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MessageQueue和Looper两个概念。
- 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。
- 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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