public interface SecondaryLoop
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();
}
}
Dialog.show(),
EventQueue.createSecondaryLoop(),
Toolkit.getSystemEventQueue()| Modifier and Type | Method and 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. |
boolean enter()
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.
true after termination of the secondary loop,
if the secondary loop was started by this call,
false otherwiseboolean exit()
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.
true if this loop was previously started and
has not yet been finished with the exit() method,
false otherwise