Android中Looper、Handler、Thread、Message的分析

关于这几个技术术语，让我们从Android的实际使用中来渐渐理清它们。

1. Looper的初始化

Application被创建时,ActivityThread的main函数会被调用,其中初始化了一个Looper

Looper.prepareMainLooper();

其实现在Looper.java中:

    /**     * Initialize the current thread as a looper, marking it as an     * application's main looper. The main looper for your application     * is created by the Android environment, so you should never need     * to call this function yourself.  See also: {@link #prepare()}     */    public static void prepareMainLooper() {        prepare(false); //进行初始化        synchronized (Looper.class) {            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }            sMainLooper = myLooper(); //赋值        }    }

prepare(false)是对Looper进行初始化, 初始化之后调用将Looper对象赋值给sMainLooper. 所以我们只需要看prepare函数:

    ......    static final ThreadLocal sThreadLocal = new ThreadLocal();    ......    private static void prepare(boolean quitAllowed) {        if(sThreadLocal.get() != null) {            throw new RuntimeException("Only one Looper may be created per thread");        } else {            sThreadLocal.set(new Looper(quitAllowed)); //创建一个Looper对象,放入ThreadLocal中        }    }

从之前的描述可以看出,Looper是一个单例,所以其构造函数是private的:

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

2. 初始化MessageQueue

上面的Looper的构造函数中又初始化了一个MessageQueue，这里又引出了一个重要的对象MessageQueue . 每个Looper都保持一个MessageQueue用来存放Message. 这里只需要记住Looper和MessageQueue的关系即可.下面是MessageQueue 的构造函数：

/** * Low-level class holding the list of messages to be dispatched by a * Looper.  Messages are not added directly to a MessageQueue, * but rather through  Handler objects associated with the Looper. *  * You can retrieve the MessageQueue for the current thread with *  Looper.myQueue(). */    MessageQueue(boolean quitAllowed) {        mQuitAllowed = quitAllowed;        mPtr = nativeInit();    }

从构造函数上面的描述可以看到,Message是不能直接加入到MessageQueue中的, 必须通过Handler来添加. 这里你可能会有疑问，为什么Message不能直接加入到MessageQueue里面呢？带着这个问题我们继续向下看。

3. 创建ActivityThread对象

Looper初始化完成后, 回到ActivityThread的main函数, 这里还创建了一个ActivityThread对象,对App来说,ActivityThread是一个非常重要的类,

        //初始化ActivityThread        ActivityThread thread = new ActivityThread();        thread.attach(false);        if (sMainThreadHandler == null) {            sMainThreadHandler = thread.getHandler();        }

ActivityThread的构造函数中初始化了ResourcesManager,保存在mResourcesManager中. thread.attach(false)这个比较重要,是进行一些状态的设置和初始化. 随后会初始化这个应用程序UI线程的Handle .下面是getHandler()

    final Handler getHandler() {        return mH;    }

其中mH的定义如下:

private class H extends Handler {        public void handleMessage(Message msg) {            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));            switch (msg.what) {                case LAUNCH_ACTIVITY: {                    final ActivityClientRecord r = (ActivityClientRecord) msg.obj;                    r.packageInfo = getPackageInfoNoCheck(                            r.activityInfo.applicationInfo, r.compatInfo);                    handleLaunchActivity(r, null);                } break;                case RELAUNCH_ACTIVITY: {                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");                    ActivityClientRecord r = (ActivityClientRecord)msg.obj;                    handleRelaunchActivity(r);                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);        }}

其handleMessage这个函数很长,仔细看就可以发现,这里调度了应用开发中各个组件的生命周期.包括activityRestart/activityPause/activityStop/activityDestroy/activityNewIntent等等.

4. 执行消息循环

随后main函数会调用Looper.loop(),其定义如下:

    /**     * 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();        ......        final MessageQueue queue = me.mQueue;        ......        for (;;) {            Message msg = queue.next(); // might block            .......            msg.target.dispatchMessage(msg);            ......            msg.recycleUnchecked();        }    }

可以看到其中调用了一个死循环for (;;),在循环开始调用

Message msg = queue.next(); // might block

来从MessageQueue中获取Message来进行处理.注意到MessageQueue的next函数会调用this.nativePollOnce(this.mPtr, nextPollTimeoutMillis); 这是一个阻塞方法. 会调用到Native层的Looper的一些方法. 当有消息的时候,就会返回一个Message.

这时候一个Activity的Handle/ Looper / MessageQueue都已经跑起来了.他们直接的关系也清楚了.

5. 自己创建的Handler与Looper和MessageQueue之间的关系

在Android开发中,我们一般会自己创建一个Handler,然后发送Message进行通信 (典型场景就是创建一个子线程, 在其中执行耗时较多的操作,然后发一个Message通知UI更新)

第一种写法:无参数的Handler

    Handler defaultHandler = new Handler() {        @Override        public void handleMessage(Message msg) {            switch (msg.what) {                case MSG_HELLO:                    Log.w(TAG, "defaultHandler ThreadName = "                            + defaultHandler.getLooper().getThread().getName());            }        }    };......//使用        new Thread(){            @Override            public void run() {                defaultHandler.sendEmptyMessage(MSG_HELLO);            }        }.start();

其打印结果如下：

Thread Name

这种写法创建的Handler,将自动与当前运行的线程相关联,也就是说这个自定义的Handler将与当前运行的线程使用同一个消息队列,并且可以处理该队列中的消息.如果不太理解这个可以查看Handler的构造函数:

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

可以看到, Handler中的mLooper和mQueue就是之前ActivityThread中初始化好的.由于Looper是单例,也就是说mLooper和mQueue都只有一个, 而Handler的sendMessage最终会调用sendMessageAtTime:

    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {        MessageQueue queue = mQueue;        ......        return enqueueMessage(queue, msg, uptimeMillis);    }

看到这里,应该对自己创建的Handler和MessageQueue以及Looper直接的关系更清楚了.

第二种写法:参数包含Looper的Handler

Handler有好几个构造函数,其中如果参数中不包含Looper,则最终会进入第一种写法中的那个构造函数,如果参数中包含Looper,则会进入到三个参数的构造函数中:

    /**     * Use the provided Looper instead of the default one and take a callback     * interface in which to handle messages.  Also set whether the handler     * should be asynchronous.     *     * Handlers are synchronous by default unless this constructor is used to make     * one that is strictly asynchronous.     *     * Asynchronous messages represent interrupts or events that do not require global ordering     * with represent to synchronous messages.  Asynchronous messages are not subject to     * the synchronization barriers introduced by MessageQueue#enqueueSyncBarrier(long).     *     * @param looper The looper, must not be null.     * @param callback The callback interface in which to handle messages, or null.     * @param async If true, the handler calls  Message#setAsynchronous(boolean) for     * each Message that is sent to it or Runnable that is posted to it.     *     * @hide     */    public Handler(Looper looper, Callback callback, boolean async) {        mLooper = looper;        mQueue = looper.mQueue;        mCallback = callback;        mAsynchronous = async;    }

这种Handler初始化的时候必须创建一个Looper对象

    Handler mHandler;    class CustomThread extends Thread {        @Override        public void run() {            Looper.prepare();            mHandler = new Handler() {                @Override                public void handleMessage(Message msg) {                    switch (msg.what) {                        case MSG_HELLO:                            Log.w(TAG, "defaultHandler ThreadName = "                            + defaultHandler.getLooper().getThread().getName());                            break;                        default:                            break;                    }                }            };            Looper.loop();        }    }//使用        CustomThread mCustomThread = new CustomThread();        mCustomThread.start();

我给TextView加了一个点击事件，点击会打印对应的线程名：

        appNameText.setOnClickListener(new View.OnClickListener() {            @Override            public void onClick(View v) {                mHandler.sendEmptyMessage(MSG_HELLO);                Log.w(TAG, "MainThreadName =" + getMainLooper().getThread().getName());            }        });

其结果如下：

Different Thread

通过上面两个例子,你应该对Thread/Looper/MessageQueue/Handler有更深入的了解了.