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Creating a Manager for Multiple Threads

The previous lesson showed how to define a task that executes on a separate thread. If you only want to run the task once, this may be all you need. If you want to run a task repeatedly on different sets of data, but you only need one execution running at a time, an IntentService suits your needs. To automatically run tasks as resources become available, or to allow multiple tasks to run at the same time (or both), you need to provide a managed collection of threads. To do this, use an instance of ThreadPoolExecutor, which runs a task from a queue when a thread in its pool becomes free. To run a task, all you have to do is add it to the queue.

A thread pool can run multiple parallel instances of a task, so you should ensure that your code is thread-safe. Enclose variables that can be accessed by more than one thread in a synchronized block. This approach will prevent one thread from reading the variable while another is writing to it. Typically, this situation arises with static variables, but it also occurs in any object that is only instantiated once. To learn more about this, read the Processes and Threads API guide.

Define the Thread Pool Class

Instantiate ThreadPoolExecutor in its own class. Within this class, do the following:

Use static variables for thread pools
You may only want a single instance of a thread pool for your app, in order to have a single control point for restricted CPU or network resources. If you have different Runnable types, you may want to have a thread pool for each one, but each of these can be a single instance. For example, you can add this as part of your global field declarations:
public class PhotoManager {
    ...
    static  {
        ...
        // Creates a single static instance of PhotoManager
        sInstance = new PhotoManager();
    }
    ...
Use a private constructor
Making the constructor private ensures that it is a singleton, which means that you don't have to enclose accesses to the class in a synchronized block:
public class 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() {
    ...
    }
Start your tasks by calling methods in the thread pool class.
Define a method in the thread pool class that adds a task to a thread pool's queue. For example:
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());
        ...
    }
Instantiate a Handler in the constructor and attach it to your app's UI thread.
A Handler allows your app to safely call the methods of UI objects such as View objects. Most UI objects may only be safely altered from the UI thread. This approach is described in more detail in the lesson Communicate with the UI Thread. For example:
    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) {
                ...
            }
        ...
        }
    }

Determine the Thread Pool Parameters

Once you have the overall class structure, you can start defining the thread pool. To instantiate a ThreadPoolExecutor object, you need the following values:

Initial pool size and maximum pool size
The initial number of threads to allocate to the pool, and the maximum allowable number. The number of threads you can have in a thread pool depends primarily on the number of cores available for your device. This number is available from the system environment:
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();
}
This number may not reflect the number of physical cores in the device; some devices have CPUs that deactivate one or more cores depending on the system load. For these devices, availableProcessors() returns the number of active cores, which may be less than the total number of cores.
Keep alive time and time unit
The duration that a thread will remain idle before it shuts down. The duration is interpreted by the time unit value, one of the constants defined in TimeUnit.
A queue of tasks
The incoming queue from which ThreadPoolExecutor takes Runnable objects. To start code on a thread, a thread pool manager takes a Runnable object from a first-in, first-out queue and attaches it to the thread. You provide this queue object when you create the thread pool, using any queue class that implements the BlockingQueue interface. To match the requirements of your app, you can choose from the available queue implementations; to learn more about them, see the class overview for ThreadPoolExecutor. This example uses the LinkedBlockingQueue class:
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>();
        ...
    }
    ...
}

Create a Pool of Threads

To create a pool of threads, instantiate a thread pool manager by calling ThreadPoolExecutor(). This creates and manages a constrained group of threads. Because the initial pool size and the maximum pool size are the same, ThreadPoolExecutor creates all of the thread objects when it is instantiated. For example:

    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);
    }
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