深入理解HandlerThread

大家好，又见面了，我是你们的朋友全栈君。

以往遇到HandlerThread，对它的认识只是停留在Message<->Looper<->Handler上，知道它有自己的消息队列，仅此而已。随着编程的深入，表面上的理解无法满足要求，所以再次翻开HandlerThread源码，做梳理记录。

HandlerThread集成Thread，并重写了Thread类的run方法(如果我们自定义一个类继承HandlerThread，就用不到run函数了)：

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

从图中我们可以看到，HandlerThread的run方法先后执行了Looper.prepare()、Looper.loop()，因为我们知道这样做的目的是为了给HandlerThread实例建立一个属于它的消息队列（MessageQueue，这一点像极了Windows系统的窗口消息分发机制）。

     /** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    public static void prepare() {
        prepare(true);
    }

    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

Looper.loop()的工作就是开始不断的从消息队列中取出消息（在loop函数里有个for循环，除非线程被系统杀死或者被调用quit函数，负责for循环会一直执行）

    /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            // This must be in a local variable, in case a UI event sets the logger
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(msg);

            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }

            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }

            msg.recycleUnchecked();
        }
    }

有人说Looper中的sThreadLocal属性(ThreadLocal<Looper>类型)是个静态常量，怎么保证每个执行过Looper.prepare()、Looper.loop()的线程都有唯一的Looper呢？这个问题刚开始我也很好奇，后来发现原因在ThreadLocal类。

/**
 * Implements a thread-local storage, that is, a variable for which each thread
 * has its own value. All threads share the same {@code ThreadLocal} object,
 * but each sees a different value when accessing it, and changes made by one
 * thread do not affect the other threads. The implementation supports
 * {@code null} values.
 *
 * @see java.lang.Thread
 * @author Bob Lee
 */
public class ThreadLocal<T> 

上述类的解释是说ThreadLocal实现了线程的本地存储，即所有的线程共同使用同一个ThreadLocal对象，但每个线程都会有一个ThreadLocal对象副本，每个线程的ThreadLocal对象副本又是相互独立的，互不影响。

推荐一篇介绍ThreadLocal的博文，写的很不错：彻底理解ThreadLocal