Communication between Servlet and Applet

[Shah Devang]

Hi,

I have a HTML page that has some links. When I click a link, I want to display the applet and its data differs depending upon the element selected. To achieve this, I made an HTTP call to the servlet and passed it the element selected as parameter. Depending upon the element selected I create a swing object that I want to render on the applet. This object I pass to the applet and add this to the applet.

But this is not working. I am getting the following exception on the Java console :

java.io.InvalidClassException: javax.swing.JComponent; local class incompatible: stream classdesc serialVersionUID = 5896976265092562486, local class serialVersionUID = 7917968344860800289
at java.io.ObjectStreamClass.initNonProxy(Unknown Source)
at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
at java.io.ObjectInputStream.readClassDesc(Unknown Source)
at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
at java.io.ObjectInputStream.readClassDesc(Unknown Source)
at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
at java.io.ObjectInputStream.readClassDesc(Unknown Source)
at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
at java.io.ObjectInputStream.readObject0(Unknown Source)
at java.io.ObjectInputStream.readObject(Unknown Source)
at com.accurev.HelloWorldApplet.readInputStream(HelloWorldApplet.java:78)
at com.accurev.HelloWorldApplet.sendObject(HelloWorldApplet.java:51)
at com.accurev.HelloWorldApplet.init(HelloWorldApplet.java:28)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
java.lang.NullPointerException
at java.awt.Container.addImpl(Unknown Source)
at java.awt.Container.add(Unknown Source)
at com.accurev.HelloWorldApplet.init(HelloWorldApplet.java:31)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

------------------------------------------------------

The code that I have implemented goes like this :

====== Applet ======
 
public class HelloWorldApplet extends Applet
{
    JPanel panel = null;
    JLabel label = null;
 
    public void init()
    {
        try {
            panel = new JPanel();
 
            panel.setLayout(new GridLayout(1,2));
            label = new JLabel("first file modified");
            label.setBackground(new Color(153, 184, 213, 100));
            panel.add(label);
 
            JComponent data = (JComponent)sendObject();
 
            //JButton button = new JButton(data);
            add(data);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
 
    public Object sendObject() {
        OutputStream out;
        ObjectOutputStream objectStream;
        Object data = null;
        try {
            URLConnection con = getServletConnection();
            out = con.getOutputStream();
            objectStream = new ObjectOutputStream(out);
            objectStream.writeObject("request from applet");
            objectStream.flush();
            objectStream.close();
 
            out.close();
 
            data = readInputStream(con);
 
        } catch(Exception e) {
            e.printStackTrace();
        }
        return data;
    }
 
    private URLConnection getServletConnection() throws MalformedURLException, IOException  {
        URL servletURL = new URL("http://10.77.5.192:8080/plugins/servlet/helloworld");
        URLConnection con = servletURL.openConnection();
        con.setDoInput(true);
        con.setDoOutput(true);
        con.setUseCaches(false);
        con.setDefaultUseCaches(false);
        ((HttpURLConnection) con).setRequestMethod("POST");
        con.setRequestProperty("Content-Type","application/x-java-serialized-object");
        con.setAllowUserInteraction(false);
        return con;
    }
 
    private Object readInputStream(URLConnection con) {
        Object object = null;
 
        try {
            InputStream in = con.getInputStream();
            ObjectInputStream objectInputStream = new ObjectInputStream(in);
            object = objectInputStream.readObject();
            in.close();
        } catch(Exception e) {
            /*try {
                    // now failing to read the inputstream does not mean the server did not send
                    // any data, here is how you can read that data, this is needed for the same
                    // reason mentioned above.
                    ((HttpURLConnection) con).getResponseCode();
                    InputStream es = ((HttpURLConnection) con).getErrorStream();
                    int ret = 0;
                    // read the response body
                    while ((ret = es.read(byteBuffer)) > 0) {
                    }
                    // close the errorstream
                    es.close();
                } catch (IOException ex) {
                    ex.printStackTrace();
                }*/
                e.printStackTrace();
        }
 
        return object;
    }
}
 
====== Servlet ======
 
public class HelloWorldServlet extends HttpServlet {
 
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        
        ObjectInputStream in = null;
 
        try {
            ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
            InputStream inStream = request.getInputStream();
            byte byteBuffer[] = new byte[1024];
            int len;
 
            while((len = inStream.read(byteBuffer)) > 0) {
                byteStream.write(byteBuffer);
            }
 
            in = new ObjectInputStream(new ByteArrayInputStream(byteStream.toByteArray()));
            Object obj = in.readObject();
 
            response.setContentType("application/x-java-serialized-object");
            OutputStream outStream = response.getOutputStream();
            ObjectOutputStream objectStream = new ObjectOutputStream(outStream);
            objectStream.writeObject(new DiffPanel());
            objectStream.flush();
            objectStream.close();
 
            //outStream.close();
            in.close();
            byteStream.reset();
        } catch(Exception e) {
            e.printStackTrace();
        }
    }
}

The DiffPanel object extends JPanel and implements Serializable.

Can anyone please help.

Thanks
Devang Shah

[Ulf Dittmer]

In order to serialize objects from one JVM to another, both need to agree on the exact version of the object transferred. It looks like the applet JVM is a different version than the servlet JVM, and the object you're trying to transfer is not the same, and thus can not be deserialized.

Instead of transferring an instantiated object, why don't you pass all the information necessary to instantiate it in the applet?

Alternatively, you could try to serialize by using the java.beans.XMLEncoder/Decoder classes, which are less prone to version conflicts.

[Shah Devang]

Hi,

So can I atleast send string objects from servlet to the applet ?


Thanks
Devang Shah

[Ulf Dittmer]

Strings are what HTTP uses anyway internally, so you can send those directly, without the need to serialize/deserialize anything.

[Shah Devang]

Hi,

What I am trying to show in Applet is a diff window. The diff tool gives me a swing based window ouput showing the diff of the two files. Now I want this window to be shown on the clients browser. Now since its diff, the data size can be huge. Can you please show some pointers how do I achieve this ?

Thanks
Devang Shah

[Ulf Dittmer]

What exactly is the problem - integrating a Swing application in an applet or retrieving the files from the server? Does the application have an API that can be used for embedding purposes?