets the container know thatCalculatorBeanis a stateless session bean.

Compiling and Packaging the example ExampleNow you are ready to compile the remote business interface (CalculatorRemote.java), the enterprise bean class (CalculatorBean.java) and the application client (CalculatorClient.java), then package the compiled classes into an enterprise bean JAR.

II. Creating the calculator Web Client

The web client is contained in the JSP page "WebClient.jsp".A JSP page is a text-based document that contains JSP elements, which construct dynamic content, and static template data, expressed in any text-based format such asHTML,WML, andXML.

The source code for the ?form.jsp?is given below.

CalculatorCalculatorEnter first value:
Enter second value:
Seclect your choice:
Addition
Subtraction
Multiplication
Division

The following statements given below in?WebClient.jsp?are used for locating the business interface, creating an enterprise bean instance, and invoking a business method.

InitialContext ic = new InitialContext();CalculatorRemote calculator = (CalculatorRemote)ic.lookup("example/CalculatorBean/remote");

The classes needed by the client are declared using a JSPpagedirective (enclosed within thecharacters). Because locating the business interface and creating the enterprise bean are performed only once, this code appears in a JSPdeclaration(enclosed within thecharacters) that contains the initialization method,jspInit, of the JSP page. Ascriptlet(enclosed within thecharacters) retrieves the parameters from the request and converts it to aFloatobject. Finally, a JSP scriptlet invokes the enterprise bean?s business methods, and JSPexpressions(enclosed within thecharacters) insert the results into the stream of data returned to the client.

The full source code for theWebClientprogram is given below.

Theresultis:

Note: The Application Server automatically compiles web clients that are JSP pages. If the web client were a servlet, you would have to compile it.

The source code for the ?index.jsp?is given below that will actual call the client-design form.

Ejb3 Stateless Tutorial

Welcome to
Ejb3-Jboss 4.2.0 TutorialClick Calculator Example to execute Calculator

III. Deploy example onto the server

To deploy the createdexampleapplication we are going to use Jboss 4.2.0 Application Server about which you have read in the previous section of this Javajazzup issue. So you first need to download the following tools to deploy this application.

JDK 1.5 or Higher apache-ant-1.7.0 JBoss 4.2.0

Do the following steps to deploy the example application:

(i)Make a directory structure. You canClick hereto extract the readymade directory structure according to this tutorial. (ii) Create the essential deployment descriptor.xmlfiles. build.xml

Put this file in the base (stateless\code)directory.

Application.xml

Stateless Session Bean Exampleexample.war/exampleexample.jar

Put this file in theStateless\code\deploymentdescriptors\eardirectory.web.xml

Put this file in theStateless\code\deploymentdescriptors\webdirectory.Put all.jspfiles in theStateless\code\webdirectory.Put all.javafiles in theStateless\code\srcdirectory.

(iii) Start command prompt, and go to theStateless\codedirectory. Then type the command as:C:\Stateless\code>ant build.xmlThe Ant tool will deploy theexample.earfile to thejboss-4.2.0.GA\server\default\deploydirectory.

V. Running the example Web Client

Open the web browser and type the following URL to run the application:

http://localhost:8080/example

Click at the given link as Calculator Example:

Give values to the textbox and choose the desire option button asAdditionthen clicks theSubmitbutton to get the result.

The result is:9.0-------------------------------------------------------------------------------------------------------In this part of Enterprise Session Beans, you will learn how to develop, deploy, and run a simple Java EE application named account using stateful session bean.

Stateful Session Bean Example

In this part of Enterprise Session Beans, you will learn how to develop, deploy, and run a simple Java EE application namedaccountusingstatefulsession bean. The purpose ofaccountis to performs two transaction operations (deposit and withdraw) for the customer.Theaccountapplication consists of an enterprise bean, which performs thetransactions, and two types of clients: anapplication clientand aweb client.

here are following steps that you have to follow to develop a account JEE application.

1. Create the enterprise bean:AccountBean2. Create the application client:AccountCustomer3. Deployaccountonto the server.

4. Run the application client.I. Creating the enterprise bean:

The enterprise bean in our example is a statelful session bean calledAccountBean.The account session bean represents an account information in an online customer account. The bean?s customer can deposit to or withdraw the amount from his account. To manage account, you need the following code:

Remote business interface (Account)

Session bean class (AccountBean)

The Business Interface:

TheAccountbusiness interface is a plain Java interface that defines all the business methods implemented in the bean class. If the bean class implements a single interface, that interface is assumed to the business interface. The business interface is a local interface unless it is annotated with thejavax.ejb.Remoteannotation.

The bean class may also implement more than one interface. If the bean class implements more than one interface, the business interfaces must be specified by decorating the bean class with@Localor@Remote.

The source code for theAccountbusiness interface is given below.

packageejbExample.stateful;

importjavax.ejb.Remote;@RemotepublicinterfaceAccount{

publicfloatdeposit(floatamount);publicfloatwithdraw(floatamount);@Removepublic voidremove();}

Coding the Session Bean Class:The session bean class for this example is calledAccountBean.This class implements the two business methods (depositandwithdraw).TheAccountBeanclass must meet these requirements:

The class is annotated@Stateful.

The class implements the business methods defined in the business interface.

The source code for theAccountBeanclass is given below.

packageejbExample.stateful;

importjavax.ejb.Stateful;importjavax.ejb.Remote;importjavax.ejb.Remove;importjavax.ejb.*;

@Stateful(name="AccountBean")@Remote(AccountRemote.class)publicclassAccountBeanimplementsAccountRemote{floatbalance=0;

publicfloatdeposit(floatamount){balance+=amount;returnbalance;}publicfloatwithdraw(floatamount){balance-=amount;returnbalance;}@Removepublicvoidremove(){balance=0;}}

The Remove Method

Business methods annotated withjavax.ejb.Removein the stateful session bean class can be invoked by enterprise bean client to remove the bean instance. The container will remove the enterprise bean after a@Removemethod completes, either normally or abnormally. Thus, we can retain the bean's state invoked by the client until we call the annotated @Remove method.

InAccountBean, the remove method is a@Remove method shown as:

@Remove

public void remove() {

balance = 0;

}

Stateful session beans also may:

Implement any optional life cycle callback methods, annotated@PostConstruct,@PreDestroy,@PostActivate, and@PrePassivate.

Implement optional business method annotated@Remote.

Life-Cycle Callback Methods:

Methods in the bean class may be declared as a life-cycle callback method by annotating the method with the following annotations:

javax.annotation.PostConstruct

javax.annotation.PreDestroy

javax.ejb.PostActivate

javax.ejb.PrePassivate

Life-cycle callback methods must returnvoidand have no parameters. Lets understands these callback annotated methods shown in the table given below:MethodDescription

@PostConstructInvoked by the container on newly constructed bean instances before the first business method is invoked on the enterprise bean and after all dependency injection has completed.

@PreDestroyInvoked, when the bean is about to be destoryed by EJB container before removing the enterprise bean instance and after any method annotated@Removehas completed.

@PostActivateInvoked by the container after the container moves the bean from secondary storage to active status.

@PrePassivateInvoked by the container before the container passivates the enterprise bean, i.e. the container temporarily removes the bean from the environment and saves it to secondary storage.

II. Coding the account Web Client

The application client source code is in theWebClient.jspfile That illustrates the basic tasks performed by the client of an enterprise bean:

Creating an enterprise bean instance

Invoking a business method

The full source code for theWebClient.jspprogram is given below.

Pleaseselectyourchoice

Pleaseentertheamount
TheTransactioniscomplete
YourCurrentBalanceis:

The source code for the ?form.jsp?is given below.

Bank Account

Bank Transaction Request Form

Enter the amount in rupees:

Select your choice:DepositWithdraw

The source code for the ?index.jsp?is given below that will actual call the client-design form.

Ejb3 Stateful Tutorial

Welcome to
Ejb3-Jboss 4.2.0 TutorialClick Bank Transaction Example to execute Bank Bean

III. Deploy account onto the server

IV. Running the account Application ClientMessage driven beans are the light weight components used for communication. In message driven beans the messaging service is in asynchronous mode because the user is not intended to get the instant result.

Message Driven Beans

Message driven beans are the light weight components used for communication. In message driven beans the messaging service is in asynchronous mode because the user is not intended to get the instant result. To understand the concept of message driven beans more clearly first we should go through the various concepts illustrated given below:

Techniques to implement messaging, message-oriented-middleware (MOM) and the asynchronous behavior.

Utilization of JMS, message-oriented-middleware to implement JMS based message-driven-beans.

Features and comparison of message-driven-beans with entity and session beans.

Techniques to develop message driven beans, including advanced topics like gotchas and possible actions.

Motivations for messaging:EJB components are used to develop the applications for the distributed computing network. Distributed computing network uses the RMI-IIOP protocol to communicate with each other. This protocol is used to call the EJB components. RMI-IIOP accepts challenges in several areas like:

Asynchrony:A typical RMI-IIOP client has to wait until the server completes its processing and returns the result to the client. After that the server enables to the client to continue its processing.

Decoupling:An RMI-IIOP server must have the knowledge about the server that it want to use. The client can address them directly through object references. It is not possible to remove a server from the server without directly impacting the clients because the client and server are closely coupled with each other.

Reliability:Data may be lost or the client can't perform any operation if the server or the network crashes.

Support for multiple senders and receivers:You can use messaging server instead of remote method invocation (RMI) but the messaging service uses the middleman between the client and the server. This middleman simply receives the messages from one or more senders and sends these messages to one or more consumers. It is not required to get the response instantly from the receiver. The receiver may sends back response back to the sender after completing all the processing. This is known as asynchronous programming.

There are four previous concerns that the messaging addresses with RMI-IIOP as follows:

Non blocking request processing:There is no need to block the messaging request while executing another request. For example suppose you are purchasing a book from Amazon.com having a single click functionality. you are enable to continue browsing the site without waiting to check the authorization of your credit card. Until and unless there is something wrong Amazon.com sends you a confirmation email. These types of systems are developed by using the messaging technique.

Decoupling:In case of message-oriented-middleware, it is not necessary for the sender to have the knowledge about the message receiver. It only interacts with the messaging system while sending messages. Thus the message senders are decoupled from consumers.

Reliability:Your message-oriented-middleware guarantees to send a message to its destination if the receiver is not temporarily available, simply by sending the message to the message-oriented-middleware which sends the message to the receiver when he is available. While it is not possible with RMI-IIOP protocol because there is no middleman in this case. If you send a message using the RMI-IIOP protocol and if server is down then it throws the exception.

Support for multiple senders and receivers:Most of the message-oriented-middleware supports this features i.e. receiving messages from multiple senders and can also broadcast them to many receivers.

Message-oriented-middleware is the term that can be given to any system that supports the messaging system. There are a number of products based on MOM architecture. BEA Tuxedo/Q, Sun Java System Messaging Server, IBM WebSphere MQ, Tibco Rendezevous, Microsoft MSMQ, FioranoMQ and Sonic Software SonicMQ are the examples of MOM based architecture.

Java Message Services (JMS):JMS API is an enterprise tool developed by Sun Microsystems used to develop the enterprise applications. JMS API supports to a framework that allows the development of portable and message based applications. JMS communicates in synchronous or in asynchronous mode by using point-to-point and the publish-subscribe models respectively. JMS has become vendor specific and many vendors such as IBM, Oracle, Hewlett-Packard, BEA Systems and Macromedia are providing the JMS implementation.

JMS API:JMS APIs play an important role as comparing the RMI-IIOP protocol. You need to aware with different interfaces, low level topology issues like structure, messaging format, networking protocol and server location.

1. Locate the JMS provider's ConnectionFactory instance:First establish the connection by using a ConnectionFactory instance to get access the JMS provider of the MOM product you are using. An administrator typically creates and configures the ConnectionFactory for the JMS client's use.

2. Create a JMS connection:A JMS connection manages the low level network communications similar to JDBC connection. Use ConnectionFactory to get a connection. For large deployment this connection may be load balance across a group of machines.

3. Create a JMS session:A JMS session object is used to send and receive the messages and works as the ConnectionFactory for message producers and message consumers. It also allows the encapsulation of messages in transactions. We use Connection to get the session object.

4. Locate the JMS Destination:A JMS destination is an object to which we are sending or from which we are receiving the messages. Locating the right destination is similar to tuning the channel in the right direction to get the clear picture while watching the television or answering the correct phone to get the desired message.

5. Create a JMS producer or a JMS consumer:If you are interested to send a message then you need an object to pass your message. This object is known as the JMS producer. If you want to receive a message you need to call a JMS object. This object is known as the Consumer object. To get hold an object Session and Destination are used.

6. Send or Receive a message:There may be different types of messages like text, streams, maps, bytes, objects and so on. To send a messages first instantiate it and then send it by using the Producer object. If you are intended to receive a message then first receive the message with the help of Consumer object and then crack it to see what it contains.

All the points described above are applicable to both point-to-point as well as publish/subscribe.

Here we are going to discuss JMS by taking the queuing system.Queuing system includes the following parts:

JMS Server:The JMS server maintains the message queue to receive emails.

JMS Queue:In case of point-to-point messaging system, a queue is used to hold the messages from clients. Messages holding this queue are of MapMessages type that stores the information about the email as name/value pair.

Email Message Client:A client of this messaging system creates the JMS message to put it on the JMS Queue. This message contains the information about the message to be sent out.

Email Message Driven Bean:It is the message driven bean that is responsible to take the JMS MapMessage to mail it out.

JMS Architecture:Talk first about JMS while talking about the message driven beans. There are a lot of messaging systems exist in the market. Messaging systems provide a mechanism to exchange data and events asynchronously. JMS API defines a standard way to access any messaging system such as JDBC that enables the user to talk to SQL Server, Oracle and Sybase simply by using the same API. The additional benefits of JMS API are loose coupling between the generated request and the code that services the request.

Here are some of the basics of the messaging server that one should know to more clearly understand the concept of JMS APIs:

Messaging Domains

JMS Messages

Messaging Domains:A messaging system includes several models of operation.JMS API provides separate domains corresponding to different models. JMS provider is free to implement one or more domains. Point-to-Point and Publish/Subscribe are the two most commonly used domains. These two domains concludes the following concepts.

Producer:Theclient, responsible for sending the message to the destination is known as the producer.

Consumer:The client, responsible for receiving the message is known as the consumer.

Destination:Destination is the object used by the client to specify the target that uses it to send the message to or to receive the message from.

Point-to-Point (PTP):An application that uses the point-to-point technique for messaging has the following characteristics.

A PTP producer is the sender.

A PTP consumer is the receiver.

PTP uses a queue as the destination.

A receiver can consume only a single message.

A call center application is the example of a PTP domain, where a phone call enters in the queue therefore only one operator takes care of this call rather than all of the operators.

Publish/Subscribe (pub/sub):An application that uses the pub/sub technique must include the following characteristics.

A pub/sub producer is the publisher

A pub/sub consumer is the subscriber

pub/sub uses the topic as the destination.

A message can have the multiple consumer.

An email newsletter application may use the pub/sub model, where an interested candidate may become the subscriber to receive a new published message.

Integrating JMS with EJB:Integration of JMS with EJB is the excellent idea because of it the EJB components gets benefit from the value proposed by messaging like multinary communications and non blocking clients. The idea behind the introduction of new type of bean is to consume messages in an EJB application.

Use a Java Object that receives the JMS messages to call an EJB client:Instead of developing the whole new bean the java community proposed an idea of object to receive a message and to call the appropriate EJB component such as the session bean or the entity bean. But the problems that come with this approach are:

You have to implement the multithreading concept just to increase the message consumption so that you can listen the messages in multiple threads. However developing the multithreaded application is not a tuff task for the developer.

Register yourself as a listener by writing the special code for JMS messages.

Hard code the JMS destination name in your java object that access the destination information required for extra effort to access the destination information from the dish like property files.

JMS message listener receives services from the EJB container just like a plain java object such as clustering, pooling, transaction, automatic life cycle management and many more. You need to hard code this yourself that is a tuff task and error prone.

Your java object is wrapped by other EJB components therefore it requires some way to start up. If the class is running inside the container then you are required to use a server specific startup class for the activation of the java object when the EJB server starts. This is not portable as the EJB container does not define an standard way to activate the given logic.

Reuse an existing type of EJB component somehow to receive JMS messages:Session beans or entity beans may receive messages but the problem with this approach is:

Life-cycle-management:The container doesn't know the way of creating a bean when a JMS message arrives.

Threading:EJB does not allow multithreading when a message arrives to the bean that is processing other request. If a bean is processing a request it does not take the new message arrive to the bean.

Message driven bean:Message driven beans are the special type of components that can receive the JMS as well as other type of messages. Message driven bean can also be used to receive the messages other than JMS. When a message is reached at the destination then the EJB container invokes the message driven bean. A message driven bean is decoupled with the client that sends the message. A client is not allowed to access the message driven bean through a business interface. The client can interact with the message driven bean only through the messaging system. To interact with the message driven bean use the message provider specific API such as JMS.

Session beans allow you to send JMS messages and to receive them synchronously, but not asynchronously.

A Message-Driven Bean Example

IntroductionSession beans allow you to send JMS messages and to receive them synchronously, but not asynchronously. To receive messages asynchronously, a Message-driven bean is used. Message driven beans are the light weight components used for communication via messages (e.g., email or IM messages). In message driven beans, the messaging service is in asynchronous mode because the user is not intended to get the instant result.Message-driven beans can implement any messaging type. Most commonly, they implement the Java Message Service (JMS) technology.

Message-driven beans have the following characteristics:

A message-driven bean's instances retain no data or conversational state for a specific client i.e. they are stateless.

A single message-driven bean can process messages from multiple clients.

They are invoked asynchronously.

They can be transaction-aware.

They do not represent directly shared data in the database, but they can access and update this data.

A message-driven bean has only a bean class i.e. unlike a session bean, the clients don't access message-driven beans through interfaces.

They don't have the remote or local interfaces that define client access.

The given diagram shows the working process of a Message driven bean.

In Message driven beans (MDB), the client components don't locate message-driven beans and invoke methods directly. Instead, The JMS clients send messages to message queues managed by the JMS server (e.g., an email inbox can be a message queue) for which thejavax.jms.MessageListenerinterface is implemented. The message queue is monitored by a special kind of EJB(s) - Message Driven Beans (MDBs) That processes the incoming messages and perform the services requested by the message. The MDBs are the end-point for JMS service request messages. You assign a message-driven bean?s destination during deployment by using Application Server resources.

JMS:The Java Message Service (JMS) is an API for Java messaging clients. JMS provides two programming models:point-to-pointandpublish-and-subscribe.In the point-to-point model, one sender puts a message on a queue that is delivered to only one receiver. The publish-and-subscribe model adds a broadcast mode in which any number of senders can add messages to atopic, and any number of recievers receive all messages posted to topics. JMS queues and topics are bound in theJNDIenvironment and made available to J2EE applications.

Example:Our application assumes the point-to-point model, and requires setting up a queue with aQueueConnectionFactoryin JMS. This section shows, how to implement an MDB with EJB 3.0 describing the source code of a simple message-driven bean application.

In this example, we are going to implement a Message-driven bean application named "massage" that has the following components:

A client application that sends several messages to a queue:MessageClient A message-driven bean that asynchronously receives and processes the messages that are sent to the queue:MessageBeanCode for the client application:packagemdb;

importjavax.jms.*;importjavax.naming.*;importjava.text.*;importjavax.annotation.*;

@Resource(mappedName="jms/Queue")

classMessageConn{privatestaticQueuequeue=null;QueueConnectionFactoryfactory=null;QueueConnectionconnection=null;QueueSendersender=null;QueueSessionsession=null;

publicMessageConn(){try{//clientcreatestheconnection,session,andmessagesender:connection=factory.createQueueConnection();session=connection.createQueueSession(false, QueueSession.AUTO_ACKNOWLEDGE);sender=session.createSender(queue);//createandsetamessagetosendTextMessagemsg=session.createTextMessage();for(inti=0;i