Handler实现线程间通信的原理

本文以Handler对象的创建和消息发送为切入点，讲述背后的实现原理。
一. Handler对象创建的背后过程

public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class 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 = callback;
        mAsynchronous = async;
    }

 public Handler(Looper looper, Callback callback, boolean async) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

  返回当前线程的Looper实例化对象
      /**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

从Handler构造方法来看，每个Handler对象，都拥有成员变量looper对象mLooper，消息队列对象mQueue，而这种消息队列对象也是mLooper成员变量，这个Looper对象要么是使用者直接传进来的，要么是通过Looper.myLooper()得到的。
下面再来看看Looper的创建，Looper对象对外提供静态方法prepare() 创建，但构造方法是私有的，不能直接实例化。
每个线程至多有一个Looper对象，否则会抛运行时异常。创建Looper对象时，会创建一个MessageQueue对象作为其成员变量。

  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));
    }
    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

MessageQueue是由Message对象组织成链表结构的消息队列，Message对象是序列化对象，除了通信的数据外，一个很关键的是成员变量就是target用来存储通信Handler对象。
由上面的代码片段可知，要实现Handler线程间通信，必须要首先通过Looper的prepare()实例化对象，但是有些同学可能平时使用Handler时都没有操作过Looper，那Looper初始化在哪里进行的呢。
其实在app启动的时候，ActivityThread main方法里都已经通过prepareMainLooper()创建了，它是主线程的Looper实例，相比子线程的初始化稍有不同。具体参看如下代码：

public static void main(String[] args) {
        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
        SamplingProfilerIntegration.start();


        // CloseGuard defaults to true and can be quite spammy.  We
        // disable it here, but selectively enable it later (via
        // StrictMode) on debug builds, but using DropBox, not logs.
        CloseGuard.setEnabled(false);


        Environment.initForCurrentUser();


        // Set the reporter for event logging in libcore
        EventLogger.setReporter(new EventLoggingReporter());


        // Make sure TrustedCertificateStore looks in the right place for CA certificates
        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
        TrustedCertificateStore.setDefaultUserDirectory(configDir);


        Process.setArgV0("");


        Looper.prepareMainLooper();


        ActivityThread thread = new ActivityThread();
        thread.attach(false);


        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }


        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }


        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();


        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

二. Handler对象触发线程通信的背后过程
当子线程通过Handler的sendMessage触发消息时，是怎么和主线程通信的呢，这个时候Looper就真正起作用了，看Looper的loop()方法：

 /**
     * 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();
        }
    }

通过上面的代码可知：这个是死循环代码，一直从MessageQueue里取msg,通过拿到msg的target对象，上面有特意强调过这个就是我们的Handler对象，调用它的dispatchMessage方法，具体实现看源代码：

/**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

看到handleMessage方法估计大家都熟悉了，这就是我们经常重写Handler类的handleMessage方法，那么loop（）方法又在哪里调用的呢，其实细心的同学在上面的ActivityThread main方法里都已经发现了，在Looper初始化成功后，就会调用loop（）方法一直检查是否有消息，有消息就调用这个handler对象的dispatchMessage方法，至此子线程和主线程通信过程结束了。

聪明的同学可能会有疑问，Android中为什么主线程不会因为Looper.loop()里的死循环卡死？
其实主线程确实会一直卡在loop()循环里，只是因为主线程又开辟了其它Binder子线程来处理，你没有感觉到而已，具体分析参看知乎牛人的回答：http://www.zhihu.com/question/34652589

总结：Thread ，Looper，MessageQueue，Message，Handler的之间关系
一个Thread至多有一个Looper，需要通过Looper.prepare()初始化和Looper.loop（）处理分发给对应的Handler对象处理
一个Looper有且有一个MessageQueue，通过Looper.loop（）取出MessageQueue的Message进行处理，但是可以服务于多个Handler对象。
MessageQueue是由Message对象组织成链表结构的消息队列，而每个Message对象都唯一对应一个Handler对象。