再一次撸Handler、Looper、Message源码
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从事Android开发的人,一定对Handler非常熟悉。Handler是一套消息处理机制,允许你发送,处理消息来实现线程间的通讯。每个Handler实例与一个线程和该线程的消息队列关联。当你创建一个Handler时,它会绑定到创建它的线程的消息队列上。将传递的消息加入到这个消息队列中,通过轮询取出消息,然后Handler可以接收并处理出队的消息。
Handler发送消息通过 post、postAtTime、postDelayed、sendEmptyMessage、sendMessage、sendMessageAtTime、sendMessageDelayed等方法来完成。使用 post的方法,允许Runnable 对象在入队后被消息队列接收时再被调用。而 sendMessage方法能够让你入队一个Message对象,并且该消息携带了一些数据,该Message对象能够被Handler中的handleMessage方法来处理。
下面我们将从源码的角度来分析它们的运行机制
源码分析我们一般从应用的角度来入手,首先在选择使用Handler来进行线程间的通讯时,我们都要先要new 一个handler对象。
Handler实例化
在Handler中,一共有四中构造方法,如下:
Handler()Handler(Callback callback)Handler(Looper looper)Handler(Looper looper, Callback callback)
其中第1和第2构造方法相似,将handler和当前线程Looper关联起来,如果这个线程中没有Looper对象,那个这个Handler将不能接收messages,从而抛出异常。
public Handler() { this(null, false);}public Handler(Callback callback) { this(callback, false); }
内部都执行的Handler(Callback callback, boolean async)方法,区别在于是否传入一个Callback接口。
/** * 使用指定的接口回调为当前线程使用Looper,设置该Handler是否异步,默认为同步。 */public Handler(Callback callback, boolean async) { ... 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;}
这个方法中对Handler的mLooper、mQueue、mCallback、mAsynchronous等属性进行了初始化。至于具体如何初始化的,以及做了哪些操作,我们稍后在讨论。
第3和第4个构造方法相似,直接提供了一个Looper对象,这个Looper对象不能为null。
public Handler(Looper looper) { this(looper, null, false);}public Handler(Looper looper, Callback callback) { this(looper, callback, false);}
方法中都是调用Handler(Looper looper, Callback callback, boolean async)方法,区别是在与是否传入一个Callback接口
public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async;}
该方法内部同上一样,也是对Handler内部的属性进行初始化。
以上就完成了Handler的初始化,接下来就是使用sendMessage来发送一个message 或者 post一个Runnable对象。
sendMessage发送消息
我们经常使用sendMessage的方式,通知主线程更新UI。
Handler.java
/** * 将消息push到消息队列的末尾 */public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0);}
内部直接调用sendMessageDelayed(Message msg, long delayMillis),进入该方法:
public final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}
该方法中直接调用sendMessageAtTime(Message msg, long uptimeMillis),以上标注的方法也可以直接在外部调用。
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);}
执行到这步,我们才知道核心方法就是enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis),首先判断MessageQueue是否为null,我们知道mQueue来自,Handler的构造函数中的Looper.mQueue,我们继续看enqueueMessage:
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis);}
上面,通过 queue.enqueueMessage方法将Message 加入队列。
继续分析MessageQueue中的enqueueMessage方法,进入这个方法中:
MessageQueue.java
boolean enqueueMessage(Message msg, long when) {//上面已经将handler 赋值给msg.target,判断为空则抛出异常 if (msg.target == null) { throw new IllegalArgumentException("Message must have a target."); }//判断该message的状态是否处于use中 if (msg.isInUse()) { throw new IllegalStateException(msg + " This message is already in use."); } synchronized (this) { if (mQuitting) { IllegalStateException e = new IllegalStateException( msg.target + " sending message to a Handler on a dead thread"); Log.w(TAG, e.getMessage(), e); msg.recycle(); return false; } msg.markInUse(); //标注该msg,处于use中 msg.when = when; //将msg的延迟时间初始化 Message p = mMessages; boolean needWake; // p为null 或者比较当前消息的执行时间小于存储的消息,将当前消息插入队列头部 if (p == null || when == 0 || when < p.when) { // New head, wake up the event queue if blocked. msg.next = p; mMessages = msg; needWake = mBlocked; } else { // 将msg插入队列。通常我们不需要唤醒事件队列,除非队列的头部有一个障碍 // 消息是队列中最早的异步消息。 needWake = mBlocked && p.target == null && msg.isAsynchronous(); Message prev; for (;;) { prev = p; p = p.next; if (p == null || when < p.when) { break; } if (needWake && p.isAsynchronous()) { needWake = false; } } msg.next = p; // invariant: p == prev.next prev.next = msg; } // We can assume mPtr != 0 because mQuitting is false. if (needWake) { nativeWake(mPtr); } } return true;}
以上就是调用handler的sendMessage时,将message加入MessageQueue的过程。接下来,我们继续分析使用post的方式,代码执行过程。
post Runnable对象
直接使用post方式,可以看到:
public final boolean post(Runnable r) { return sendMessageDelayed(getPostMessage(r), 0);}public final boolean postAtTime(Runnable r, long uptimeMillis) { return sendMessageAtTime(getPostMessage(r), uptimeMillis);}public final boolean postDelayed(Runnable r, long delayMillis) { return sendMessageDelayed(getPostMessage(r), delayMillis);}
以上都能看到,里面同样执行的是sendMessage中相同的方法,区别是通过方法getPostMessage(r),将Runnable对象转为Message对象,如下:
private static Message getPostMessage(Runnable r) { Message m = Message.obtain(); m.callback = r; return m;}
由此可见将传入的Runnable对象,作为Message 的callback 属性。
从开始到现在,我们介绍了send Message 和 post 的方式,通过这样将Message加入到MessageQueue中来。但是我们知道,我们可以从Handler的handleMessage(Message msg)方法中,取到之前发送的消息。这样怎样做到的呢??下面我们将说明怎么从MessageQueue中取到对应的消息。
Looper的loop
我们回到前面Handler的构造方法中,知道Handler持有mLooper的对象引用,通过默认构造方法中初始化,或者接收外部传入的Looper对象,在Handler的构造方法中初始化。先来看默认的初始化是怎样的:
public Handler(Callback callback, boolean async) {.....mLooper = Looper.myLooper();.....}
直接到Looper查看该方法:
Looper.java
/** * 返回与当前线程关联的Looper对象,如果调用的线程与Looper无关,则为null * */public static @Nullable Looper myLooper() { return sThreadLocal.get();}
从ThreadLocal中取出一个Looper,这样做可以避免并发访问时,线程安全问题。
// sThreadLocal.get() will return null unless you've called prepare().static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
sThreadLocal 采用static修饰,直接取值时,返回为空,必须先调用prepare()方法。
/** 初始化当前当前线程作为一个Looper * 在实际开始loop之前,这样将会给你一个机会来创建handler来引用这个looper *调用这个方法之后,确保要调用loop()方法,调用quit()方法来结束它 。*/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));}
上面的方法都是static方法,要在类初始化之前,需要先被调用来先实例化一个Looper对象到sThreadLocal 中,这样使得在初始化Handler时,通过Looper.myLooper() 就能获得当前的Looper对象。
然后我们继续查看Looper实例化时,做了哪些操作:
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread();}
从中可以看出,进行了两个初始化,创建一个MessageQueue实例对象,获取当前的Thead对象。再对照Handler的Handler(Callback callback, boolean async)方法,得知获得Looper实例后,然后将Looper中的MessageQueue引用直接赋值给Handler的mQueue中。
以上这些只是完成了Looper调用的准备工作,那么到底在哪里执行了prepare方法呢?
这让我们想起了,以前学习Handler时,当在子线程中创建Handler时,程序直接crash,如:
new Thread(new Runnable() { @Override public void run() { Handler handler = new Handler(){ @Override public void handleMessage(Message msg) { Toast.makeText(getApplicationContext(), "handler msg", Toast.LENGTH_LONG).show(); } }; handler.sendEmptyMessage(1); }}).start();
报错提示:java.lang.RuntimeException: Can’t create handler inside thread that has not called Looper.prepare();
这时我们只要在Handler 实例化之前执行Looper.prepare();这时,便不会报错了,但是handleMessage也同样不会执行。这时,我们还需要在handler.sendEmptyMessage(1) 后,继续执行Looper.loop(); 这样程序才能正确的运行。
到这里,我们可能都会产生疑惑,为什么在主线程中不需要执行这些操作,而在子线程中必须要这样呢?
首先我们来分析下报错的原因,根据错误提示,我们到源码中查看知道,在子线程中实例化Handler对象时,里面执行了mLooper = Looper.myLooper();(上面已经分析过),在判断mLooper 为null时,才抛出这样的异常提示。而在Looper的myLooper()方法中,只是从sThreadLocal中取出Looper的对象。说明sThreadLocal中并没有Looper的对象,也就是说Looper中没有调用prepare()方法来给sThreadLocal中set一个Looper对象。到这里我们知道了为什么在子线程调用Handler构造方法之前,添加了Looper.prepare()后为什了不报错了。
现在我们回到问题的原点,为什么主线程中不需要这样做呢?
我们在Activity的源码中,可以看到类中存在一个ActivityThread的引用,在ActivityThread的main方法中,可以看到相关的Looper调用:
ActivityThread.java
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()); AndroidKeyStoreProvider.install(); // Make sure TrustedCertificateStore looks in the right place for CA certificates final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); Process.setArgV0("<pre-initialized>"); 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");}
由此可知,在启动Activity时候,通过ActivityThread调用prepareMainLooper方法,里面执行了prepare()方法。
上面完成了获取Looper对象,剩下就是将MessageQueue中的消息对象取出来,然后由Handler中的handleMessage方法接处理。完成这个操作的方法就是loop()。
Looper.java
public static void loop() { //判断当前Looer是否为空 final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } //当前Looper的MessageQueue 赋值 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(); }}
在for的无限循环中,每次都执行方法queue.next(),取出一个Message,如果为null的话,则跳出循环。否则就执行方法:msg.target.dispatchMessage(msg);从上面我们可以知道这个方法其实就是调用的Handler类中的dispatchMessage方法,进入该方法:
Handler.java
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); }}
该方法里面有几个关键的执行路径,分别对应你的实例化Handler时传入的参数和调用Handler的方法。如:
当你使用post(Runnable r)方法时,这时调用的方法就是handleCallback(msg);
当你使用sendMessage(Message msg)方法时,这时根据你实例化Handler,是否传入回调接口Callback来选择代码执行,当Callback为null时,执行handleMessage(msg),这时在你重写的handleMessage方法中,就能接收到该Message。
下面我们再分析下MessageQueue的next方法是如何将Message从队列中取出的:
MessageQueue.java
Message next() { // Return here if the message loop has already quit and been disposed. // This can happen if the application tries to restart a looper after quit // which is not supported. final long ptr = mPtr; if (ptr == 0) { return null; } int pendingIdleHandlerCount = -1; // -1 only during first iteration int nextPollTimeoutMillis = 0; for (;;) { if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } nativePollOnce(ptr, nextPollTimeoutMillis); synchronized (this) { // Try to retrieve the next message. Return if found. final long now = SystemClock.uptimeMillis(); Message prevMsg = null; Message msg = mMessages; if (msg != null && msg.target == null) { // Stalled by a barrier. Find the next asynchronous message in the queue. do { prevMsg = msg; msg = msg.next; } while (msg != null && !msg.isAsynchronous()); } if (msg != null) { if (now < msg.when) { // Next message is not ready. Set a timeout to wake up when it is ready. nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE); } else { // Got a message. mBlocked = false; if (prevMsg != null) { prevMsg.next = msg.next; } else { mMessages = msg.next; } msg.next = null; if (DEBUG) Log.v(TAG, "Returning message: " + msg); msg.markInUse(); return msg; } } else { // No more messages. nextPollTimeoutMillis = -1; } // Process the quit message now that all pending messages have been handled. if (mQuitting) { dispose(); return null; } // If first time idle, then get the number of idlers to run. // Idle handles only run if the queue is empty or if the first message // in the queue (possibly a barrier) is due to be handled in the future. if (pendingIdleHandlerCount < 0 && (mMessages == null || now < mMessages.when)) { pendingIdleHandlerCount = mIdleHandlers.size(); } if (pendingIdleHandlerCount <= 0) { // No idle handlers to run. Loop and wait some more. mBlocked = true; continue; } if (mPendingIdleHandlers == null) { mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)]; } mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers); } // Run the idle handlers. // We only ever reach this code block during the first iteration. for (int i = 0; i < pendingIdleHandlerCount; i++) { final IdleHandler idler = mPendingIdleHandlers[i]; mPendingIdleHandlers[i] = null; // release the reference to the handler boolean keep = false; try { keep = idler.queueIdle(); } catch (Throwable t) { Log.wtf(TAG, "IdleHandler threw exception", t); } if (!keep) { synchronized (this) { mIdleHandlers.remove(idler); } } } // Reset the idle handler count to 0 so we do not run them again. pendingIdleHandlerCount = 0; // While calling an idle handler, a new message could have been delivered // so go back and look again for a pending message without waiting. nextPollTimeoutMillis = 0; } }
通过循环遍历,取出等待时间最短的Message,于是选择将该Message移除队列。
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