详解Android,Handler机制和Looper,Handler,Message关系

tangjin

作者

概述

我们就从以下六个问题来探讨Handler 机制和Looper、Handler、Message之前的关系?

1.一个线程有几个Handler?

详解Android,Handler机制和Looper,Handler,Message关系

2.一个线程有几个Looper?如何保证?

3.Handler内存泄漏原因?为什么其他的内部类没有说过这个问题?

4.为何主线程可以new Handler?如果在想要在子线程中new Handler 要做些什么准备?

5.子线程中维护的Looper,消息队列无消息的时候的处理方案是什么?有什么用?

6.Looper死循环为什么不会导致应用卡死?

一、源码解析

1.Looper

对于Looper主要是prepare()和loop()两个方法

首先看prepare()方法

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)); }

可以看出sThreadLocal是一个ThreadLocal对象,ThreadLocal 并不是线程,而是一个线程内部的存储类,可以在线程内存储数据.在第5行可以看到,将一个Looper实例放入了

ThreadLocal,并且在第2~4行判断了sThreadLocal是否为空,否则抛出异常.这也Looper.prepare()方法不能被调用两次.这也对应了上面的第二个问题.

下面来看Looper的构造方法:

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

在Looper的构造方法中创建了一个MessageQueue(消息队列)

然后我们在看loop()方法:

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(); // Allow overriding a threshold with a system prop. e.g. // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start' final int thresholdOverride = SystemProperties.getInt("log.looper." + Process.myUid() + "." + Thread.currentThread().getName() + ".slow", 0); boolean slowDeliveryDetected = false; 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 final Printer logging = me.mLogging; if (logging != null) { logging.println(">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what); } final long traceTag = me.mTraceTag; long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs; long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs; if (thresholdOverride > 0) { slowDispatchThresholdMs = thresholdOverride; slowDeliveryThresholdMs = thresholdOverride; } final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0); final boolean logSlowDispatch = (slowDispatchThresholdMs > 0); final boolean needStartTime = logSlowDelivery || logSlowDispatch; final boolean needEndTime = logSlowDispatch; if (traceTag != 0 && Trace.isTagEnabled(traceTag)) { Trace.traceBegin(traceTag, msg.target.getTraceName(msg)); } final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0; final long dispatchEnd; try { msg.target.dispatchMessage(msg); dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0; } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } if (logSlowDelivery) { if (slowDeliveryDetected) { if ((dispatchStart - msg.when) <= 10) { Slog.w(TAG, "Drained"); slowDeliveryDetected = false; } } else { if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery", msg)) { // Once we write a slow delivery log, suppress until the queue drains. slowDeliveryDetected = true; } } } if (logSlowDispatch) { showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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(); } }

第2行:final Looper me = myLooper();

public static @Nullable Looper myLooper() { return sThreadLocal.get(); }

第6行:拿到改Looper实例中的mQueue(消息队列)

第23~98行:进入了一个死循环,

第24行:Message msg = queue.next(); next()方法里会一直去取消息,然后会加锁,就会一直堵塞进程,这也就是我们经常说的Looper死循环为什么不会导致死机.在这next()源码就不粘贴了,后面会说这个为什么不会死机的问题.

第57行: 调用msg.target.dispatchMessage(msg); 把消息交给msg的target的dispatchMessage()方法去处理.msg的target是什么呢?其实就是handler对象,下面会分析.

第97行:释放消息占用的资源

Looper的主要作用:

与当前线程绑定,保证一个线程只会有一个Looper实例,同时一个Looper实例也是只有一个MessageQueue.

loop()方法,不断从MessageQueue中去取消息,交给消息的target属性的dispatchMessage()去处理.

2.Handler

使用Handler之前,我们都是初始化一个实例,比如用于更新UI线程,我们会在声明的时候直接初始化,或者在onCreate中初始化Handler实例.所以我们首先看Handler的构造方法,

看其如何与MessageQueue联系上的,它的子线程中发送的消息(一般发送的消息都是在非UI线程)怎么发送到MessageQueue中的.

public Handler(Callback callback) { this(callback, false); } 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 " + Thread.currentThread() + " that has not called Looper.prepare()"); } mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async; }

第15行:通过Looper.myLooper()获取了当前线程保存的Looper实例,然后在19行又获取了这个Looper实例中保存的MessageQueue(消息队列)

这样就保证了handler的实例与我们Looper实例中MessageQueue关联上了,

然后我们再看最常用的sendMessage方法:

public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 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); }

看到最后我们发现最后调用了sendMessageAtTime,在此方法内部有直接获取MessageQueue然后调用了enqueueMessage方法,我们再来看此方法:

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }

enqueueMessage中首先为meg.target赋值为this, 在Looper的loop()方法会取出每个msg然后交给msg,target.dispatchMessage(msg)去处理消息,也就是把当前的Handler作为

msg的target属性,最终会调用queue的enqueueMessage的方法,也就是说Handler发出饿消息,最终会保存到消息队列中去.

现在已经很清楚了:Looper会调用Prepare()和loop()方法,在当前执行的线程中保存一个Looper实例,这个实例会保存一个MessageQueue对象,然后在当前的线程进入一个

无限循环中去,不断地从MessageQueue中读取Handler发来的消息.然后在回调创建这个消息的handler的dispatchMessage()方法.下面看一下dispathMessage方法:

public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } }

第10行: 调用了handleMessage()方法,下面我们看这个方法:

/** * Subclasses must implement this to receive messages. */ public void handleMessage(Message msg) { }

可以看到这个是一个空方法,为什么呢?因为消息的最终回调是由我们控制的,我们在创建handler的时候都是重写handleMessage方法,然后根据msg.what进行消息处理的

例如:

private Handler mHandler = new Handler() { public void handleMessage(android.os.Message msg) { switch (msg.what) { case value: break; default: break; } }; };

整个流程已经说完了,总结一哈:

1.首先Looper,prepare()方法在本线程中保存了一个Looper实例,然后该实例中保存一个MessageQueue对象;因为Looper.prepare()在一个线程中只能调用一次,

所以MessageQueue在一个线程中只会存在一个.

2.Looper.loop()会让当前的线程进入一个无限循环,不断地从MessageQueue的实例中读取消息,然后回调,msg.target.dispatchMessage(msg)方法.

3.Handler的构造方法,会首先得到当前线程中保存的Looper实例,进而与Looper实例的MessageQueue相关联.

4.Handler的sendMessage()方法,会给msg的target赋值为handler自身,然后加入MessageQueue中.

5.在构造Handler实例时,我们会重写handlerMessage方法.也就是msg.target,dispatchMessage(msg)最终调用的方法.

回过头来来看我们的之前的六个问题:

二、分析问题

1.一个线程有几个Handler?

我相信大家应该都使用过Handler,所以这个问题的答案:多个

这个问题没有什么好分析的,大家也亲身使用过!

2.一个线程有几个Looper?如何保证?

一个线程能有多个Handler,那么会产生多少个Looper呢? 答案: 1个

为什么?如何保证呢?

在源码分析中,可以看到sTheadLocal会实例一个Looper,如果在同一个线程中再次调用Looper.prepare方法,会抛出异常:Only one Looper may be created per thread

说明了同一个线程只能实例Looper对象.

3.Handler内存泄漏原因?

为什么其他的内部类没有说过这个问题?

Handler内存泄漏原因? 答案: 内部类引用外部类方法

private Handler mHandler =new Handler(){ @Override public void handleMessage(Message msg) { super.handleMessage(msg); switch (msg.what){ case 0: setLog(); break; default: break; } } }; private void setLog() { Log.d(TAG,"This is Log!"); } @Override public void onClick(View v) { switch (v.getId()){ case R.id.create_xml: Log.d(TAG,"create_xml"); mHandler.sendMessageDelayed(0,1000*60); break; default: break; }

创建一个匿名内部类Handler, 这时候我发延迟sendMessageDelayed()执行setLog()方法,但这个时候我如果强行关闭Activity,这个时候Activity会被销毁,但是这个Handler得不到

释放,因为还要延迟一分钟才能执行setLog()方法,这个时候就会造成内存泄漏.

其他的内部类为什么不会?

很简单,比如ListView的ViewHolder这个常用的匿名内部类,如果当主Activity销毁,这个时候ViewHolder内部类,也是直接被销毁的!所以不会出现内存泄漏问题!

4.为何主线程可以new Handler?

如果在想要在子线程中new Handler 要做些什么准备?

由前面的讲解，可以看出new Handler的条件是需要一个Looper对象，而Looper对象需要调用两个方法prepare()和loop()方法，大家可以看下面主线程的Main方法

public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); // Install selective syscall interception AndroidOs.install(); // 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(); // 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(); // Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line. // It will be in the format "seq=114" long startSeq = 0; if (args != null) { for (int i = args.length - 1; i >= 0; --i) { if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) { startSeq = Long.parseLong( args[i].substring(PROC_START_SEQ_IDENT.length())); } } } ActivityThread thread = new ActivityThread(); thread.attach(false, startSeq); 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"); }

这个Main方法，是所有的程序启动之前，都要走的这个main方法

第20行：调用了一个Looper.prepareMainLooper();

第47行：调用了一个Looper.loop()；

而Looper.prepareMainLooper()源码：

public static void prepareMainLooper() { prepare(false); synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } }

第2行：可以看到调用了Looper里的prepare()方法；

所以说可以在一个主线程中直接new Handler

那如果在一个子线程new Handler的话，需要做什么准备？

当然是要需要：调用一个Looper.prepar()和Looper.loop()方法了。