SecondaryLoop (Java SE 26 & JDK 26)
Description: declaration: module: java.desktop, package: java.awt, interface: SecondaryLoop
Keywords: java.awt.SecondaryLoop interface

# Interface SecondaryLoop

public interface SecondaryLoop
A helper interface to run the nested event loop.

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();
          }
      }

Since:
  1.7
See Also:
  - `Dialog.show()`
  - `EventQueue.createSecondaryLoop()`
  - `Toolkit.getSystemEventQueue()`

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `enter()`
  Blocks the execution of the current thread and enters a new secondary event loop on the event dispatch thread.
  `boolean`
  `exit()`
  Unblocks the execution of the thread blocked by the `enter()` method and exits the secondary loop.

- ## Method Details

  - ### enter

    boolean enter()
    Blocks the execution of the current thread and enters a new secondary event loop on the event dispatch thread.

    This method can be called by any thread including the event dispatch thread. This thread will be blocked until the `exit()` method is called or the loop is terminated. A new secondary loop will be created on the event dispatch thread for dispatching events in either case.

    This method can only start one new event loop at a time per object. If a secondary event loop has already been started by this object and is currently still running, this method returns `false` to indicate that it was not successful in starting a new event loop. Otherwise, this method blocks the calling thread and later returns `true` when the new event loop is terminated. At such time, this object can again be used to start another new event loop.

    Returns:
      `true` after termination of the secondary loop, if the secondary loop was started by this call, `false` otherwise

  - ### exit

    boolean exit()
    Unblocks the execution of the thread blocked by the `enter()` method and exits the secondary loop.

    This method resumes the thread that called the `enter()` method and exits the secondary loop that was created when the `enter()` method was invoked.

    Note that if any other secondary loop is started while this loop is running, the blocked thread will not resume execution until the nested loop is terminated.

    If this secondary loop has not been started with the `enter()` method, or this secondary loop has already finished with the `exit()` method, this method returns `false`, otherwise `true` is returned.

    Returns:
      `true` if this loop was previously started and has not yet been finished with the `exit()` method, `false` otherwise