浅谈Handler机制

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一、前言

  大家对Handler一定不陌生,它主要用于多线程间的通信,常见的就是在UI线程中创建了Handler对象,在异步线程中做数据请求等耗时的操作,耗时操作后再通过Handler.sendMessage(message)等接口通知UI线程刷新界面等。

二、概念了解

  说到Handler的内部实现,一定要知道MessageQueueLooper两个概念。

  1. MessageQueue:字面理解就是消息队列,用来存放消息,Handler就是将message发送到该队列中。
  2. Looper:字面理解就是循环处理机,内部会循环去MessageQueue获取消息message,如果没有消息就锁住;有则通知Handler处理该消息,最终便会调用Handler.handleMessage(message)(如果通过Handler.post(runnable),则最终执行runnable里面的内容)。

三、源码分析

  首先看看Handler的构造,其实最终会执行以下两个构造函数:

public Handler(Callback callback, boolean async) {    if (FIND_POTENTIAL_LEAKS) {        final Class<? extends Handler> 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;}

  后者会传入Looper对象,前者则用当前线程的Looper,如果没有(mLooper == null)则会抛出异常 "Can't create handler inside thread that has not called Looper.prepare()",是不是有的印象哈?曾经我也犯过该错误,也许这时有人会说,“我每次都是直接new Handler创建,都没有执行Looper.prepare(),从来都没遇到过该报错”,这是因为当前线程是主线程(即所谓的UI线程)。主线程就是ActivityThread,和普通的java类一样,入口是一个main方法,见如下源码:

public static void main(String[] args) {    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());     Security.addProvider(new AndroidKeyStoreProvider());     Process.setArgV0("<pre-initialized>");     Looper.prepareMainLooper();     ActivityThread thread = new ActivityThread();    thread.attach(false);     if (sMainThreadHandler == null) {        sMainThreadHandler = thread.getHandler();    }     AsyncTask.init();     if (false) {        Looper.myLooper().setMessageLogging(new                LogPrinter(Log.DEBUG, "ActivityThread"));    }     Looper.loop();     throw new RuntimeException("Main thread loop unexpectedly exited");}

  根据源码,主线程启动就会调用了Looper.prepareMainLooper(),这时便创建了主线程的Looper。因此,当我们创建Handler对象又没传入特定Looper时,需要确保当前线程已创建了Looper,即已调用了Looper.prepare()。接下来,我们来看看Looper的相关方法及成员。

/** 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));} private Looper(boolean quitAllowed) {    mQueue = new MessageQueue(quitAllowed);    mThread = Thread.currentThread();} /** * 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();} // sThreadLocal.get() will return null unless you've called prepare().static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); /** * 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);    synchronized (Looper.class) {        if (sMainLooper != null) {            throw new IllegalStateException("The main Looper has already been prepared.");        }        sMainLooper = myLooper();    }} /** * Returns the application's main looper, which lives in the main thread of the application. */public static Looper getMainLooper() {    synchronized (Looper.class) {        return sMainLooper;    }}

  根据源码知道,Looper有个静态成员sThreadLocal,通过它保存每个线程的Looper(可以把它当做一个Map,其中key就是线程,value就是Looper,它的内部实现就不再深入解析了)。执行Looper.prepare(),最终便会为当前线程创建Looper对象,其中会创建MessageQueue(所以,一个Looper就对应有一个MessageQueue),需要注意,已创建了Looper则不能再执行该方法,否则会抛出异常"Only one Looper may be created per thread"。在方法Looper.prepare()的注释中,我们知道,当创建完Looper对象需要执行Looper.loop()。继续看源码,看Looper.loop()做了什么。

/** * 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();    }}

  终于看到Looper最核心的代码了,其实就是一个死循环。现在,我们对Handler、Looper、MessageQueue有了初步的了解,接下来通过时序图看看他们三者是如何工作的。

  其实,三者的工作流程还是比较好理解的,现在看看内部的源码实现。

public final boolean sendEmptyMessage(int what){    return sendEmptyMessageDelayed(what, 0);} public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {    Message msg = Message.obtain();    msg.what = what;    return sendMessageDelayed(msg, delayMillis);} public final boolean sendMessageDelayed(Message msg, long delayMillis){    if (delayMillis < 0) {        delayMillis = 0;    }    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);} public boolean sendMessageAtTime(Message msg, long uptimeMillis) {    MessageQueue queue = mQueue;    if (queue == null) {        RuntimeException e = new RuntimeException(                this + " sendMessageAtTime() called with no mQueue");        Log.w("Looper", e.getMessage(), e);        return false;    }    return enqueueMessage(queue, msg, uptimeMillis);} private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {  //等下Looper就是通过该target回调通知处理消息    msg.target = this;    if (mAsynchronous) {        msg.setAsynchronous(true);    }    return queue.enqueueMessage(msg, uptimeMillis);}

  上面展示的是通过sendMessage(message)的方式发送消息,我们有时也会通过post(runnable)发送消息,源码如下:

public final boolean post(Runnable r){   return  sendMessageDelayed(getPostMessage(r), 0);}private static Message getPostMessage(Runnable r) {    Message m = Message.obtain();    m.callback = r;    return m;}

其实就是进一步封装成带有callback的Message,后面的流程还是一样。消息发送完后,接下来就是Looper去获取消息,见上面的源码Looper.loop(),通过queue.next()获得有效消息后,执行msg.target.dispatchMessage(msg),其中target就是Handler了,看看dispatchMessage()方法做了什么。

/** * 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);    }}private static void handleCallback(Message message) {    message.callback.run();}

  说到这里,是不是有种峰回路转的感觉哈!Handler机制表面上理解还是比较简单的,能力不足就只能解析到这里了。

四、总结

1、每个线程只有一个Looper,可通过Looper.prepare()创建,其中主线程启动就通过Looper.prepareMainLooper()创建了,创建完Looper记得执行Looper.loop()。

2、创建Handler对象时,需要保证当前线程已创建Looper,如果确定是主线程的Handler,可通过Looper.getMainLooper()传入Looper对象构造。

3、一般我们会在类通过new Handler()并重写handleMessage(messgae)方法来构造,其实这时我们就是创建了一个匿名内部类,而内部类默认是持有外部类的对象,所以这时需要注意是否存在内存泄露的问题。一般我们在退出activity时会通过handler.removeCallbacksAndMessages(null)来移除所有待处理的消息。


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