HandlerThread

1. 正常情况下使用线程我们都是直接new Thread.start来开启，但是这种方式是执行完run 方法后线程结束了，当我们有多个线程的时候意味着我们需要开启多个线程去执行，这样是很耗费CPU资源的。

2. HandlerThread解决了这个问题，它是一个继承于Thread的类，内部的run方法实现了一套looper机制，可以不让这个子线程结束。内部的looper里面有一个死循环，如果有消息就把任务发送到子线程中处理，没有则挂起等待新的消息到来。

使用场景
如果你有多个耗时操作需要开启多个子线程操作的话，你可以考虑使用它，它的好处是一个线程可以持续的执行多个任务队列，节省资源方便维护。 坏处是不适合那些需要马上响应的耗时操作，比如网络请求，如果网络请求走队列的话，任务一多请求就会一直堵塞等待执行。

注意事项
Handler如果要在子线程中创建必须调用 Looper.prepare();初始化一个looper，Handler会在构造检查looper没有会抛出异常，但是在主线程中就不需要，因为主线程中默认已经包含了一个MainLooper对象。

检查Looper的代码

 mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()"); 
        }

使用起来

HandlerThread handlerThread = new HandlerThread("mainhander");
        handlerThread.start();
        Handler handler = new Handler(handlerThread.myLooper()) {

            @Override
            public void dispatchMessage(Message msg) {
                LogU.Log("pId=" + Process.myTid());
            }
        };
        handler.sendEmptyMessage(1);

这个时候handlerThread子线程已经具备了队列的功能，而且线程是一直在的，内部的Looper一直处理挂起状态等带新的消息到来。

结束handlerThread线程

handlerThread自带了二个结束线程队列的方法， 一个是线程安全一个线程不安全，当我们不需要在用这个子线程的时候需要及时的结束它，免的占用资源。

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

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

Looper初始化特点
1.子线程的Looper就变成子线程回调
2.主线程的Looper就变成主线程回调

/**
 * 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 {
    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;
    }

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

    /**
     * Quits the handler thread's looper.
     * 

     * Causes the handler thread's looper to terminate without processing any
     * more messages in the message queue.
     * 

     * 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.
     * 

     * 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.
     * 
     *
     * @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();
        if (looper != null) {
            looper.quit();
            return true;
        }
        return false;
    }

    /**
     * Quits the handler thread's looper safely.
     * 

     * 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.
     * 

     * 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.
     * 

     * 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.
     * 
     *
     * @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;
    }

    /**
     * Returns the identifier of this thread. See Process.myTid().
     */
    public int getThreadId() {
        return mTid;
    }
}