上一篇文章對HandlerThread的概念及用法做了一個基本的介紹，并且使用HandlerThread改寫了相冊圖片加載模塊中的ImageLoader，可以看到，改寫后的代碼變得更加清晰和簡潔了。本篇文章將延續(xù)之前的主題，從源代碼層面對HandlerThread的底層原理進行深入的剖析，讓大家不僅知其然，更知其所以然，好了，話不多說，就讓我們開始今天美妙的探索之旅吧_
如果讀者已經(jīng)較好地掌握了Android異步消息處理的底層機制，那么理解HandlerThread的源碼將會變得異常簡單。對Android異步消息處理底層機制尚未完全掌握的朋友可先閱讀我之前的博文Android異步消息處理機制深度解析。
HandlerThread源碼如下：
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 }
代碼不多，算上注釋才149行。
HandlerThread中最重要的是它的run方法和getLooper方法，我們先來看run方法：
首先掃一眼run方法，突然感覺好熟悉啊！熟悉的Looper.prepare()，熟悉的Looper.loop()，這不是我們在子線程創(chuàng)建Handler時所需要的一些操作嗎？沒錯，雖然我們在當(dāng)前的run方法里并沒有創(chuàng)建Handler，但是讀完本文，你應(yīng)該能體會到它和我們在子線程創(chuàng)建Handler的異曲同工之妙。
在run方法中，首先會去調(diào)用Looper.prepare()，Looper.prepare()會去檢查sThreadLocal中是否存儲了Looper對象，若已經(jīng)存儲了，則報異常(一個線程最多只能有一個Looper對象)，若尚未存儲，則新建一個Looper對象并將其放入sThreadLocal中。之后會進入一個同步代碼塊，取出當(dāng)前的Looper對象賦給HandlerThread對象的成員變量mLooper并調(diào)用notifyAll()方法。接著便會去調(diào)用Looper.loop()，在Looper.loop()中有一個死循環(huán)，會不斷地從MessageQueue中取出下一條消息，如果拿不到消息則阻塞。拿到消息后，如果消息不為空且消息的target屬性不為空，則調(diào)用消息的target屬性的dispatchMessage方法。
再來看getLooper方法,首先會調(diào)用isAlive()方法測試線程是否還活著，如果不是，則直接返回null。接著會進入一個同步代碼塊，當(dāng)線程還存活著且成員變量mLooper為null時，會執(zhí)行wait操作，等待run方法中的notifyAll()，由此可知，只要之前我們的HandlerThread被啟動了，這個方法會一直阻塞直至mLooper初始化完成，最后將初始化完成的mLooper返回出去。
通常情況下，HandlerThread啟動后，會通過getLooper()方法取出Looper對象并將其作為Handler的初始化參數(shù)。此時，getLooper()方法是運行在主線程的，而Looper對象的初始化是位于子線程(HandlerThread)的run方法中的，getLooper()方法中的 wait()與run方法中的notifyAll()共同協(xié)作，實現(xiàn)了兩個線程之間的同步。
到這里，相信大家對HandlerThread的源碼已經(jīng)有了一個非常深入的理解了，下一篇文章將介紹與HandlerThread關(guān)聯(lián)甚大的IntentService,大家一起期待吧！
參考：http://blog.csdn.net/lmj623565791/article/details/47079737
