1page.title=Running Code on a Thread Pool Thread
2
3trainingnavtop=true
4@jd:body
5
6<div id="tb-wrapper">
7<div id="tb">
8
9<!-- table of contents -->
10<h2>This lesson teaches you to</h2>
11<ol>
12  <li><a href="#RunRunnable">Run a Runnable on a Thread in the Thread Pool</a></li>
13  <li><a href="#StopThread">Interrupt Running Code</a></li>
14</ol>
15
16<h2>You should also read</h2>
17<ul>
18  <li><a href="{@docRoot}guide/components/processes-and-threads.html">Processes and Threads</a></li>
19</ul>
20
21
22<h2>Try it out</h2>
23<div class="download-box">
24    <a href="{@docRoot}shareables/training/ThreadSample.zip" class="button">Download the sample</a>
25    <p class="filename">ThreadSample.zip</p>
26</div>
27
28</div>
29</div>
30
31<p>
32    The previous lesson showed you how to define a class that manages thread pools and the tasks
33    that run on them. This lesson shows you how to run a task on a thread pool. To do this,
34    you add the task to the pool's work queue. When a thread becomes available, the
35    {@link java.util.concurrent.ThreadPoolExecutor} takes a task from the queue and runs it on the
36    thread.
37</p>
38<p>
39    This lesson also shows you how to stop a task that's running. You might want to do this if a
40    task starts, but then discovers that its work isn't necessary. Rather than wasting processor
41    time, you can cancel the thread the task is running on. For example, if you are downloading
42    images from the network and using a cache, you probably want to stop a task if it detects that
43    an image is already present in the cache. Depending on how you write your app, you may not be
44    able to detect this before you start the download.
45</p>
46<h2 id="RunRunnable">Run a Task on a Thread in the Thread Pool</h2>
47<p>
48    To start a task object on a thread in a particular thread pool, pass the
49    {@link java.lang.Runnable} to {@link java.util.concurrent.ThreadPoolExecutor#execute
50    ThreadPoolExecutor.execute()}. This call adds the task to the thread pool's work queue. When an
51    idle thread becomes available, the manager takes the task that has been waiting the longest and
52    runs it on the thread:
53</p>
54<pre>
55public class PhotoManager {
56    public void handleState(PhotoTask photoTask, int state) {
57        switch (state) {
58            // The task finished downloading the image
59            case DOWNLOAD_COMPLETE:
60            // Decodes the image
61                mDecodeThreadPool.execute(
62                        photoTask.getPhotoDecodeRunnable());
63            ...
64        }
65        ...
66    }
67    ...
68}
69</pre>
70<p>
71    When {@link java.util.concurrent.ThreadPoolExecutor} starts a {@link java.lang.Runnable} on a
72    thread, it automatically calls the object's {@link java.lang.Runnable#run run()} method.
73</p>
74<h2 id="StopThread">Interrupt Running Code</h2>
75<p>
76    To stop a task, you need to interrupt the task's thread. To prepare to do this, you need to
77    store a handle to the task's thread when you create the task. For example:
78</p>
79<pre>
80class PhotoDecodeRunnable implements Runnable {
81    // Defines the code to run for this task
82    public void run() {
83        /*
84         * Stores the current Thread in the
85         * object that contains PhotoDecodeRunnable
86         */
87        mPhotoTask.setImageDecodeThread(Thread.currentThread());
88        ...
89    }
90    ...
91}
92</pre>
93<p>
94    To interrupt a thread, call {@link java.lang.Thread#interrupt Thread.interrupt()}. Notice that
95    {@link java.lang.Thread} objects are controlled by the system, which can modify them outside of
96    your app's process. For this reason, you need to lock access on a thread before you
97    interrupt it, by placing the access in a <code>synchronized</code> block. For example:
98</p>
99<pre>
100public class PhotoManager {
101    public static void cancelAll() {
102        /*
103         * Creates an array of Runnables that's the same size as the
104         * thread pool work queue
105         */
106        Runnable[] runnableArray = new Runnable[mDecodeWorkQueue.size()];
107        // Populates the array with the Runnables in the queue
108        mDecodeWorkQueue.toArray(runnableArray);
109        // Stores the array length in order to iterate over the array
110        int len = runnableArray.length;
111        /*
112         * Iterates over the array of Runnables and interrupts each one's Thread.
113         */
114        synchronized (sInstance) {
115            // Iterates over the array of tasks
116            for (int runnableIndex = 0; runnableIndex &lt; len; runnableIndex++) {
117                // Gets the current thread
118                Thread thread = runnableArray[taskArrayIndex].mThread;
119                // if the Thread exists, post an interrupt to it
120                if (null != thread) {
121                    thread.interrupt();
122                }
123            }
124        }
125    }
126    ...
127}
128</pre>
129<p>
130    In most cases, {@link java.lang.Thread#interrupt Thread.interrupt()} stops the thread
131    immediately. However, it only stops threads that are waiting, and will not interrupt CPU or
132    network-intensive tasks. To avoid slowing down or locking up the system, you should test for
133    any pending interrupt requests before attempting an operation :
134</p>
135<pre>
136/*
137 * Before continuing, checks to see that the Thread hasn't
138 * been interrupted
139 */
140if (Thread.interrupted()) {
141    return;
142}
143...
144// Decodes a byte array into a Bitmap (CPU-intensive)
145BitmapFactory.decodeByteArray(
146        imageBuffer, 0, imageBuffer.length, bitmapOptions);
147...
148</pre>
149