概要介绍
HandlerThread是一种特殊的Thread，也就是有Looper的thread，既然有Looper的话，那我们就可以用此Looper来创建一个Handler，从而实现和它的交互。比如你可以通过与它关联的Handler对象在UI线程中发消息给它处理。HandlerThread一般可以用来执行某些background的操作，比如读写文件、进行一些计算（在此HandlerThread而非UI线程中）。既然还是一个Thread，那么和一般的Thread一样，也要通过调用其start()方法来启动它。它只是Android替我们封装的一个Helper类，其源码相当简洁，有一些经典的标准写法，我们一起来看看。
源码分析
和以往一样，我们先来看看其字段和构造函数：
int mPriority; // 线程优先级
int mTid = -1; // 线程id
Looper mLooper; // 与此线程关联的Looper
public HandlerThread(String name) { // 提供个名字，方便debug
super(name);
mPriority = Process.THREAD_PRIORITY_DEFAULT; // 没提供，则使用默认优先级
}
/**
* 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; // 使用用户提供的优先级，取Process.java中定义的常量
}
代码很简单，相关的分析都直接写在代码的注释里了，值得注意的是这里的priority是基于Linux优先级的，而不是Java Thread类里的MIN_PRIORITY，NORM_PRIORITY，MAX_PRIORITY之类，请注意区分（其实认真阅读方法的doc即可）。
接下来看看此类的3个关键方法：
/**
* Call back method that can be explicitly overridden if needed to execute some
* setup before Looper loops.
*/
protected void onLooperPrepared() {
// callback方法，如果你愿意可以Override放自己的逻辑；其在looper开始前执行
}
@Override
public void run() {
mTid = Process.myTid();
Looper.prepare(); // 此方法我们前面介绍过，会创建与线程关联的Looper对象
synchronized (this) { // 进入同步块，当mLooper变的可用的使用，调用notifyAll通知其他可能block在当前对象上的线程
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority); // 设置线程优先级
onLooperPrepared(); // 调用回调函数
Looper.loop(); // 开始loop
mTid = -1; // reset为invalid值
}
/**
* 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()) { // 如果线程不是在alive状态则直接返回null，有可能是你忘记调start方法了。。。
return null;
}
// If the thread has been started, wait until the looper has been created.
synchronized (this) {
while (isAlive() && mLooper == null) { // 进入同步块，当条件不满足时无限等待，
try { // 直到mLooper被设置成有效值了才退出while（当然也可能是线程状态不满足）；
wait(); // run方法里的notifyAll就是用来唤醒这里的
} catch (InterruptedException e) { // 忽略InterruptedException
}
}
}
return mLooper; // 最后返回mLooper，此时可以保证是有效值了。
}
当你new一个HandlerThread的对象时记得调用其start()方法，然后你可以接着调用其getLooper()方法来new一个Handler对象，最后你就可以利用此Handler对象来往HandlerThread发送消息来让它为你干活了。
最后来看2个退出HandlerThread的方法，其实对应的是Looper的2个退出方法：
/**
* 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(); // 注意这里是调用getLooper而不是直接使用mLooper，
if (looper != null) { // 因为mLooper可能还没初始化完成，而调用方法可以
looper.quit(); // 等待初始化完成。
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;
}
通过代码我们可以看到其内部都是delegate给了Looper对象，而Looper我们在前面也介绍过了，感兴趣的同学可以翻看前面的分析或者查看这2个方法的doc，写的都很详细。
至此这个简单的Handy class就算分析完毕了。在实际的开发中，如果你只是要做某些后台的操作（短暂的，比如把某些设置文件load到内存中），而不需要更新UI的话，那你可以优先使用HandlerThread而不是AsyncTask。
