This chapter describes how to access EJBs running in JBoss from an Applet running in a browser.

When a JVM runs an Applet, it verifies its conformity to a set of security rules which have been established in order to protect the user from malicious code. Some of these rules are:

These restrictions do have consequences on what can be done from a client Applet that want to access EJBs over the network.

The main consequence of the sandbox on EJB access from an Applet is that we will need to download the Applet from a Web server that has the same IP address as the J2EE server running the EJB we want to access. Nevertheless, it is still possible, thanks to IP traffic load-balancers for example, to have more than one server hidden behind a single IP address.

In our example, we will build a stateless session bean that simply returns the server date and hostname in a friendly message. To try this example, you’d better install the Java plug-in: support for Java in Web browsers in somewhat hazardous.

We first build our remote interface:

public interface TestApplet extends EJBObject
{
   public String getMessage() throws RemoteException;
}

And the Home interface:

public interface TestAppletHome extends EJBHome
{
   public TestApplet create() throws RemoteException, CreateException;
}

Now the simple EJB implementation:

public class TestAppletBean implements SessionBean
{
   private SessionContext sessionContext;

   public void ejbCreate() {}
   public void ejbRemove() {}
   public void ejbActivate() {}
   public void ejbPassivate(){}
   public void setSessionContext(SessionContext context)
   {
      sessionContext = context;
   }
   public String getMessage()
   {
      String hostname = "Unknown host";

      try
      {
         hostname = java.net.InetAddress.getLocalHost ().getHostName ();
      } catch (java.net.UnknownHostException uhe) { }

      String now = java.text.DateFormat.getDateInstance().format (new java.util.Date());

      System.out.println ("getMessage has been called from Applet!");

      return "Hello from " + hostname + ", here it is: " + now + ".";
   }
}

And its associated ejb-jar.xml descriptor:

<?xml version="1.0"?>

<!DOCTYPE ejb-jar PUBLIC '-//Sun Microsystems, Inc.//DTD Enterprise JavaBeans 1.1//EN' 'http://java.sun.com/j2ee/dtds/ejb-jar_1_1.dtd'>

<ejb-jar>
  <enterprise-beans>
    <session>
      <ejb-name>TestAppletBean</ejb-name>
      <home>org.jboss.docs.appletclient.interfaces.TestAppletHome</home>
      <remote>org.jboss.docs.appletclient.interfaces.TestApplet</remote>
      <ejb-class>org.jboss.docs.appletclient.ejb.TestAppletBean</ejb-class>
      <session-type>Stateless</session-type>
      <transaction-type>Container</transaction-type>
    </session>
  </enterprise-beans>
  <assembly-descriptor>
    <container-transaction>
      <method>
	<ejb-name>TestAppletBean</ejb-name>
	<method-intf>Remote</method-intf>
	<method-name>*</method-name>
      </method>
      <trans-attribute>Required</trans-attribute>
    </container-transaction>
  </assembly-descriptor>
</ejb-jar>

We now implements a simple Applet that will have a single button. When pressed, the Applet will call the EJB and display its message.

public class AppletEjbCaller extends JApplet
{
   JPanel buttonPanel = new JPanel();
   JButton callEjbButton = new JButton();
   JPanel labelPanel = new JPanel();
   JLabel ejbMessageLabel = new JLabel();


   // Construct the Applet
   //
   public AppletEjbCaller () {}

   // Initialize the applet
   //
   public void init()
   {
      try
      {
         this.setSize(new Dimension(400, 65));
         callEjbButton.setText("Get server message");
         callEjbButton.addActionListener(new java.awt.event.ActionListener()
         {
            public void actionPerformed(ActionEvent e)
            {
               callEjbButton_actionPerformed(e);
            }
         });
         ejbMessageLabel.setText("no call performed yet.");
         this.getContentPane().add(buttonPanel, BorderLayout.CENTER);
         buttonPanel.add(callEjbButton, null);
         this.getContentPane().add(labelPanel, BorderLayout.SOUTH);
         labelPanel.add(ejbMessageLabel, null);
      }
      catch(Exception e)
      {
         e.printStackTrace();
      }
   }

   /**Get Applet information*/
   public String getAppletInfo()
   {
      return "JBoss EJB client Applet demo";
   }

   void callEjbButton_actionPerformed(ActionEvent e)
   {
      try
      {
         Properties jndiProps = new Properties() ;
         String myServer = this.getCodeBase().getHost ();
         jndiProps.setProperty("java.naming.factory.initial", "org.jnp.interfaces.NamingContextFactory" ) ;
         jndiProps.setProperty("java.naming.provider.url", myServer ) ;
         jndiProps.setProperty("java.naming.factory.url.pkgs", "org.jboss.naming:org.jnp.interfaces" ) ;
         TestAppletHome home = (TestAppletHome)PortableRemoteObject.narrow( new InitialContext(jndiProps).lookup( "TestAppletBean" ),
                                                                            TestAppletHome.class) ;
         TestApplet remote = home.create() ;
         ejbMessageLabel.setText( remote.getMessage() ) ;
      }
      catch ( SecurityException se )
      {
         se.printStackTrace ();
         ejbMessageLabel.setText (se.toString ());
      }
      catch( Exception ex )
      {
         System.err.println( "APPLET" );
         ex.printStackTrace();
      }
   }
}

One of the most important part of this Applet are these two lines:

String myServer = this.getCodeBase().getHost ();
jndiProps.setProperty("java.naming.provider.url", myServer ) ;

We make sure that we access the server from which we have just been downloaded. Attempt to access another server would raise a security exception.

We also need an HTML page from which the Applet will be launched:

<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=windows-1252">
<TITLE>
HTML Test Page
</TITLE>
</HEAD>
<BODY>
Here is the demo Applet for EJB access coming below...<BR>

<!--"CONVERTED_APPLET"-->
<!-- CONVERTER VERSION 1.3 -->
<SCRIPT LANGUAGE="JavaScript"><!--
    var _info = navigator.userAgent; var _ns = false;
    var _ie = (_info.indexOf("MSIE") > 0 && _info.indexOf("Win") > 0 && _info.indexOf("Windows 3.1") < 0);
//--></SCRIPT>
<COMMENT><SCRIPT LANGUAGE="JavaScript1.1"><!--
    var _ns = (navigator.appName.indexOf("Netscape") >= 0 && ((_info.indexOf("Win") > 0 && 
               _info.indexOf("Win16") < 0 && java.lang.System.getProperty("os.version").indexOf("3.5") < 0) 
               || (_info.indexOf("Sun") > 0) || (_info.indexOf("Linux") > 0)));
//--></SCRIPT></COMMENT>

<SCRIPT LANGUAGE="JavaScript"><!--
    if (_ie == true) document.writeln('<OBJECT classid="clsid:8AD9C840-044E-11D1-B3E9-00805F499D93" WIDTH = 400 
                     HEIGHT = 100 NAME = "AppletEjbCaller"
                     codebase="http://java.sun.com/products/plugin/1.3/jinstall-13-win32.cab#Version=1,3,0,0">
                     <NOEMBED><XMP>');
    else if (_ns == true) document.writeln('<EMBED type="application/x-java-applet;version=1.3"  
                          CODE = "org.jboss.docs.appletclient.applet.AppletEjbCaller" CODEBASE = "." 
                          ARCHIVE = "AppletClient.jar, jboss-client.jar, jboss-j2ee.jar, jbosssx-client.jar,
                          jnp-client.jar, jndi.jar, jaas.jar" NAME = "AppletEjbCaller" WIDTH = 400 HEIGHT = 100  
                          scriptable=false pluginspage="http://java.sun.com/products/plugin/1.3/plugin-install.html">
                          <NOEMBED><XMP>');
//--></SCRIPT>
<APPLET  CODE = "org.jboss.docs.appletclient.applet.AppletEjbCaller" CODEBASE = "." ARCHIVE = "AppletClient.jar, 
jboss-client.jar, jboss-j2ee.jar, jbosssx-client.jar, jnp-client.jar, jndi.jar, jaas.jar" WIDTH = 400 
HEIGHT = 100 NAME = "AppletEjbCaller"></XMP>
<PARAM NAME = CODE VALUE = "org.jboss.docs.appletclient.applet.AppletEjbCaller" >
<PARAM NAME = CODEBASE VALUE = "." >
<PARAM NAME = ARCHIVE VALUE = "AppletClient.jar, jboss-client.jar, jboss-j2ee.jar, jbosssx-client.jar,
jnp-client.jar, jndi.jar, jaas.jar" >
<PARAM NAME = NAME VALUE = "AppletEjbCaller" >
<PARAM NAME="type" VALUE="application/x-java-applet;version=1.3">
<PARAM NAME="scriptable" VALUE="false">

</APPLET>

</NOEMBED></EMBED></OBJECT>


<!--
<APPLET CODE = "org.jboss.docs.appletclient.applet.AppletEjbCaller" CODEBASE = "." ARCHIVE = "AppletClient.jar, 
jboss-client.jar, jboss-j2ee.jar, jbosssx-client.jar, jnp-client.jar, jndi.jar, jaas.jar" 
WIDTH = 400 HEIGHT = 100 NAME = "AppletEjbCaller">


</APPLET>
-->
<!--"END_CONVERTED_APPLET"-->



</BODY>
</HTML>

This HTML file has been transformed, from a standard HTML file with a standard <applet> tag definition by the SUN HTML converter for their Java plugin. This tools reads HTML files and replaces standard applet definition by browser specific declarations that will call the Java Plugin at runtime instead of the embedded JVM.

You then need to:

While building the provided example, if the JBoss distribution is correctly referenced in the build script, the build process will automatically deploy the bean in JBoss and copy the Applet JAR and HTML file to the JBoss client folder.

One way to easily use Applets for EJB access is to circumvents the sandbox! This can be done by signing the Applet and all code it uses. When downloaded from a browser, it indicates to the user that this Applet is signed by a particular entity and ask for the permission to suppress the sandbox restriction.

The main problem with Applet signing is that almost each JVM vendor/browser requires its own method! For this reason, we will concentrate on the JDK 1.3 plugin from SUN that can be plugged in any browser on many platforms. Thus, use of signed Applets is more interesting in a controlled environment such as an intranet.

jarsigner -keystore KEY_STORE -genkey -storepass STORE_PASS JAR_FILE KEY_ALIAS_NAME

<signjar jar="${build.lib.dir}/monitron_Applet_client_bean.jar" keystore="${key.store.db}" alias="${key.store.alias}" storepass="${key.store.pw}"/>