Android HandlerThread使用及原理分析

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子线程的消息循环

很明显的一点就是，我们要在子线程中调用Looper.prepare() 为一个线程开启一个消息循环，默认情况下Android中新诞生的线程是没有开启消息循环的。（主线程除外，主线程系统会自动为其创建Looper对象，开启消息循环。） Looper对象通过MessageQueue来存放消息和事件。一个线程只能有一个Looper，对应一个MessageQueue。然后通过Looper.loop() 让Looper开始工作，从消息队列里取消息，处理消息。

public class MainActivity extends AppCompatActivity {    private Handler handler1 ;    private Handler handler2 ;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        new MyThread1().start();        new MyThread2().start();    }    class MyThread1 extends Thread {        @Override        public void run() {            super.run();            Looper.prepare();            handler1 = new Handler(){                @Override                public void handleMessage(Message msg) {                    super.handleMessage(msg);                    System.out.println( "threadName--" + Thread.currentThread().getName() + "messageWhat-"+ msg.what );                }            };            try {                sleep( 3000 );            } catch (InterruptedException e) {                e.printStackTrace();            }            handler2.sendEmptyMessage( 2 ) ;            Looper.loop();        }    }    class MyThread2 extends Thread {        @Override        public void run() {            super.run();            Looper.prepare();            handler2 = new Handler(){                @Override                public void handleMessage(Message msg) {                    super.handleMessage(msg);                    System.out.println( "threadName--" + Thread.currentThread().getName() + "messageWhat-"+ msg.what );                }            };            try {                sleep( 4000 );            } catch (InterruptedException e) {                e.printStackTrace();            }            handler1.sendEmptyMessage( 5 ) ;            Looper.loop();        }    }}

然而这一切都可以用HandlerThread类来帮我们做这些逻辑操作。

HandlerThread

HandlerThread本质上就是一个普通Thread,只不过内部建立了Looper.

HandlerThread的常规用法

创建一个HandlerThread

  mThread = new HandlerThread("handler_thread");`

启动一个HandlerThread

  mThread.start();

退出循环
Looper是通过调用loop方法驱动着消息循环的进行: 从MessageQueue中阻塞式地取出一个消息，然后让Handler处理该消息，周而复始，loop方法是个死循环方法。

那如何终止消息循环呢？我们可以调用Looper的quit方法或quitSafely方法，二者稍有不同。

public void quit() {    mQueue.quit(false);}public void quitSafely() {    mQueue.quit(true);}

相同点：
将不在接受新的事件加入消息队列。

不同点
当我们调用Looper的quit方法时，实际上执行了MessageQueue中的removeAllMessagesLocked方法，该方法的作用是把MessageQueue消息池中所有的消息全部清空，无论是延迟消息（延迟消息是指通过sendMessageDelayed或通过postDelayed等方法发送的需要延迟执行的消息）还是非延迟消息。

当我们调用Looper的quitSafely方法时，实际上执行了MessageQueue中的removeAllFutureMessagesLocked方法，通过名字就可以看出，该方法只会清空MessageQueue消息池中所有的延迟消息，并将消息池中所有的非延迟消息派发出去让Handler去处理，quitSafely相比于quit方法安全之处在于清空消息之前会派发所有的非延迟消息。

无论是调用了quit方法还是quitSafely方法只会，Looper就不再接收新的消息。即在调用了Looper的quit或quitSafely方法之后，消息循环就终结了，这时候再通过Handler调用sendMessage或post等方法发送消息时均返回false，表示消息没有成功放入消息队列MessageQueue中，因为消息队列已经退出了。

需要注意的是Looper的quit方法从API Level 1就存在了，但是Looper的quitSafely方法从API Level 18才添加进来。

小例子

import android.os.Bundle;import android.os.Handler;import android.os.HandlerThread;import android.os.Message;import android.support.v7.app.AppCompatActivity;import android.util.Log;public class MainActivity extends AppCompatActivity {    private HandlerThread myHandlerThread ;    private Handler handler ;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        //创建一个线程,线程名字：handler-thread        myHandlerThread = new HandlerThread( "handler-thread") ;        //开启一个线程        myHandlerThread.start();        //在这个线程中创建一个handler对象        handler = new Handler( myHandlerThread.getLooper() ){            @Override            public void handleMessage(Message msg) {                super.handleMessage(msg);                //这个方法是运行在 handler-thread 线程中的 ，可以执行耗时操作                Log.d( "handler " , "消息： " + msg.what + "  线程： " + Thread.currentThread().getName()  ) ;            }        };        //在主线程给handler发送消息        handler.sendEmptyMessage( 1 ) ;        new Thread(new Runnable() {            @Override            public void run() {             //在子线程给handler发送数据             handler.sendEmptyMessage( 2 ) ;            }        }).start() ;    }    @Override    protected void onDestroy() {        super.onDestroy();        //释放资源        myHandlerThread.quit() ;    }}

运行效果：

/com.app D/handler: 消息： 1  线程： handler-thread/com.app D/handler: 消息： 2  线程： handler-thread

HandlerThread源码分析

** * Handy class for starting a new thread that has a looper. The looper can then be  * used to create handler classes. Note that start() must still be called. */public class HandlerThread extends Thread {    int mPriority;    int mTid = -1;    Looper mLooper;    public HandlerThread(String name) {        super(name);        mPriority = Process.THREAD_PRIORITY_DEFAULT;    }    /**     * Constructs a HandlerThread.     * @param name     * @param priority The priority to run the thread at. The value supplied must be from      * {@link android.os.Process} and not from java.lang.Thread.     */    public HandlerThread(String name, int priority) {        super(name);        mPriority = priority;    }    /**     * Call back method that can be explicitly overridden if needed to execute some     * setup before Looper loops.     */    protected void onLooperPrepared() {    }    @Override    public void run() {        mTid = Process.myTid();        Looper.prepare();        synchronized (this) {            mLooper = Looper.myLooper();            notifyAll();        }        Process.setThreadPriority(mPriority);        onLooperPrepared();        Looper.loop();        mTid = -1;    }    /**     * This method returns the Looper associated with this thread. If this thread not been started     * or for any reason is isAlive() returns false, this method will return null. If this thread      * has been started, this method will block until the looper has been initialized.       * @return The looper.     */    public Looper getLooper() {        if (!isAlive()) {            return null;        }        // If the thread has been started, wait until the looper has been created.        synchronized (this) {            while (isAlive() && mLooper == null) {                try {                    wait();                } catch (InterruptedException e) {                }            }        }        return mLooper;    }    /**     * Quits the handler thread's looper.     * <p>     * Causes the handler thread's looper to terminate without processing any     * more messages in the message queue.     * </p><p>     * Any attempt to post messages to the queue after the looper is asked to quit will fail.     * For example, the {@link Handler#sendMessage(Message)} method will return false.     * </p><p class="note">     * Using this method may be unsafe because some messages may not be delivered     * before the looper terminates.  Consider using {@link #quitSafely} instead to ensure     * that all pending work is completed in an orderly manner.     * </p>     *     * @return True if the looper looper has been asked to quit or false if the     * thread had not yet started running.     *     * @see #quitSafely     */    public boolean quit() {        Looper looper = getLooper();        if (looper != null) {            looper.quit();            return true;        }        return false;    }    /**     * Quits the handler thread's looper safely.     * <p>     * Causes the handler thread's looper to terminate as soon as all remaining messages     * in the message queue that are already due to be delivered have been handled.     * Pending delayed messages with due times in the future will not be delivered.     * </p><p>     * Any attempt to post messages to the queue after the looper is asked to quit will fail.     * For example, the {@link Handler#sendMessage(Message)} method will return false.     * </p><p>     * If the thread has not been started or has finished (that is if     * {@link #getLooper} returns null), then false is returned.     * Otherwise the looper is asked to quit and true is returned.     * </p>     *     * @return True if the looper looper has been asked to quit or false if the     * thread had not yet started running.     */    public boolean quitSafely() {        Looper looper = getLooper();        if (looper != null) {            looper.quitSafely();            return true;        }        return false;    }    /**     * Returns the identifier of this thread. See Process.myTid().     */    public int getThreadId() {        return mTid;    }}

HandlerThread的特点

HandlerThread将loop转到子线程中处理，说白了就是将分担MainLooper的工作量，降低了主线程的压力，使主界面更流畅。

开启一个线程起到多个线程的作用。处理任务是串行执行，按消息发送顺序进行处理。HandlerThread本质是一个线程，在线程内部，代码是串行处理的。

但是由于每一个任务都将以队列的方式逐个被执行到，一旦队列中有某个任务执行时间过长，那么就会导致后续的任务都会被延迟处理。

HandlerThread拥有自己的消息队列，它不会干扰或阻塞UI线程。

对于网络IO操作，HandlerThread并不适合，因为它只有一个线程，还得排队一个一个等着。

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