上一篇文章对HandlerThread的概念及用法做了一个基本的介绍，并且使用HandlerThread改写了相册图片加载模块中的ImageLoader，可以看到，改写后的代码变得更加清晰和简洁了。本篇文章将延续之前的主题，从源代码层面对HandlerThread的底层原理进行深入的剖析，让大家不仅知其然，更知其所以然，好了，话不多说，就让我们开始今天美妙的探索之旅吧_
如果读者已经较好地掌握了Android异步消息处理的底层机制，那么理解HandlerThread的源码将会变得异常简单。对Android异步消息处理底层机制尚未完全掌握的朋友可先阅读我之前的博文Android异步消息处理机制深度解析。
HandlerThread源码如下：
1 /*
2 * Copyright (C) 2006 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 package android.os;
18
19 /**
20 * Handy class for starting a new thread that has a looper. The looper can then be
21 * used to create handler classes. Note that start() must still be called.
22 */
23 public class HandlerThread extends Thread {
24 int mPriority;
25 int mTid = -1;
26 Looper mLooper;
27
28 public HandlerThread(String name) {
29 super(name);
30 mPriority = Process.THREAD_PRIORITY_DEFAULT;
31 }
32
33 /**
34 * Constructs a HandlerThread.
35 * @param name
36 * @param priority The priority to run the thread at. The value supplied must be from
37 * {@link android.os.Process} and not from java.lang.Thread.
38 */
39 public HandlerThread(String name, int priority) {
40 super(name);
41 mPriority = priority;
42 }
43
44 /**
45 * Call back method that can be explicitly overridden if needed to execute some
46 * setup before Looper loops.
47 */
48 protected void onLooperPrepared() {
49 }
50
51 @Override
52 public void run() {
53 mTid = Process.myTid();
54 Looper.prepare();
55 synchronized (this) {
56 mLooper = Looper.myLooper();
57 notifyAll();
58 }
59 Process.setThreadPriority(mPriority);
60 onLooperPrepared();
61 Looper.loop();
62 mTid = -1;
63 }
64
65 /**
66 * This method returns the Looper associated with this thread. If this thread not been started
67 * or for any reason is isAlive() returns false, this method will return null. If this thread
68 * has been started, this method will block until the looper has been initialized.
69 * @return The looper.
70 */
71 public Looper getLooper() {
72 if (!isAlive()) {
73 return null;
74 }
75
76 // If the thread has been started, wait until the looper has been created.
77 synchronized (this) {
78 while (isAlive() && mLooper == null) {
79 try {
80 wait();
81 } catch (InterruptedException e) {
82 }
83 }
84 }
85 return mLooper;
86 }
87
88 /**
89 * Quits the handler thread's looper.
90 * <p>
91 * Causes the handler thread's looper to terminate without processing any
92 * more messages in the message queue.
93 * </p><p>
94 * Any attempt to post messages to the queue after the looper is asked to quit will fail.
95 * For example, the {@link Handler#sendMessage(Message)} method will return false.
96 * </p><p class="note">
97 * Using this method may be unsafe because some messages may not be delivered
98 * before the looper terminates. Consider using {@link #quitSafely} instead to ensure
99 * that all pending work is completed in an orderly manner.
100 * </p>
101 *
102 * @return True if the looper looper has been asked to quit or false if the
103 * thread had not yet started running.
104 *
105 * @see #quitSafely
106 */
107 public boolean quit() {
108 Looper looper = getLooper();
109 if (looper != null) {
110 looper.quit();
111 return true;
112 }
113 return false;
114 }
115
116 /**
117 * Quits the handler thread's looper safely.
118 * <p>
119 * Causes the handler thread's looper to terminate as soon as all remaining messages
120 * in the message queue that are already due to be delivered have been handled.
121 * Pending delayed messages with due times in the future will not be delivered.
122 * </p><p>
123 * Any attempt to post messages to the queue after the looper is asked to quit will fail.
124 * For example, the {@link Handler#sendMessage(Message)} method will return false.
125 * </p><p>
126 * If the thread has not been started or has finished (that is if
127 * {@link #getLooper} returns null), then false is returned.
128 * Otherwise the looper is asked to quit and true is returned.
129 * </p>
130 *
131 * @return True if the looper looper has been asked to quit or false if the
132 * thread had not yet started running.
133 */
134 public boolean quitSafely() {
135 Looper looper = getLooper();
136 if (looper != null) {
137 looper.quitSafely();
138 return true;
139 }
140 return false;
141 }
142
143 /**
144 * Returns the identifier of this thread. See Process.myTid().
145 */
146 public int getThreadId() {
147 return mTid;
148 }
149 }
代码不多，算上注释才149行。
HandlerThread中最重要的是它的run方法和getLooper方法，我们先来看run方法：
首先扫一眼run方法，突然感觉好熟悉啊！熟悉的Looper.prepare()，熟悉的Looper.loop()，这不是我们在子线程创建Handler时所需要的一些操作吗？没错，虽然我们在当前的run方法里并没有创建Handler，但是读完本文，你应该能体会到它和我们在子线程创建Handler的异曲同工之妙。
在run方法中，首先会去调用Looper.prepare()，Looper.prepare()会去检查sThreadLocal中是否存储了Looper对象，若已经存储了，则报异常(一个线程最多只能有一个Looper对象)，若尚未存储，则新建一个Looper对象并将其放入sThreadLocal中。之后会进入一个同步代码块，取出当前的Looper对象赋给HandlerThread对象的成员变量mLooper并调用notifyAll()方法。接着便会去调用Looper.loop()，在Looper.loop()中有一个死循环，会不断地从MessageQueue中取出下一条消息，如果拿不到消息则阻塞。拿到消息后，如果消息不为空且消息的target属性不为空，则调用消息的target属性的dispatchMessage方法。
再来看getLooper方法,首先会调用isAlive()方法测试线程是否还活着，如果不是，则直接返回null。接着会进入一个同步代码块，当线程还存活着且成员变量mLooper为null时，会执行wait操作，等待run方法中的notifyAll()，由此可知，只要之前我们的HandlerThread被启动了，这个方法会一直阻塞直至mLooper初始化完成，最后将初始化完成的mLooper返回出去。
通常情况下，HandlerThread启动后，会通过getLooper()方法取出Looper对象并将其作为Handler的初始化参数。此时，getLooper()方法是运行在主线程的，而Looper对象的初始化是位于子线程(HandlerThread)的run方法中的，getLooper()方法中的 wait()与run方法中的notifyAll()共同协作，实现了两个线程之间的同步。
到这里，相信大家对HandlerThread的源码已经有了一个非常深入的理解了，下一篇文章将介绍与HandlerThread关联甚大的IntentService,大家一起期待吧！
参考：http://blog.csdn.net/lmj623565791/article/details/47079737
