关于HandlerThread的分析

Android中的Thread没有对java中的Thread做任何封装，而Android提供了一个遍历方法HandlerThread，他继承于Thread，实现了对遍历系统的一些封装，下面研究一下HandlerThread的源码：

 /*
  * Copyright (C) 2006 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 package android.os;

 /**
  * 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;
     }
 }

下面来分析一下该类的实现原理：

1、首先该类有两个构造方法：一个传入一个字符串名称，任意选取，优先级设置为默认的，第二个多了设置进程优先级的参数

2、

实际上简单理解就是：HandlerThread可以让Handler运行于其他线程中，在平常应用中，以Activity为例，新建一个Handler对象，默认是运行于UI线程中的，如果需要在Handler中执行一些耗时的任务，可以重新开启一个子线程来执行，而不可以直接在handler中执行；

那具体用途在于哪呢？

如果需要在子线程中使用Handler类，首先需要创建Looper类实例，这时可以通过Looper.prepare()和Looper.loop()函数来实现的。阅读Framework层源码发现，Android为我们提供了一个HandlerThread类，该类继承Thread类，并使用上面两个函数创建Looper对象，而且使用wait/notifyAll解决了多线程中子线程1获取子线程2的Looper对象为空的问题

为了方便描述，这里附上一段demo：

 void useHanlerThread() {

     // two ways to construct HandlerThread
     HandlerThread hThread = new HandlerThread(”AnyName“,

             // a property in android.os.Process
             Process.THREAD_PRIORITY_BACKGROUND);

     // HandlerThread hThread2 = new HandlerThread(”AnyName“);
     hThread.start();

     // get Looper object of hThread(HandlerThread)
     Looper looper = hThread.getLooper();

     Handler h = new Handler(looper) {
         @Override
         public void handleMessage(Message msg) {
             // TODO Auto-generated method stub
             super.handleMessage(msg);
         }
     };

 }

由上面代码第4行可以看到，这里采用HandlerThread新建一个thread对象，实际上他就是一个Thread，只不过经过了一些封装，它封装了looper等，所以该实例也就是一个worker thread，也即新建了一个线程，只是不需要维护Looper，内部封装好了，当调用HandlerThread的start方法时，就会执行重写的run方法，进而可以直接通过第13行代码的getLooper获取，然后根据获取的这个looper来实例化Handler，这样这个Handler就工作于前面创建的HandlerThread实例中，也就是说相比传统的方式，这里可以直接处理耗时逻辑而无需再新建线程

这里再附上一段样例代码来辅助理解：

 public void startWorkerHandler(){
         HandlerThread handlerThread = new HandlerThread("handler-thread");
         handlerThread.start();

         final Handler handler = new Handler(handlerThread.getLooper());
         handler.post(new Runnable() {
             @Override
             public void run() {

                 npb.increaseProgressBy(1);
                 handler.postDelayed(this, 100);
             }
         });
     }

实例：

 package com.example.randy.helloworld;

 import android.app.Activity;
 import android.os.Bundle;
 import android.os.Handler;
 import android.os.HandlerThread;
 import android.os.Looper;

 /**
  * Created by randy on 2015/12/7.
  */
 public class HandlerDemo extends Activity {
     private Handler handler1;

     private Handler handler2;

     private HandlerThread handlerThread;
     private Handler handler3;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         handler1 = new Handler();
         new Thread(new Runnable() {
             @Override
             public void run() {
                 Looper.prepare();
                 handler2 = new Handler();
                 Looper.loop();
             }
         }).start();

        /* new Thread(new Runnable(){
             @Override
             public void run(){
                 handlerThread = new HandlerThread("handlerThread");
                 handlerThread.start();
             }
         }).start();*/

         new HandlerThread("handlerThread"){

             @Override
             public void run(){

                 handler3 = new Handler();
             }
         }.start();
     }
 }