Creating the Application Client - The J2EE 1.4 Tutorial

Creating the Application Client

An application client is a program written in the Java programming language. At runtime, the client program executes in a different virtual machine than the Application Server. For detailed information on the appclient command-line tool, see the man page at http://java.sun.com/j2ee/1.4/docs/relnotes/cliref/index.html.

The application client in this example requires two JAR files. The first JAR file is for the J2EE component of the client. This JAR file contains the client's deployment descriptor and class files; it is created when you run the New Application Client wizard. Defined by the J2EE Specification, this JAR file is portable across all compliant application servers.

The second JAR file contains stub classes that are required by the client program at runtime. These stub classes enable the client to access the enterprise beans that are running in the Application Server. The JAR file for the stubs is created by deploytool when you deploy the application. Because this JAR file is not covered by the J2EE specification, it is implementation-specific, intended only for the Application Server.

The application client source code is in the ConverterClient.java file, which is in this directory:

<INSTALL>/j2eetutorial14/examples/ejb/converter/src/ 

You compiled this code along with the enterprise bean code in the section Compiling the Source Files.

Coding the Application Client

The ConverterClient.java source code illustrates the basic tasks performed by the client of an enterprise bean:

Locating the home interface
Creating an enterprise bean instance
Invoking a business method

Locating the Home Interface

The ConverterHome interface defines life-cycle methods such as create and remove. Before the ConverterClient can invoke the create method, it must locate and instantiate an object whose type is ConverterHome. This is a four-step process.

Create an initial naming context.

Context initial = new InitialContext();

The Context interface is part of the Java Naming and Directory Interface (JNDI). A naming context is a set of name-to-object bindings. A name that is bound within a context is the JNDI name of the object.

An InitialContext object, which implements the Context interface, provides the starting point for the resolution of names. All naming operations are relative to a context.

Obtain the environment naming context of the application client.

Context myEnv = (Context)initial.lookup("java:comp/env");

The java:comp/env name is bound to the environment naming context of the ConverterClient component.

Retrieve the object bound to the name ejb/SimpleConverter.

Object objref = myEnv.lookup("ejb/SimpleConverter");

The ejb/SimpleConverter name is bound to an enterprise bean reference, a logical name for the home of an enterprise bean. In this case, the ejb/SimpleConverter name refers to the ConverterHome object. The names of enterprise beans should reside in the java:comp/env/ejb subcontext.

Narrow the reference to a ConverterHome object.

ConverterHome home =
  (ConverterHome) PortableRemoteObject.narrow(objref,
  ConverterHome.class);

Creating an Enterprise Bean Instance

To create the bean instance, the client invokes the create method on the ConverterHome object. The create method returns an object whose type is Converter. The remote Converter interface defines the business methods of the bean that the client can call. When the client invokes the create method, the EJB container instantiates the bean and then invokes the ConverterBean.ejbCreate method. The client invokes the create method as follows:

Converter currencyConverter = home.create(); 

Invoking a Business Method

Calling a business method is easy: you simply invoke the method on the Converter object. The EJB container will invoke the corresponding method on the ConverterBean instance that is running on the server. The client invokes the dollarToYen business method in the following lines of code.

BigDecimal param = new BigDecimal ("100.00");
BigDecimal amount = currencyConverter.dollarToYen(param); 

ConverterClient Source Code

The full source code for the ConverterClient program follows.

import javax.naming.Context;
import javax.naming.InitialContext;
import javax.rmi.PortableRemoteObject;
import java.math.BigDecimal;

public class ConverterClient {

  public static void main(String[] args) {

    try {
      Context initial = new InitialContext();
      Context myEnv = 
        (Context)initial.lookup("java:comp/env");
      Object objref = myEnv.lookup("ejb/SimpleConverter");

      ConverterHome home = 
        (ConverterHome)PortableRemoteObject.narrow(objref, 
                            ConverterHome.class);

      Converter currencyConverter = home.create();

      BigDecimal param = new BigDecimal ("100.00");
      BigDecimal amount =
        currencyConverter.dollarToYen(param);
      System.out.println(amount);
      amount = currencyConverter.yenToEuro(param);
      System.out.println(amount);

      System.exit(0);

    } catch (Exception ex) {
      System.err.println("Caught an unexpected exception!");
      ex.printStackTrace();
    }
  } 
} 

Compiling the Application Client

The application client files are compiled at the same time as the enterprise bean files, as described in Compiling the Source Files.

Packaging the Application Client

To package an application client component, you run the New Application Client wizard of deploytool. During this process the wizard performs the following tasks.

Creates the application client's deployment descriptor
Puts the deployment descriptor and client files into a JAR file
Adds the JAR file to the application's ConverterApp.ear file

To start the New Application Client wizard, select FileNewApplication Client. The wizard displays the following dialog boxes.

Introduction dialog box
Read the explanatory text for an overview of the wizard's features.
Click Next.
JAR File Contents dialog box
Select the button labeled Create New AppClient Module in Application.
In the combo box below this button, select ConverterApp.
In the AppClient Display Name field, enter ConverterClient.
Click Edit Contents.
In the tree under Available Files, locate this directory:

<INSTALL>/j2eetutorial14/examples/ejb/converter/build/

Select the ConverterClient.class file.
Click Add.
Click OK.
Click Next.
General dialog box
In the Main Class combo box, select ConverterClient.
Click Next.
Click Finish.

Specifying the Application Client's Enterprise Bean Reference

When it invokes the lookup method, the ConverterClient refers to the home of an enterprise bean:

Object objref = myEnv.lookup("ejb/SimpleConverter"); 

You specify this reference in deploytool as follows.

In the tree, select ConverterClient.
Select the EJB Ref's tab.
Click Add.
In the Coded Name field, enter ejb/SimpleConverter.
In the EJB Type field, select Session.
In the Interfaces field, select Remote.
In the Home Interface field enter, converter.ConverterHome.
In the Local/Remote Interface field, enter converter.Converter.
In the JNDI Name field, select ConverterBean.
Click OK.

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