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：
public Handler() { if (FIND_POTENTIAL_LEAKS) { final Class<? extends Handler> klass = getClass(); if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) && (klass.getModifiers() & Modifier.STATIC) == 0) { Log.w(TAG, "The following Handler class should be static or leaks might occur: " + klass.getCanonicalName()); } } mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = null; }
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。HandlerThread类完整代码如下：
/** * 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ID private 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; } }