Android消息机制总结

Android Android 评论

消息机制相关知识点

  • Android消息机制涉及相关的知识点，handler 、ThreadLocal 、looper、 MessageQueue、Message、对象池使用

    Handler

  • handler在Android里常用在子线程的数据抛给主线程使用，常见操作更新UI。但是它也能实现任意两个线程的数据传递。
  • 在子线程上创建Handler时需要Looper.prepare()和Looper.loop() , UI线程也是需要的可以从ActivityThread的main里看到，所以默认系统为主线程已经调用过
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class MyThread extends Thread {
    public Handler mHandler;
  
     public void run() {
        Looper.prepare();
       mHandler = new Handler() {
             public void handleMessage(Message msg) {
                 // process incoming messages here
             }
         };
         Looper.loop();
     }

创建Handler为什么需要 Looper.prepare()和Looper.loop()

  • Handle 实例创建的源码
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public Handler(@Nullable Callback callback, boolean async) {
   // 核心代码块
      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;
  }
  • 从代码里看到获取mLooper时会当未空，需要先进行 Looper.prepare() , 查看Looper里的方法
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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));
}
  • 可以看到prepare的过程会在 Looper里的sThreadLocal创建出来Looper的实例，并进行保存。
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private Looper(boolean quitAllowed) {
     mQueue = new MessageQueue(quitAllowed);
     mThread = Thread.currentThread();
 }

  • 创建Looper的时候可以看到引入了MessageQueue， 并在Looper实例里创建了MessageQueue的实例，用于存储Message

  • Looper prepare() 是为了创建出来Looper，并存放在ThreadLocal里，在构建Looper实例时候，也创建出来消息队列MessageQueue。而Looper.loop()则从消息队列里取出来消息，进行执行。

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/**
 * 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();

    // 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);
        }
        // Make sure the observer won't change while processing a transaction.
        final Observer observer = sObserver;

        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;
        Object token = null;
        if (observer != null) {
            token = observer.messageDispatchStarting();
        }
        long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
        try {
            msg.target.dispatchMessage(msg);
            if (observer != null) {
                observer.messageDispatched(token, msg);
            }
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } catch (Exception exception) {
            if (observer != null) {
                observer.dispatchingThrewException(token, msg, exception);
            }
            throw exception;
        } finally {
            ThreadLocalWorkSource.restore(origWorkSource);
            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();
    }
}
  • Message在MessageQueue的存储是通过Message.next 来存放的，类似单链表的存储结构。在取出的时候通过 MessageQueue.next()方法取出某个message, message之间也是通过next属性，形成链表存储在MessageQueue里。

消息机制总结

  • Handler可以在主线程创建也可以在子线程创建，主线程创建时已在系统启动时（ActivityThread的main方法里），调用过Looper.prepare和Loop，所以创建完直接使用。但是子线程创建Handler需要先prepare()，创建出来Looper对象，以及消息队列，之后进行Loop()运行起来，从消息队列死循环取消息出来，并进行分发出去进行执行，即到handler里进行dispatchMessage
  • 一个线程可以有多个handler，但是只能有一个Looper和一个MessageQueue， 每个线程对应一个Looper，每个线程的Looper通过ThreadLocal来存储保证，Looper对象的内部又维护有唯一的一个MessageQueue
  • Handler提供创建消息的方法obtainMessage和 sendMessage的方法，通过Handler持有的mQueue（消息队列引用），放入消息到队列（MessageQueue）。 mQueue = mLooper.mQueue;

Handler的消息发生

  • Handler的obtainMessage方法通过对象复用方式，减少对象的创建, 
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/**
 * Return a new Message instance from the global pool. Allows us to
 * avoid allocating new objects in many cases.
 */
public static Message obtain() {
    synchronized (sPoolSync) {
        if (sPool != null) {
            Message m = sPool;
            sPool = m.next;
            m.next = null;
            m.flags = 0; // clear in-use flag
            sPoolSize--;
            return m;
        }
    }
    return new Message();
}

  • 在Message中有一个static Message变量sPool，这个变量是用于缓存Message对象的, 当sPool不为空就取出, 相应个数减一，并通过next 设定下一个对象，重新赋值到sPool

  • sPool中缓存的Message是哪里来回收来的

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/**
    * Return a Message instance to the global pool.
    * <p>
    * You MUST NOT touch the Message after calling this function because it has
    * effectively been freed.  It is an error to recycle a message that is currently
    * enqueued or that is in the process of being delivered to a Handler.
    * </p>
    */
   public void recycle() {
       if (isInUse()) {
           if (gCheckRecycle) {
               throw new IllegalStateException("This message cannot be recycled because it "
                       + "is still in use.");
           }
           return;
       }
       recycleUnchecked();
   }

   /**
    * Recycles a Message that may be in-use.
    * Used internally by the MessageQueue and Looper when disposing of queued Messages.
    */
   @UnsupportedAppUsage
   void recycleUnchecked() {
       // Mark the message as in use while it remains in the recycled object pool.
       // Clear out all other details.
       flags = FLAG_IN_USE;
       what = 0;
       arg1 = 0;
       arg2 = 0;
       obj = null;
       replyTo = null;
       sendingUid = UID_NONE;
       workSourceUid = UID_NONE;
       when = 0;
       target = null;
       callback = null;
       data = null;

       synchronized (sPoolSync) {
           if (sPoolSize < MAX_POOL_SIZE) {
               next = sPool;
               sPool = this;
               sPoolSize++;
           }
       }
   }
  • 使用obtain获取Message对象是因为Message内部维护了一个数据缓存池，回收的Message不会被立马销毁，而是放入了缓存池，在获取Message时会先从缓存池中去获取，缓存池为null才会去创建新的Message。

HandlerThread

  • Handler可以在主线程上创建也可以在子线程上创建，HandlerThread 继承自Thread ,本质是一个Thread , 在run方法里创建了Looper和MessageQueue对象，并开启了Looper轮询消息。

  • 通过获取HandlerThread的looper对象传递给主线程的Handler对象（构造handler时传入），然后在handleMessage()方法中执行异步任务。Handler虽然是在住线程创建，但是它的handleMessage接收到消息是在HandlerThread线程，达到收到消息，执行异步任务的操作。与以往常用handleMessage里主线程操作不同，因为传入的looper是HandlerThread里构造的，是一个子线程。

  • HandlerThread相当于在子线程上创建的Handler，android做了层封装为提供了现成的使用。可以在handleMessage里处理异步任务

  • 模板用法：

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//步骤1：创建HandlerThread的实例对象=已经创建了一个新线程
//参数=线程名字，作用是标记该线程
HandlerThread mHandlerThread = new HandlerThread("handlerThread");

//步骤2：启动线程
mHandlerThread.start();

//步骤3：创建工作线程Handler，传入 handlerThread.getLooper() , 实现消息处理的操作，并与其他线程进行通信
Handler mHandler = new Handler( handlerThread.getLooper() ) {
            @Override
            public boolean handleMessage(Message msg) {
              //运行HandlerThread子线程，用于实现自己的消息处理
                return true;
            }
        });

//步骤4：结束线程，即停止线程的消息循环
mHandlerThread.quit();
  • 使用场景：
  • 存在多个耗时的任务需要放到开启子线程依次去处理（串行处理任务）
  • HandlerThread是一个子线程，适合处理耗时的任务，其次，Handler分发消息是通过MessageQueue顶部的Message不断的通过Message的next依次取出Message，符合任务的按顺序串行处理的要求，所以使用HandlerThread就能完美的解决
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