单例模式-服务器套接字

Question

Im using this code:

    final int LOCK_PORT= 54321;
    ServerSocket ss = new ServerSocket(LOCK_PORT);

The thing is that in the same port you cannot listen to 2 different applications (teachers theory).

This code was implemented into an application and the same instance ran more than 1 time. The objective is that the same instance is not suposed to run more than 1 time in the same port. However this isnt working and it does run...

// Edited, more code...

public VentanaPropiedades() {
    initFirst();
    initComponents(); //graphic components of Matisse
}

private void initFirst() {
    loadProperties(); //just internal values of the program, nothing to do

    activateInstance();
}

private void activateInstance() throws Exception {
    try {
    final int LOCK_PORT= 54321;
    ServerSocket ss = new ServerSocket(LOCK_PORT);
    } catch (Exception e) {
        System.out.println(e);
        throw e;
    }
}

private void killProgram() {
    setVisible(false);
    dispose();
    System.exit(0);
}

private void validateInstance() {
    try {
        activateInstance();
    } catch (Exception ex) {
        killProgram();
    }
}

--------------------------Supposed Solution---------------------------
The error catched when the 2nd instance DOES NOT RUN is this one:

 java.net.BindException: Address already in use: JVM_Bind 

However, this error not always happens and you can run more than 1 instance of the same program.

Answer 1

It doesn't work. You should get a BindException the second time you try to create the socket. See if you accidentally catch it somewhere or if the port actually is different or something similar.

Answer 2

The ServerSocket must be declared outside the method, right after main:

public class VentanaPropiedades extends javax.swing.JFrame {
    ServerSocket ss = null;
    // ... more code
}

And the activation method should use the reference:

private void activateInstance() throws Exception {
    try {
        final int LOCK_PORT= 54321;
        ss = new ServerSocket(LOCK_PORT); // note the missing "ServerSocket" before ss
    } catch (Exception e) {
        System.out.println(e);
        throw e;
    }  
}

The problem is that if you create the variable ServerSocket inside a method, the garbage collector will clean it once the method is done. If the variable is declared above, the garbage collector wont collect and clean it because the declared variable will stay instantiated but with NO reference.