Objects that implement this interface are created with the EventQueue.createSecondaryLoop() method. The interface provides two methods, enter() and exit(), which can be used to start and stop the event loop.

When the enter() method is called, the current thread is blocked until the loop is terminated by the exit() method. Also, a new event loop is started on the event dispatch thread, which may or may not be the current thread. The loop can be terminated on any thread by calling its exit() method. After the loop is terminated, the SecondaryLoop object can be reused to run a new nested event loop.

A typical use case of applying this interface is AWT and Swing modal dialogs. When a modal dialog is shown on the event dispatch thread, it enters a new secondary loop. Later, when the dialog is hidden or disposed, it exits the loop, and the thread continues its execution.

The following example illustrates a simple use case of secondary loops:

   SecondaryLoop loop;

   JButton jButton = new JButton("Button");
   jButton.addActionListener(new ActionListener() {
       @Override
       public void actionPerformed(ActionEvent e) {
           Toolkit tk = Toolkit.getDefaultToolkit();
           EventQueue eq = tk.getSystemEventQueue();
           loop = eq.createSecondaryLoop();

           // Spawn a new thread to do the work
           Thread worker = new WorkerThread();
           worker.start();

           // Enter the loop to block the current event
           // handler, but leave UI responsive
           if (!loop.enter()) {
               // Report an error
           }
       }
   });

   class WorkerThread extends Thread {
       @Override
       public void run() {
           // Perform calculations
           doSomethingUseful();

           // Exit the loop
           loop.exit();
       }
   }