Android中HandlerThread类的解析
Android应用中的消息循环由Looper和Handler配合完成，Looper类用于封装消息循环，类中有个MessageQueue消息队列；Handler类封装了消息投递和消息处理等功能。
系统默认情况下只有主线程（即UI线程）绑定Looper对象，因此在主线程中可以直接创建Handler的实例，但是在子线程中就不能直接new出Handler的实例了，因为子线程默认并没有Looper对象，此时会抛出RuntimeException异常：
浏览下Handler的默认构造函数就一目了然了：
/** * 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 {private int mPriority; // 线程优先级private int mTid = -1; // 线程IDprivate Looper mLooper; // 我们需要的Looper对象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;}/** * 在Looper.loop()之前执行动作的回调函数 */protected void onLooperPrepared() {}public void run() {mTid = Process.myTid();Looper.prepare(); // 创建本线程的Looper对象synchronized (this) {mLooper = Looper.myLooper();notifyAll(); //通知所有等待该线程Looper对象的其他子线程，本线程的Looper对象已就绪}Process.setThreadPriority(mPriority);onLooperPrepared(); //回调函数Looper.loop(); //开始消息队列循环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(); //Looper对象未创建好，等待} catch (InterruptedException e) {}}}return mLooper;}/** * Ask the currently running looper to quit. 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. */public boolean quit() {Looper looper = getLooper();if (looper != null) {looper.quit();return true;}return false;}/** * Returns the identifier of this thread. See Process.myTid(). */public int getThreadId() {return mTid;}}