training/multiple-threads/run-code.jd

page.title=Running Code on a Thread Pool Thread

trainingnavtop=true
@jd:body

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<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#RunRunnable">Run a Runnable on a Thread in the Thread Pool</a></li>
  <li><a href="#StopThread">Interrupt Running Code</a></li>
</ol>

<h2>You should also read</h2>
<ul>
  <li><a href="{@docRoot}guide/components/processes-and-threads.html">Processes and Threads</a></li>
</ul>


<h2>Try it out</h2>
<div class="download-box">
    <a href="{@docRoot}shareables/training/ThreadSample.zip" class="button">Download the sample</a>
    <p class="filename">ThreadSample.zip</p>
</div>

</div>
</div>

<p>
    The previous lesson showed you how to define a class that manages thread pools and the tasks
    that run on them. This lesson shows you how to run a task on a thread pool. To do this,
    you add the task to the pool's work queue. When a thread becomes available, the
    {@link java.util.concurrent.ThreadPoolExecutor} takes a task from the queue and runs it on the
    thread.
</p>
<p>
    This lesson also shows you how to stop a task that's running. You might want to do this if a
    task starts, but then discovers that its work isn't necessary. Rather than wasting processor
    time, you can cancel the thread the task is running on. For example, if you are downloading
    images from the network and using a cache, you probably want to stop a task if it detects that
    an image is already present in the cache. Depending on how you write your app, you may not be
    able to detect this before you start the download.
</p>
<h2 id="RunRunnable">Run a Task on a Thread in the Thread Pool</h2>
<p>
    To start a task object on a thread in a particular thread pool, pass the
    {@link java.lang.Runnable} to {@link java.util.concurrent.ThreadPoolExecutor#execute
    ThreadPoolExecutor.execute()}. This call adds the task to the thread pool's work queue. When an
    idle thread becomes available, the manager takes the task that has been waiting the longest and
    runs it on the thread:
</p>
<pre>
public class PhotoManager {
    public void handleState(PhotoTask photoTask, int state) {
        switch (state) {
            // The task finished downloading the image
            case DOWNLOAD_COMPLETE:
            // Decodes the image
                mDecodeThreadPool.execute(
                        photoTask.getPhotoDecodeRunnable());
            ...
        }
        ...
    }
    ...
}
</pre>
<p>
    When {@link java.util.concurrent.ThreadPoolExecutor} starts a {@link java.lang.Runnable} on a
    thread, it automatically calls the object's {@link java.lang.Runnable#run run()} method.
</p>
<h2 id="StopThread">Interrupt Running Code</h2>
<p>
    To stop a task, you need to interrupt the task's thread. To prepare to do this, you need to
    store a handle to the task's thread when you create the task. For example:
</p>
<pre>
class PhotoDecodeRunnable implements Runnable {
    // Defines the code to run for this task
    public void run() {
        /*
         * Stores the current Thread in the
         * object that contains PhotoDecodeRunnable
         */
        mPhotoTask.setImageDecodeThread(Thread.currentThread());
        ...
    }
    ...
}
</pre>
<p>
    To interrupt a thread, call {@link java.lang.Thread#interrupt Thread.interrupt()}. Notice that
    {@link java.lang.Thread} objects are controlled by the system, which can modify them outside of
    your app's process. For this reason, you need to lock access on a thread before you
    interrupt it, by placing the access in a <code>synchronized</code> block. For example:
</p>
<pre>
public class PhotoManager {
    public static void cancelAll() {
        /*
         * Creates an array of Runnables that's the same size as the
         * thread pool work queue
         */
        Runnable[] runnableArray = new Runnable[mDecodeWorkQueue.size()];
        // Populates the array with the Runnables in the queue
        mDecodeWorkQueue.toArray(runnableArray);
        // Stores the array length in order to iterate over the array
        int len = runnableArray.length;
        /*
         * Iterates over the array of Runnables and interrupts each one's Thread.
         */
        synchronized (sInstance) {
            // Iterates over the array of tasks
            for (int runnableIndex = 0; runnableIndex &lt; len; runnableIndex++) {
                // Gets the current thread
                Thread thread = runnableArray[taskArrayIndex].mThread;
                // if the Thread exists, post an interrupt to it
                if (null != thread) {
                    thread.interrupt();
                }
            }
        }
    }
    ...
}
</pre>
<p>
    In most cases, {@link java.lang.Thread#interrupt Thread.interrupt()} stops the thread
    immediately. However, it only stops threads that are waiting, and will not interrupt CPU or
    network-intensive tasks. To avoid slowing down or locking up the system, you should test for
    any pending interrupt requests before attempting an operation :
</p>
<pre>
/*
 * Before continuing, checks to see that the Thread hasn't
 * been interrupted
 */
if (Thread.interrupted()) {
    return;
}
...
// Decodes a byte array into a Bitmap (CPU-intensive)
BitmapFactory.decodeByteArray(
        imageBuffer, 0, imageBuffer.length, bitmapOptions);
...
</pre>