作为Android开发者都知道在子线程中使用Handler必须要创建Looper，其实HandlerThread就是在线程中封装了Looper的创建和循环，不用我们开发者自己去创建它，下面我们来看看源码

源码

public class HandlerThread extends Thread {
    int mPriority;
    int mTid = -1;
    Looper mLooper;

    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;
    }
    
    /**
     * Call back method that can be explicitly overridden if needed to execute some
     * setup before Looper loops.
     */
    protected void onLooperPrepared() {
    }

    @Override
    public void run() {
        mTid = Process.myTid();
        Looper.prepare();
        synchronized (this) {
            mLooper = Looper.myLooper();
            notifyAll();
        }
        Process.setThreadPriority(mPriority);
        onLooperPrepared();
        Looper.loop();
        mTid = -1;
    }

可以看出它就是一个普通的线程，创建的时候设置优先级，我们来看看它的run方法，挑重点看

 //创建looper，保存到ThreadLocal线程中
 Looper.prepare();
 synchronized (this) {
      //得到创建的Looper
      mLooper = Looper.myLooper();
      notifyAll();
  }
  Process.setThreadPriority(mPriority);
  onLooperPrepared();
  //启动循环Looper中的消息队列
  Looper.loop();

很简单，就是创建了Looper并且启动循环消息队列。

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();
             } catch (InterruptedException e) {
            }
          }
     }
        return mLooper;
 }

得到刚才创建的Looper对象。

public boolean quit() {
      Looper looper = getLooper();
      if (looper != null) {
          looper.quit();
          return true;
      }
        return false;
 }

释放Looper消息队列里的消息。

到此HandlerThread的源码就解析完了。