Android的消息循环——线程间通信
Android的线程可分为UI线程和工作线程,UI线程默认开启消息循环,但是创建的工作线程默认是没有消息循环和消息队列的,如果想让该 线程具有消息队列和消息循环,需要在线程中首先调用Looper.prepare()来创建消息队列,然后调用Looper.loop()进入消息循环。 如下例所示:
class LooperThread extends Thread { public Handler mHandler; public void run() { Looper.prepare(); mHandler = new Handler() { public void handleMessage(Message msg) { // process incoming messages here } }; Looper.loop(); } }
Android的Looper和Handler
Looper:每一个线程都可以产生一个Looper,用来管理线程的Message,Looper对象会建立一个MessgaeQueue数据结构来存放message。
Handler:与Looper沟通的对象,可以push消息或者runnable对象到MessgaeQueue,也可以从MessageQueue得到消息。
Activity是一个UI线程,运行于主线程中,Android系统在启动的时候会为Activity创建一个消息队列和消息循环(Looper)。详细实现请参考ActivityThread.java文件
Android应用程序进程在启动的时候,会在进程中加载ActivityThread类,并且执行这个类的main函数,应用程序的消息循环过程就是在这个main函数里面实现的
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"); }
贴一篇别人分析的源码和自己的理解
//Looper类分析 public class Looper { //static变量,判断是否打印调试信息。 private static final boolean DEBUG = false; private static final boolean localLOGV = DEBUG ? Config.LOGD : Config.LOGV; // sThreadLocal.get() will return null unless you've called prepare(). //线程本地存储功能的封装,TLS,thread local storage,什么意思呢?因为存储要么在栈上,例如函数内定义的内部变量。要么在堆上,例如new或者malloc出来的东西 //但是现在的系统比如Linux和windows都提供了线程本地存储空间,也就是这个存储空间是和线程相关的,一个线程内有一个内部存储空间,这样的话我把线程相关的东西就存储到 //这个线程的TLS中,就不用放在堆上而进行同步操作了。 //本人理解:线程局部变量,内部是类map存储,key为它本身,value是当前线程相关的变量副本 private static final ThreadLocal sThreadLocal = new ThreadLocal(); //消息队列,MessageQueue,看名字就知道是个queue.. final MessageQueue mQueue; volatile boolean mRun; //和本looper相关的那个线程,初始化为null Thread mThread; private Printer mLogging = null; //static变量,代表一个UI Process(也可能是service吧,这里默认就是UI)的主线程 private static Looper mMainLooper = null; /** 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()}. */ //往TLS中设上这个Looper对象的,如果这个线程已经设过了looper的话就会报错 //这说明,一个线程只能设一个looper public static final void prepare() { if (sThreadLocal.get() != null) { throw new RuntimeException("Only one Looper may be created per thread"); } sThreadLocal.set(new 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. * {@link #prepare()} */ //由framework设置的UI程序的主消息循环,注意,这个主消息循环是不会主动退出的 // public static final void prepareMainLooper() { prepare(); setMainLooper(myLooper()); //判断主消息循环是否能退出.... //通过quit函数向looper发出退出申请 if (Process.supportsProcesses()) { myLooper().mQueue.mQuitAllowed = false; } } private synchronized static void setMainLooper(Looper looper) { mMainLooper = looper; } /** Returns the application's main looper, which lives in the main thread of the application. */ public synchronized static final Looper getMainLooper() { return mMainLooper; } /** * Run the message queue in this thread. Be sure to call * {@link #quit()} to end the loop. */ //消息循环,整个程序就在这里while了。 //这个是static函数喔! public static final void loop() { Looper me = myLooper();//从该线程中取出对应的looper对象 MessageQueue queue = me.mQueue;//取消息队列对象... while (true) { Message msg = queue.next(); // might block取消息队列中的一个待处理消息.. //if (!me.mRun) {//是否需要退出?mRun是个volatile变量,跨线程同步的,应该是有地方设置它。 // break; //} if (msg != null) { if (msg.target == null) { // No target is a magic identifier for the quit message. return; } if (me.mLogging!= null) me.mLogging.println( ">>>>> Dispatching to " + msg.target + " " + msg.callback + ": " + msg.what ); //msg.target目标handler msg.target.dispatchMessage(msg); if (me.mLogging!= null) me.mLogging.println( "<<<<< Finished to " + msg.target + " " + msg.callback); msg.recycle(); } } } /** * Return the Looper object associated with the current thread. Returns * null if the calling thread is not associated with a Looper. */ //返回和线程相关的looper public static final Looper myLooper() { return (Looper)sThreadLocal.get(); } /** * Control logging of messages as they are processed by this Looper. If * enabled, a log message will be written to <var>printer</var> * at the beginning and ending of each message dispatch, identifying the * target Handler and message contents. * * @param printer A Printer object that will receive log messages, or * null to disable message logging. */ //设置调试输出对象,looper循环的时候会打印相关信息,用来调试用最好了。 public void setMessageLogging(Printer printer) { mLogging = printer; } /** * Return the {@link MessageQueue} object associated with the current * thread. This must be called from a thread running a Looper, or a * NullPointerException will be thrown. */ public static final MessageQueue myQueue() { return myLooper().mQueue; } //创建一个新的looper对象, //内部分配一个消息队列,设置mRun为true private Looper() { mQueue = new MessageQueue(); mRun = true; mThread = Thread.currentThread(); } public void quit() { Message msg = Message.obtain(); // NOTE: By enqueueing directly into the message queue, the // message is left with a null target. This is how we know it is // a quit message. mQueue.enqueueMessage(msg, 0); } /** * Return the Thread associated with this Looper. */ public Thread getThread() { return mThread; } //后面就简单了,打印,异常定义等。 public void dump(Printer pw, String prefix) { pw.println(prefix + this); pw.println(prefix + "mRun=" + mRun); pw.println(prefix + "mThread=" + mThread); pw.println(prefix + "mQueue=" + ((mQueue != null) ? mQueue : "(null")); if (mQueue != null) { synchronized (mQueue) { Message msg = mQueue.mMessages; int n = 0; while (msg != null) { pw.println(prefix + " Message " + n + ": " + msg); n++; msg = msg.next; } pw.println(prefix + "(Total messages: " + n + ")"); } } } public String toString() { return "Looper{" + Integer.toHexString(System.identityHashCode(this)) + "}"; } static class HandlerException extends Exception { HandlerException(Message message, Throwable cause) { super(createMessage(cause), cause); } static String createMessage(Throwable cause) { String causeMsg = cause.getMessage(); if (causeMsg == null) { causeMsg = cause.toString(); } return causeMsg; } } }
class Handler{ .......... //handler默认构造函数 public Handler() { //这个if是干嘛用的暂时还不明白,涉及到java的深层次的内容了应该 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()); } } //获取本线程的looper对象 //如果本线程还没有设置looper,这回抛异常 mLooper = Looper.myLooper(); if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.prepare()"); } //无耻啊,直接把looper的queue和自己的queue搞成一个了 //这样的话,我通过handler的封装机制加消息的话,就相当于直接加到了looper的消息队列中去了 mQueue = mLooper.mQueue; mCallback = null; } //还有好几种构造函数,一个是带callback的,一个是带looper的 //由外部设置looper public Handler(Looper looper) { mLooper = looper; mQueue = looper.mQueue; mCallback = null; } // 带callback的,一个handler可以设置一个callback。如果有callback的话, //凡是发到通过这个handler发送的消息,都有callback处理,相当于一个总的集中处理 //待会看dispatchMessage的时候再分析 public Handler(Looper looper, Callback callback) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; } // //通过handler发送消息 //调用了内部的一个sendMessageDelayed public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } //FT,又封装了一层,这回是调用sendMessageAtTime了 //因为延时时间是基于当前调用时间的,所以需要获得绝对时间传递给sendMessageAtTime 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) { boolean sent = false; MessageQueue queue = mQueue; if (queue != null) { //把消息的target设置为自己,然后加入到消息队列中 //对于队列这种数据结构来说,操作比较简单了 msg.target = this; sent = queue.enqueueMessage(msg, uptimeMillis); } else { RuntimeException e = new RuntimeException( this + " sendMessageAtTime() called with no mQueue"); Log.w("Looper", e.getMessage(), e); } return sent; } //还记得looper中的那个消息循环处理吗 //从消息队列中得到一个消息后,会调用它的target的dispatchMesage函数 //message的target已经设置为handler了,所以 //最后会转到handler的msg处理上来 //这里有个处理流程的问题 public void dispatchMessage(Message msg) { //如果msg本身设置了callback,则直接交给这个callback处理了 if (msg.callback != null) { handleCallback(msg); } else { //如果该handler的callback有的话,则交给这个callback处理了---相当于集中处理 if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } //否则交给派生处理,基类默认处理是什么都不干 handleMessage(msg); } } .......... }
总结:android线程间通信可通过handler来解决,工作线程向UI线程发送消息,handler.sendMessage(Message msg)或者post(Runnable obj)一个runnable对象,UI线程向工作线程发送消息,首先要手动开启工作线程的消息循环,handler必须在工作线程的run方法中创建,然后由UI线程发送消息或runnable对象到工作线程。
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