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This example creates abean. A stateless session bean is similar to a servlet without sessions. The server bean itself can have state, but doesn't remember the client from one request to the next.
A Hello, World example for EJB is fairly complicated. To implement the EJB you need to implement:
To configure Resin to be a server for the EJB you need to:
To use the EJB from a client you need to:
In this tutorial, a simple "Hello" EJB is created and deployed within Resin. The use of the Hello EJB from a client is also demonstrated.
The remote interface defines the client view of the bean. It extendsand declares all the business methods. Our only business method is the hello method.
The sole responsibility of the Home interface is to create a remote object. The client uses the Home to obtain instances of the bean.
The third class for EJB's is the bean implementation class. It implements the functionality provided by the remote interface.
A stateless session bean needs to have exactly onemethod with no arguments. It must also define the business methods.
The bean implementation class for all EJB Session bean's must implement the SessionBean interface. Most users will create a com.foo.AbstractSessionBean class that provides default method implementations. This example extends Resin's AbstractSessionBean to simplify the implementation.
A deployment descriptor is used to describe and define the EJB bean. In general, configuration of EJB's belongs in anfile. In a jar, it belongs in .
This name, of course, only makes sense if the EJB is put into a jar. Jars are inconvenient during development, so Resin lets you rename the ejb-jar.xml file as *.ejb and put it in WEB-INF. We'll name it. You can have as many *.ejb files as you want, Resin will use them all (for example WEB-INF/hello.ejb, WEB-INF/goodbye.ejb).
is the top element for a deployment descriptor for the bean. It's part of the EJB spec. The configuration for the stateless session example looks like the following:
All of the above steps are needed to define and implement an EJB. Once the bean has bean defined and implemented, it is deployed within the Resin server. Through deployment, Resin becomes aware of the EJB and how it is meant to be used.
<ejb-server> is used to configure the Resin EJB server. The EJB server is responsible for reading the *.ejb descriptor files and deploying the beans within the server.
The ejb-server automatically searches for all *.ejb files in WEB-INF. It will find the hello.ejb file and deploy the "hello" EJB bean.
The server needs to make the EJB's available to remote clients using a. The protocol defines the language that is used by the client and the server to communicate.
This example uses the Hessian EJB Servlet to make the EJB's available to remote clients using the.
When the EJB is used by a Hessian client, the client adds thefrom the hello.ejb file to the url that points to the Hessian servlet. In this example, it is .
The whole point of EJB stateless session beans is to provide a means of communication between a client and a server. Typically, the client will be on a different machine, or at least a different Java virtual machine, than the server.
The client needs to be provided with some things to be able to accomplish this. It needs:
EJB clients perform the following steps in obtaining and using a remote object:
The factory is the link between the client and the server. Once the factory is created, it is used to obtain Home's for particular EJB's. The factory, and the Home's, do not change, Usually the factory and the Home for each EJB are created once and then stored for future use. An applet, for example, creates the factory on startup and stores it as a member variable. Individual classes within the applet, that need particular Home's, create them once and store them as member variables.
The home is used to create a stub to a particular remote instance of an object. A stub is a placeholder or proxy object. The client uses the stub just like any other object, the actual execution of the methods provided by the stub happens on the server.
In this example, the client is a servlet and the Home is obtained in the init() method and stored as a member variable. A HessianProxyFactory is created and then used to obtain the HelloHome for the Hello ejb.
The url of the server is passed as an init-param to the servlet. This allows for easy configuration if the url of the server adress changes. The reulsting url that is passed to the factory will look something like http://localhost:8080/hessian/home.
The preceding example used Hessian directly. This is a good idea for applets or other clients that do not have good support for the use of JNDI.
More often, clients will use, the Java Naming and Directory Interface, to get the Home. JNDI is used to configure the , the factory is obtained with a JNDI lookup and is used to obtain Home's for the different EJB's.
JNDI is based on naming. Contexts are like paths on a file system, but store objects instead of files. By convention, the path is used for EJB.
Typically, the client will be on a different machine, or at least a different Java virtual machine, than the server. Each client environment will provide a different means of configuring a JNDI factory.
Resin uses jndi-link to configure the JNDI factory.
The url of the server is passed as an init-param to the HessianContextFactory. This allows for easy configuration if the url of the server address changes.
HessianContextFactory can be used in any server or other Java environment that supports JNDI.
The client code does a JNDI lookup to get the factory. As in the previous client example, the client is a servlet and the Home is obtained in the init() method and stored as a member variable.
Only the init() method is differet in the HelloJndiServlet, the rest of the class is identical to the previous HelloServlet.