public class

EventQueue

extends Object
java.lang.Object
   ↳ java.awt.EventQueue
Known Direct Subclasses

Class Overview

EventQueue is a platform-independent class that queues events, both from the underlying peer classes and from trusted application classes.

It encapsulates asynchronous event dispatch machinery which extracts events from the queue and dispatches them by calling dispatchEvent(AWTEvent) method on this EventQueue with the event to be dispatched as an argument. The particular behavior of this machinery is implementation-dependent. The only requirements are that events which were actually enqueued to this queue (note that events being posted to the EventQueue can be coalesced) are dispatched:

Sequentially.
That is, it is not permitted that several events from this queue are dispatched simultaneously.
In the same order as they are enqueued.
That is, if AWTEvent A is enqueued to the EventQueue before AWTEvent B then event B will not be dispatched before event A.

Some browsers partition applets in different code bases into separate contexts, and establish walls between these contexts. In such a scenario, there will be one EventQueue per context. Other browsers place all applets into the same context, implying that there will be only a single, global EventQueue for all applets. This behavior is implementation-dependent. Consult your browser's documentation for more information.

For information on the threading issues of the event dispatch machinery, see AWT Threading Issues.

Summary

Public Constructors
EventQueue()
Public Methods
static AWTEvent getCurrentEvent()
Returns the the event currently being dispatched by the EventQueue associated with the calling thread.
static long getMostRecentEventTime()
Returns the timestamp of the most recent event that had a timestamp, and that was dispatched from the EventQueue associated with the calling thread.
AWTEvent getNextEvent()
Removes an event from the EventQueue and returns it.
static void invokeAndWait(Runnable runnable)
Causes runnable to have its run method called in the dispatch thread of the system EventQueue.
static void invokeLater(Runnable runnable)
Causes runnable to have its run method called in the dispatch thread of the system EventQueue.
static boolean isDispatchThread()
Returns true if the calling thread is the current AWT EventQueue's dispatch thread.
synchronized AWTEvent peekEvent(int id)
Returns the first event with the specified id, if any.
synchronized AWTEvent peekEvent()
Returns the first event on the EventQueue without removing it.
void postEvent(AWTEvent theEvent)
Posts a 1.1-style event to the EventQueue.
synchronized void push(EventQueue newEventQueue)
Replaces the existing EventQueue with the specified one.
Protected Methods
void dispatchEvent(AWTEvent event)
Dispatches an event.
void pop()
Stops dispatching events using this EventQueue.
[Expand]
Inherited Methods
From class java.lang.Object

Public Constructors

public EventQueue ()

Public Methods

public static AWTEvent getCurrentEvent ()

Returns the the event currently being dispatched by the EventQueue associated with the calling thread. This is useful if a method needs access to the event, but was not designed to receive a reference to it as an argument. Note that this method should only be invoked from an application's event dispatching thread. If this method is invoked from another thread, null will be returned.

Returns
  • the event currently being dispatched, or null if this method is invoked on a thread other than an event dispatching thread

public static long getMostRecentEventTime ()

Returns the timestamp of the most recent event that had a timestamp, and that was dispatched from the EventQueue associated with the calling thread. If an event with a timestamp is currently being dispatched, its timestamp will be returned. If no events have yet been dispatched, the EventQueue's initialization time will be returned instead.In the current version of the JDK, only InputEvents, ActionEvents, and InvocationEvents have timestamps; however, future versions of the JDK may add timestamps to additional event types. Note that this method should only be invoked from an application's event dispatching thread. If this method is invoked from another thread, the current system time (as reported by System.currentTimeMillis()) will be returned instead.

Returns
  • the timestamp of the last InputEvent, ActionEvent, or InvocationEvent to be dispatched, or System.currentTimeMillis() if this method is invoked on a thread other than an event dispatching thread
See Also

public AWTEvent getNextEvent ()

Removes an event from the EventQueue and returns it. This method will block until an event has been posted by another thread.

Returns
  • the next AWTEvent
Throws
InterruptedException if any thread has interrupted this thread

public static void invokeAndWait (Runnable runnable)

Causes runnable to have its run method called in the dispatch thread of the system EventQueue. This will happen after all pending events are processed. The call blocks until this has happened. This method will throw an Error if called from the event dispatcher thread.

Parameters
runnable the Runnable whose run method should be executed synchronously on the EventQueue
Throws
InterruptedException if any thread has interrupted this thread
InvocationTargetException if an throwable is thrown when running runnable
See Also

public static void invokeLater (Runnable runnable)

Causes runnable to have its run method called in the dispatch thread of the system EventQueue. This will happen after all pending events are processed.

Parameters
runnable the Runnable whose run method should be executed synchronously on the EventQueue
See Also

public static boolean isDispatchThread ()

Returns true if the calling thread is the current AWT EventQueue's dispatch thread. Use this call the ensure that a given task is being executed (or not being) on the current AWT EventDispatchThread.

Returns
  • true if running on the current AWT EventQueue's dispatch thread

public synchronized AWTEvent peekEvent (int id)

Returns the first event with the specified id, if any.

Parameters
id the id of the type of event desired
Returns
  • the first event of the specified id or null if there is no such event

public synchronized AWTEvent peekEvent ()

Returns the first event on the EventQueue without removing it.

Returns
  • the first event

public void postEvent (AWTEvent theEvent)

Posts a 1.1-style event to the EventQueue. If there is an existing event on the queue with the same ID and event source, the source Component's coalesceEvents method will be called.

Parameters
theEvent an instance of java.awt.AWTEvent, or a subclass of it
Throws
NullPointerException if theEvent is null

public synchronized void push (EventQueue newEventQueue)

Replaces the existing EventQueue with the specified one. Any pending events are transferred to the new EventQueue for processing by it.

Parameters
newEventQueue an EventQueue (or subclass thereof) instance to be use
Throws
NullPointerException if newEventQueue is null
See Also

Protected Methods

protected void dispatchEvent (AWTEvent event)

Dispatches an event. The manner in which the event is dispatched depends upon the type of the event and the type of the event's source object:

Event Type Source Type Dispatched To
ActiveEvent Any event.dispatch()
Other Component source.dispatchEvent(AWTEvent)
Other MenuComponent source.dispatchEvent(AWTEvent)
Other Other No action (ignored)

Parameters
event an instance of java.awt.AWTEvent, or a subclass of it
Throws
NullPointerException if event is null

protected void pop ()

Stops dispatching events using this EventQueue. Any pending events are transferred to the previous EventQueue for processing.

Warning: To avoid deadlock, do not declare this method synchronized in a subclass.

Throws
EmptyStackException if no previous push was made on this EventQueue
See Also