Android线程和线程池

Translated From Google Android.

class PhotoDecodeRunnable implements Runnable {
...
    /*
     * Defines the code to run for this task.
     */
    @Override
    public void run() {
        // Moves the current Thread into the background
        android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_BACKGROUND);
        ...
        /*
         * Stores the current Thread in the the PhotoTask instance,
         * so that the instance
         * can interrupt the Thread.
         */
        mPhotoTask.setImageDecodeThread(Thread.currentThread());
        ...
    }
...
}

android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_BACKGROUND);
这句话可以将你的线程移动到后台。（该优先级低于主线程，避免与主线程竞争CPU）

管理多线程

这里以PhotoManager为例:

1.既然是个管理者，最好是单例：（至于如何创建单例，不解释了）

public class PhotoManager {
    ...
    static  {
        ...
        // Creates a single static instance of PhotoManager
        sInstance = new PhotoManager();
    }

   /**
     * Constructs the work queues and thread pools used to download
     * and decode images. Because the constructor is marked private,
     * it's unavailable to other classes, even in the same package.
     */
    private PhotoManager() {
    ...
    }
    ...

2.启动一个照片下载任务，通过线程池，从队列中取一个runnable也就是任务对象：

public class PhotoManager {
    ...
    // Called by the PhotoView to get a photo
    static public PhotoTask startDownload(
        PhotoView imageView,
        boolean cacheFlag) {
        ...
        // Adds a download task to the thread pool for execution
        sInstance.
                mDownloadThreadPool.
                execute(downloadTask.getHTTPDownloadRunnable());
        ...
    }

3.自定义Handler来更新UI

    private PhotoManager() {
    ...
        // Defines a Handler object that's attached to the UI thread
        mHandler = new Handler(Looper.getMainLooper()) {
            /*
             * handleMessage() defines the operations to perform when
             * the Handler receives a new Message to process.
             */
            @Override
            public void handleMessage(Message inputMessage) {
                ...
            }
        ...
        }
    }

4.决定线程池的参数：初始化的线程池大小和最大池大小

首先ThreadPoolExcutor是android定义的类,不是java.

那么线程池的大小如何决定？最大又该多少呢？

答：省事点：通过Runtime.getRuntime().availableProcessors()

public class PhotoManager {
...
    /*
     * Gets the number of available cores
     * (not always the same as the maximum number of cores)
     */
    private static int NUMBER_OF_CORES =
            Runtime.getRuntime().availableProcessors();
}

5.决定线程池的参数：空闲线程持续时间。（keep alive time）,单位通过TimeUnit。

6.任务队列：可以使用LinkedBlockingQueue<Runnable>

public class PhotoManager {
    ...
    private PhotoManager() {
        ...
        // A queue of Runnables
        private final BlockingQueue<Runnable> mDecodeWorkQueue;
        ...
        // Instantiates the queue of Runnables as a LinkedBlockingQueue
        mDecodeWorkQueue = new LinkedBlockingQueue<Runnable>();
        ...
    }
    ...
}

7.好，下面进入创建线程池：

private PhotoManager() {
        ...
   // Sets the amount of time an idle thread waits before terminating
   private static final int KEEP_ALIVE_TIME = 1;
   // Sets the Time Unit to seconds
   private static final TimeUnit KEEP_ALIVE_TIME_UNIT = TimeUnit.SECONDS;
   // Creates a thread pool manager
   mDecodeThreadPool = new ThreadPoolExecutor(
        NUMBER_OF_CORES,     // Initial pool size
        NUMBER_OF_CORES,    // Max pool size
        KEEP_ALIVE_TIME,
        KEEP_ALIVE_TIME_UNIT,
        mDecodeWorkQueue);
}

8.让线程池执行一个任务试试：ThreadPoolExecutor.execute()会将该任务加入到线程池的队列中。当一个空闲的线程变得可用时，Manager会将等待时间最长的任务运行在这个线程中。

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());
            ...
        }
        ...
    }
    ...
}

9.打断正在执行任务的某个或者所有线程

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 < 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();
                }
            }
        }
    }
    ...
}

注意：大多数情况，Thread.interrupt()可以马上把线程打断掉。但是只能打断在等待的线程，不能打断CPU或者网络任务的线程。为了避免减缓或则锁住系统。你应该在尝试操作前，验证是否被打断。

/*
 * 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);
...