Package android.os

Class MessageQueue

  • public final class MessageQueue
extends Object
    Low-level class holding the list of messages to be dispatched by a Looper. Messages are not added directly to a MessageQueue, but rather through Handler objects associated with the Looper.

    You can retrieve the MessageQueue for the current thread with Looper.myQueue().

    • Method Detail

      • finalize

        protected void finalize()
                 throws Throwable
        Description copied from class: Object
        Invoked when the garbage collector has detected that this instance is no longer reachable. The default implementation does nothing, but this method can be overridden to free resources.

        Note that objects that override finalize are significantly more expensive than objects that don't. Finalizers may be run a long time after the object is no longer reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup. Note also that finalizers are run on a single VM-wide finalizer thread, so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary for a class that has a native peer and needs to call a native method to destroy that peer. Even then, it's better to provide an explicit close method (and implement Closeable), and insist that callers manually dispose of instances. This works well for something like files, but less well for something like a BigInteger where typical calling code would have to deal with lots of temporaries. Unfortunately, code that creates lots of temporaries is the worst kind of code from the point of view of the single finalizer thread.

        If you must use finalizers, consider at least providing your own ReferenceQueue and having your own thread process that queue.

        Unlike constructors, finalizers are not automatically chained. You are responsible for calling super.finalize() yourself.

        Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer thread. See Effective Java Item 7, "Avoid finalizers" for more.

        Overrides:
        finalize in class Object
        Throws:
        Throwable

      • isIdle

        public boolean isIdle()
        Returns true if the looper has no pending messages which are due to be processed.

        This method is safe to call from any thread.

        Returns:
        True if the looper is idle.

      • isPolling

        public boolean isPolling()
        Returns whether this looper's thread is currently polling for more work to do. This is a good signal that the loop is still alive rather than being stuck handling a callback. Note that this method is intrinsically racy, since the state of the loop can change before you get the result back.

        This method is safe to call from any thread.

        Returns:
        True if the looper is currently polling for events.

      • postSyncBarrier

        public int postSyncBarrier()
        Posts a synchronization barrier to the Looper's message queue. Message processing occurs as usual until the message queue encounters the synchronization barrier that has been posted. When the barrier is encountered, later synchronous messages in the queue are stalled (prevented from being executed) until the barrier is released by calling removeSyncBarrier(int) and specifying the token that identifies the synchronization barrier. This method is used to immediately postpone execution of all subsequently posted synchronous messages until a condition is met that releases the barrier. Asynchronous messages (see Message.isAsynchronous() are exempt from the barrier and continue to be processed as usual. This call must be always matched by a call to removeSyncBarrier(int) with the same token to ensure that the message queue resumes normal operation. Otherwise the application will probably hang!
        Returns:
        A token that uniquely identifies the barrier. This token must be passed to removeSyncBarrier(int) to release the barrier.

      • removeSyncBarrier

        public void removeSyncBarrier​(int token)
        Removes a synchronization barrier.
        Parameters:
        token - The synchronization barrier token that was returned by postSyncBarrier().
        Throws:
        IllegalStateException - if the barrier was not found.