Class EventQueue

java.lang.Object
java.awt.EventQueue
public class EventQueue extends Object
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.

Since:
1.1

Constructor Summary

Constructor Description
EventQueue()
Initializes a new instance of EventQueue.

Method Summary

Modifier and Type Method Description
SecondaryLoop createSecondaryLoop()
Creates a new secondary loop associated with this event queue.
protected void dispatchEvent(AWTEvent event)
Dispatches an event.
static AWTEvent getCurrentEvent()
Returns 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.
AWTEvent peekEvent()
Returns the first event on the EventQueue without removing it.
AWTEvent peekEvent(int id)
Returns the first event with the specified id, if any.
protected void pop()
Stops dispatching events using this EventQueue.
void postEvent(AWTEvent theEvent)
Posts a 1.1-style event to the EventQueue.
void push(EventQueue newEventQueue)
Replaces the existing EventQueue with the specified one.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

EventQueue

public EventQueue()
Initializes a new instance of EventQueue.

Method Details

postEvent

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

getNextEvent

public AWTEvent getNextEvent() throws InterruptedException
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

peekEvent

public AWTEvent peekEvent()
Returns the first event on the EventQueue without removing it.
Returns:
the first event

peekEvent

public 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

dispatchEvent

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 types, source types, and dispatch methods
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
Since:
1.2

getMostRecentEventTime

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
Since:
1.4
See Also:

getCurrentEvent

public static AWTEvent getCurrentEvent()
Returns 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
Since:
1.4

push

public 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
Since:
1.2
See Also:

pop

protected void pop() throws EmptyStackException
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
Since:
1.2
See Also:

createSecondaryLoop

public SecondaryLoop createSecondaryLoop()
Creates a new secondary loop associated with this event queue. Use the SecondaryLoop.enter() and SecondaryLoop.exit() methods to start and stop the event loop and dispatch the events from this queue.
Returns:
secondaryLoop A new secondary loop object, which can be used to launch a new nested event loop and dispatch events from this queue
Since:
1.7
See Also:

isDispatchThread

public static boolean isDispatchThread()
Returns true if the calling thread is the current AWT EventQueue's dispatch thread. Use this method to ensure that a particular task is being executed (or not being) there.

Note: use the invokeLater(java.lang.Runnable) or invokeAndWait(java.lang.Runnable) methods to execute a task in the current AWT EventQueue's dispatch thread.

Returns:
true if running in the current AWT EventQueue's dispatch thread
Since:
1.2
See Also:

invokeLater

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 asynchronously in the event dispatch thread of the system EventQueue
Since:
1.2
See Also:

invokeAndWait

public static void invokeAndWait(Runnable runnable) throws InterruptedException, InvocationTargetException
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 in the event dispatch thread of the system EventQueue
Throws:
InterruptedException - if any thread has interrupted this thread
InvocationTargetException - if an throwable is thrown when running runnable
Since:
1.2
See Also:

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https://docs.oracle.com/en/java/javase/17/docs/api/java.desktop/java/awt/EventQueue.html