Provide a means of exchanging messages between the JavaScript context of the

page hosting the applet and the launched application through a simple server
socket bound to the loopback address on any available port (where the port
is passed to the application as a system property).  Not tested yet, but it
shouldn't affect any deployments that don't specifically enable it.

build.xml

@@ -15,7 +15,7 @@
   <condition property="maven-ant.exists"><available file="${maven-ant.jar}"/></condition>
   <target name="-download-maven-ant" unless="maven-ant.exists">
     <mkdir dir="${maven-ant.dir}"/>
-    <get src="${maven-ant.url}/maven-ant-tasks-${maven-ant.vers}.jar" 
+    <get src="${maven-ant.url}/maven-ant-tasks-${maven-ant.vers}.jar"
          dest="${maven-ant.jar}" usetimestamp="true"/>
   </target>
 
@@ -23,12 +23,16 @@
     <taskdef resource="org/apache/maven/artifact/ant/antlib.xml"
              uri="urn:maven-artifact-ant" classpath="${maven-ant.jar}"/>
     <artifact:pom id="pom" file="pom.xml"/>
-    <artifact:dependencies pathId="compile.classpath" pomRefId="pom" useScope="compile"/>
+    <artifact:dependencies pathId="pom-compile.classpath" pomRefId="pom" useScope="compile"/>
     <artifact:dependencies pathId="testlibs.classpath" pomRefId="pom" useScope="test"/>
   </target>
 
   <target name="-prepare" depends="-init-maven-ant">
     <mkdir dir="${deploy.dir}"/>
+    <path id="compile.classpath">
+      <path refid="pom-compile.classpath"/>
+      <pathelement location="${java.home}/lib/plugin.jar"/>
+    </path>
     <path id="built.classpath">
       <path refid="compile.classpath"/>
       <pathelement location="${deploy.dir}/classes"/>
@@ -124,13 +128,14 @@
     <proguard>
       <injar path="${getdown.jar}"/>
       <injar path="${jregistrykey:jregistrykey:jar}" filter="!META-INF/**"/>
-      <injar path="${com.samskivert:samskivert:jar}" filter="!META-INF/**, 
+      <injar path="${com.samskivert:samskivert:jar}" filter="!META-INF/**,
         !**/Log4JLogger*,!**/*.java,com/samskivert/Log.class,**/samskivert/io/**,
         **/samskivert/swing/**,**/samskivert/text/**,**/samskivert/util/**"/>
       <injar path="${commons-codec:commons-codec:jar}" filter="!META-INF/**"/>
       <outjar path="${deploy.dir}/getdown-pro${version.suff}.jar"/>
       <libraryjar name="${rt.jar}"/>
       <libraryjar name="${org.apache.ant:ant:jar}"/>
+      <libraryjar name="${java.home}/lib/plugin.jar"/>
 
       <!-- keep the whole nine yards, just trim/obfuscate our depends -->
       <keep name="com.threerings.getdown.**">
@@ -149,7 +154,7 @@
   </target>
 
   <property name="maven.deploy.repo" value="file://${user.home}/.m2/repository"/>
-  <target name="maven-deploy" depends="dist" 
+  <target name="maven-deploy" depends="dist"
           description="Deploys our build artifacts to a Maven repository.">
     <jar basedir="${src.dir}" destfile="${deploy.dir}/${ant.project.name}-sources.jar"/>
     <artifact:deploy file="${deploy.dir}/${ant.project.name}.jar" uniqueVersion="false">

src/main/java/com/threerings/getdown/data/Properties.java

@@ -32,4 +32,8 @@ public class Properties
 {
     /** This property will be set to "true" on the application when it is being run by getdown. */
     public static final String GETDOWN = "com.threerings.getdown";
+
+    /** If accepting connections from the launched application, this property
+     * will be set to the connection server port. */
+    public static final String CONNECT_PORT = "com.threerings.getdown.connectPort";
 }

src/main/java/com/threerings/getdown/launcher/GetdownApplet.java

@@ -27,12 +27,17 @@ package com.threerings.getdown.launcher;
 
 import java.awt.Container;
 import java.awt.Image;
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.PrintStream;
+import java.net.InetAddress;
 import java.net.MalformedURLException;
+import java.net.ServerSocket;
+import java.net.Socket;
 import java.net.URL;
 import java.util.ArrayList;
 import java.util.List;
@@ -44,6 +49,11 @@ import java.security.cert.CertificateFactory;
 import javax.swing.JApplet;
 import javax.swing.JPanel;
 
+import netscape.javascript.JSObject;
+
+import com.threerings.getdown.data.Application;
+import com.threerings.getdown.data.Properties;
+
 import static com.threerings.getdown.Log.log;
 
 /**
@@ -52,6 +62,35 @@ import static com.threerings.getdown.Log.log;
 public class GetdownApplet extends JApplet
     implements ImageLoader
 {
+    /**
+     * Sets the JavaScript callback to invoke when a message is received from the launched app.
+     * The callback should be a function that accepts a single string parameter (the received
+     * message).
+     */
+    public synchronized void setMessageCallback (JSObject callback)
+    {
+        _messageCallback = callback;
+    }
+
+    /**
+     * Attempts to send a message to the launched app.
+     *
+     * @return true if we succeeded in sending the message, false if the launched app has not (yet)
+     * established a connection to Getdown, or the send failed.
+     */
+    public synchronized boolean sendMessage (String message)
+    {
+        if (_connectOut != null) {
+            try {
+                _connectOut.writeUTF(message);
+                return true;
+            } catch (IOException e) {
+                log.warning("Error sending message to app.", "message", message, e);
+            }
+        }
+        return false;
+    }
+
     @Override // documentation inherited
     public void init ()
     {
@@ -108,6 +147,19 @@ public class GetdownApplet extends JApplet
                     return GetdownApplet.this;
                 }
                 @Override
+                protected void launch () {
+                    // if so configured, create a server socket to listen
+                    // for a connection from the app
+                    if (_config.allowConnect) {
+                        try {
+                            startConnectServer();
+                        } catch (IOException e) {
+                            log.warning("Failed to start connect server.", e);
+                        }
+                    }
+                    super.launch();
+                }
+                @Override
                 protected void exit (int exitCode) {
                     _app.releaseLock();
                     _config.redirect();
@@ -128,6 +180,62 @@ public class GetdownApplet extends JApplet
         }
     }
 
+    /**
+     * Attempts to start the server that will accept a connection from the launched app, allowing
+     * it to exchange messages with the JavaScript context.
+     */
+    protected void startConnectServer ()
+        throws IOException
+    {
+        // bind and set a property with the local port that will be passed through to the app
+        _serverSocket = new ServerSocket(0, 0, InetAddress.getByName(null));
+        System.setProperty(Application.PROP_PASSTHROUGH_PREFIX + Properties.CONNECT_PORT,
+            String.valueOf(_serverSocket.getLocalPort()));
+        Thread thread = new Thread("ConnectServer") {
+            @Override
+            public void run () {
+                while (true) {
+                    try {
+                        acceptConnection();
+                    } catch (IOException e) {
+                        if (!_serverSocket.isClosed()) {
+                            log.warning("Error accepting connection.", e);
+                        }
+                        break;
+                    }
+                }
+            }
+            protected void acceptConnection () throws IOException {
+                Socket socket = _serverSocket.accept();
+                DataInputStream connectIn = new DataInputStream(socket.getInputStream());
+                synchronized (GetdownApplet.this) {
+                    _connectOut = new DataOutputStream(socket.getOutputStream());
+                }
+                while (true) {
+                    try {
+                        String message = connectIn.readUTF();
+                        synchronized (GetdownApplet.this) {
+                            if (_messageCallback != null) {
+                                _messageCallback.call("call",
+                                    new Object[] { _messageCallback, message });
+                            }
+                        }
+                    } catch (IOException e) {
+                        if (!socket.isClosed()) {
+                            log.warning("Error reading message.", e);
+                        }
+                        break;
+                    }
+                }
+                synchronized (GetdownApplet.this) {
+                    _connectOut = null;
+                }
+            }
+        };
+        thread.setDaemon(true);
+        thread.start();
+    }
+
     // implemented from ImageLoader
     public Image loadImage (String path)
     {
@@ -163,6 +271,25 @@ public class GetdownApplet extends JApplet
         _getdown._app.releaseLock();
     }
 
+    @Override // documentation inherited
+    public synchronized void destroy ()
+    {
+        if (_serverSocket != null) {
+            try {
+                _serverSocket.close();
+            } catch (IOException e) {
+                log.warning("Error closing server socket.", e);
+            }
+        }
+        if (_connectOut != null) {
+            try {
+                _connectOut.close();
+            } catch (IOException e) {
+                log.warning("Error closing connect socket/output stream.", e);
+            }
+        }
+    }
+
     /**
      * Creates the specified file and writes the supplied contents to it.
      */
@@ -207,4 +334,13 @@ public class GetdownApplet extends JApplet
 
     /** An error encountered during initialization. */
     protected String _errmsg;
+
+    /** The message callback registered by JavaScript on the containing page, if any. */
+    protected JSObject _messageCallback;
+
+    /** The server socket on which we listen for connections, if any. */
+    protected ServerSocket _serverSocket;
+
+    /** The output stream to the launched app, if a connection has been established. */
+    protected DataOutputStream _connectOut;
 }

src/main/java/com/threerings/getdown/launcher/GetdownAppletConfig.java

@@ -57,6 +57,7 @@ public class GetdownAppletConfig
     public static final String ERRORBGIMAGE = "errorbgimage";
     public static final String PARAM_DELIMITER = ".";
     public static final String DIRECT = "direct";
+    public static final String CONNECT = "connect";
     public static final String REDIRECT_ON_FINISH = "redirect_on_finish";
     public static final String REDIRECT_ON_FINISH_TARGET = "redirect_on_finish_target";
 
@@ -77,7 +78,7 @@ public class GetdownAppletConfig
      * }</pre>
      * When a number is reached for which no value exists, we stop looking. */
     public static final String APPARG_PREFIX = "appargs";
- 
+
     public String appbase;
 
     public String appname;
@@ -107,6 +108,11 @@ public class GetdownAppletConfig
      * separate java process (false). */
     public boolean invokeDirect;
 
+    /** Indicates whether Getdown should allow the launched app to connect to it through a server
+     * socket bound to localhost on any available port in order to allow interaction with
+     * JavaScript code on the page containing the applet. */
+    public boolean allowConnect;
+
     /** Optional default bounds for the status panel. */
     public Rectangle statusBounds;
 
@@ -183,6 +189,9 @@ public class GetdownAppletConfig
         String direct = getParameter(DIRECT, "false");
         invokeDirect = Boolean.valueOf(direct);
 
+        String connect = getParameter(CONNECT, "false");
+        allowConnect = Boolean.valueOf(connect);
+
         String redirectURL = getParameter(REDIRECT_ON_FINISH);
         if (redirectURL != null) {
             try {