Android-Handler

概述

• Handler是Android比较基础，也是非常重要的一个组成部分；整个App运行就是基于消息驱动运行起来的，也可以理解为消费者-生产者模式；
• 原理及实现是相对比较简单的，主要涉及四个类：Message，Handler，MessageQueue，Looper；Message是消息的载体；Handler是消息的发送者及处理者；MessageQueue是消息队列，核心操作是入列和出列；Looper从MessageQueue获取Message并进行分发；
• 消息的执行是异步的；Message分为同步Message和异步Message，可以向MessageQueue中发送同步栅栏，阻止同步Message执行，但是异步Message不受影响；
• Handler可以用于异步/延迟/定时执行，也可用于线程间通信；
• 源码基于Android-SDK-29；

源码

Message

• Message是消息的载体，生命周期从发送（获取Message对象）到处理（Handler处理结束）；
• Message是单向链式结构；
• Message

  public final class Message implements Parcelable {
    public int what;
    Handler target;
    public long when;
    Runnable callback;
    public int arg1;
    public int arg2;
    public Object obj;
    Bundle data;
  }
  

  • what：用来标识什么消息，作用域为对应的Handler，只要Handler能区分所有的what即可；
  • target：Message的发送者和处理者；
  • when：Message执行的时间，自Android系统启动以来的时间（不包括系统睡眠时间）为起点；Message不一定正好在when时间执行（可能前面积压了很多Message），但是肯定不会早于when时间；
  • callback：callback也是Message的处理者；
  • arg1/arg2/obj：可携带的数据；是data的另一种替换方案，不涉及内存分配；
  • data：复杂数据或者跨进程携带数据；
• 对象池

  @UnsupportedAppUsage
    /*package*/ Message next; //链式结构
    /** @hide */
    public static final Object sPoolSync = new Object();
    private static Message sPool;
    private static int sPoolSize = 0;
  
    private static final int MAX_POOL_SIZE = 50;
  

  • Message是生命周期很短（从发送到处理完毕）的对象，并且很多地方都会用到；大量创建生命周期短的对象这种场景下，创建对象本身需要比较耗时，也可能触发GC，导致系统卡顿不稳定；所以需要对象池；
  • Message是单向链式结构，对象池只要持有一个Message对象即可；
  • 获取Message对象尽量通过Message.obtain/Handler.obtain方法；源码中会对sPool加锁，防止并发调用；
• 跨进程

  /**
     * Optional Messenger where replies to this message can be sent.  The
     * semantics of exactly how this is used are up to the sender and
     * receiver.
     */
    public Messenger replyTo;
    /**
     * Indicates that the uid is not set;
     *
     * @hide Only for use within the system server.
     */
    public static final int UID_NONE = -1;
    /**
     * Optional field indicating the uid that sent the message.  This is
     * only valid for messages posted by a {@link Messenger}; otherwise,
     * it will be -1.
     */
    public int sendingUid = UID_NONE;
    /**
     * Optional field indicating the uid that caused this message to be enqueued.
     *
     * @hide Only for use within the system server.
     */
    public int workSourceUid = UID_NONE;
  

  • 跨进程这块后面再补；

Handler

• Handler是Message的发送者和处理者；
• 每个Handler对象都绑定到Looper对象，Handler发送Message最终发送到绑定的Looper对象的MessageQueue中；如果创建Handler时未指定了Looper对象，那么就绑定到创建Handler对象时所处的线程的Looper对象；
• 创建Handler对象

  public Handler(@Nullable Callback callback, boolean async) {
        //提醒可能内存泄漏
        if (FIND_POTENTIAL_LEAKS) {
            final Class 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());
            }
        }
        //获取当前线程的Looper
        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;
    }
  
  public Handler(@NonNull Looper looper, @Nullable Callback callback, boolean async) {
        mLooper = looper;
        mQueue = looper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }
  

  • 如果构造函数没有指定Looper，那么获取当前线程的Looper对象；构造函数就是为了生成Looper，MessageQueue，Callback（Handler的Callback，不是Message的Callback，类似于Handler.handleMessage方法），Asynchronous（为true时，该Handler发送的都是异步Message）；
• 发送消息

  public boolean sendMessageAtTime(@NonNull 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(@NonNull MessageQueue queue, @NonNull Message msg,
            long uptimeMillis) {
        msg.target = this;
        msg.workSourceUid = ThreadLocalWorkSource.getUid();
  
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }
  

  • 发送Message和Runnable，最终都会调用sendMessageAtTime；Runnable会被封装为Message对象；sendMessage，sendMessageDelayed通过时间转化最终都会转化为sendMessageAtTime；
  • Message入列是直接调用MessageQueue.enqueueMessage方法，如果Handler.mAsynchronous为true，则该Handler所有的Message都是异步Message；

  public final boolean runWithScissors(@NonNull Runnable r, long timeout) {
        if (Looper.myLooper() == mLooper) {
            调用线程和执行线程是同一个线程时，直接run
            r.run();
            return true;
        }
  
        BlockingRunnable br = new BlockingRunnable(r);
        return br.postAndWait(this, timeout);
    }
  

  • 阻塞发送消息：如果Handler绑定的Looper和调用者所在的线程Looper是同一个，直接调用Runnable.run；否则调用者所在的线程阻塞，直到执行结束；
  • 发送立即处理消息：postAtFrontOfQueue/sendMessageAtFrontOfQueue方法，设置Message.when为0，Message会排在MessageQueue的最前面，Looper取出的下一个Message就是该Message，所以是立即处理消息；
• 删除消息
  • Handler的删除消息都会调用MessageQueue对应的方法；
• 处理消息

  public void dispatchMessage(@NonNull Message msg) {
        if (msg.callback != null) {
            handleCallback(msg); // Runnable转化的Message
        } else {
            if (mCallback != null) {
                //Handler.Callback优先处理Message，如果返回true，表示处理结束
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
  
  private static void handleCallback(Message message) {
        message.callback.run();
    }
  
  public void handleMessage(@NonNull Message msg) {
    }
  

  • 如果Message的callback不为null（Runnable转化的Message），直接调用callback处理；否则，优先Handler.Callback处理，如果返回true，表示处理结束；否则，handleMessage处理；

Looper

• Looper从MessageQueue获取Message并进行分发；主要功能就是消息循环以及监控；
• Looper是保存在ThreadLocal中，MessageQueue是Looper的实例变量，所以MessageQueue对象也是线程内唯一；
• 线程本地存储

    static final ThreadLocal sThreadLocal = new ThreadLocal();
  
    final MessageQueue mQueue;
    final Thread mThread;
  

  • Looper保存在ThreadLocal中，确保线程内单例；MessageQueue是Looper的成员变量，外部不可创建，所以MessageQueue也是线程内单例；
• 初始化

  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();
    }
  

  • Looper构造函数中，初始化了MessageQueue，并将Looper保存在ThreadLocal中；
  • 初始化必须在对应的线程中调用；
  • 默认是可退出的（Looper/MessageQueue）；
• 循环

  public static void loop() {
        final Looper me = myLooper();
        final MessageQueue queue = me.mQueue;
  
        // 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;
        //死循环
        for (;;) {
            Message msg = queue.next(); // 可阻塞
            if (msg == null) { //MessageQueue已退出
                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);
            }
            msg.recycleUnchecked(); //Message回收
        }
    }
  

  • 主要代码就是从MessageQueue中取出要执行的下一个Message（可能阻塞），然后调用Handler.dispatchMessage进行分发；最后回收Message对象；
• 监控
  • 在loop方法中，分发Message前后，根根据设置的阈值/Printer/全局Observer，执行对应的Log以及回调；
  • 主要用于系统监控，业务层只能设置Printer，但是返回的只有一个拼接好的String，用法有限；

MessageQueue

• MessageQueue是消息队列，负责Message入列，出列，空闲时间IdleHandler的处理，以及线程的挂起；
• 初始化

  MessageQueue(boolean quitAllowed) {
        mQuitAllowed = quitAllowed;
        mPtr = nativeInit();
    }
  
  private native static long nativeInit();
  

  • 初始化Looper对象时，也会初始化MessageQueue，进而在Native层也初始化一个NativeMessageQueue，并在Java层保存起来（mPtr）；
  • Native层也有消息队列，和Java层类似，MessageQueue是Native和Java层消息队列的纽带；
• 入列

  boolean enqueueMessage(Message msg, long when) {
  
        synchronized (this) { //加锁
            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();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            if (p == null || when == 0 || when < p.when) {
                // 插入到头部
                msg.next = p;
                mMessages = msg;
                needWake = mBlocked;
            } else {
                // Inserted within the middle of the queue.  Usually we don't have to wake
                // up the event queue unless there is a barrier at the head of the queue
                // and the message is the earliest asynchronous message in the queue.
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                //根据when插入到合适位置
                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) { //唤醒
                nativeWake(mPtr);
            }
        }
        return true;
    }
  

  • 可能并发，所以在MessageQueue对象上加锁；
  • 根据Message.when插入到合适位置；
  • 如果需要唤醒，则从Native唤醒；
• 出列

  Message next() {
        final long ptr = mPtr;
        if (ptr == 0) { //消息队列已经退出
            return null;
        }
  
        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        int nextPollTimeoutMillis = 0;
        for (;;) {
  
            nativePollOnce(ptr, nextPollTimeoutMillis);
  
            synchronized (this) { //加锁
                // 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) {
                    //同步栅栏，获取下一个异步Message
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                if (msg != null) {
                    if (now < msg.when) {
                        //头部Message还未到执行时间
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        //成功获取到要分发的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 (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;
        }
    }
  

  • 整体是个死循环，循环获取Message（Java层）；
  • 调用nativePollOnce，每个迭代优先处理Native层，Native消息处理完，判断nextPollTimeoutMillis，如果等于0直接返回，如果等于-1进入休眠，如果大于0进入超时休眠；
  • 处理Java层，如果头部Message执行时间已到（包括同步栅栏逻辑）则直接返回Message，否则处理IdleHandler，进入下个迭代；
• 删除
  • 从头部开始迭代，删除符合参数的Message；
• IdleHandler

  public void addIdleHandler(@NonNull IdleHandler handler) {
        if (handler == null) {
            throw new NullPointerException("Can't add a null IdleHandler");
        }
        synchronized (this) {
            mIdleHandlers.add(handler);
        }
    }
  
  public void removeIdleHandler(@NonNull IdleHandler handler) {
        synchronized (this) {
            mIdleHandlers.remove(handler);
        }
    }
  
  public boolean isIdle() {
        synchronized (this) {
            final long now = SystemClock.uptimeMillis();
            return mMessages == null || now < mMessages.when;
        }
    }
  

  • IdleHandler用于在消息队列空闲的时候，执行任务；执行逻辑在next方法中
  • IdleHandler.queueIdle：返回true表示保留任务，返回false表示删除任务只执行一次；
• 同步栅栏

  private int postSyncBarrier(long when) {
        // Enqueue a new sync barrier token.
        // We don't need to wake the queue because the purpose of a barrier is to stall it.
        synchronized (this) {
            final int token = mNextBarrierToken++;
            //同步栅栏Message（target = null）
            final Message msg = Message.obtain();
            msg.markInUse();
            msg.when = when;
            msg.arg1 = token;
            //插入同步栅栏Message
            Message prev = null;
            Message p = mMessages;
            if (when != 0) {
                while (p != null && p.when <= when) {
                    prev = p;
                    p = p.next;
                }
            }
            if (prev != null) { // invariant: p == prev.next
                msg.next = p;
                prev.next = msg;
            } else {
                msg.next = p;
                mMessages = msg;
            }
            //用于删除同步栅栏
            return token;
        }
    }
  
  public void removeSyncBarrier(int token) {
        // Remove a sync barrier token from the queue.
        // If the queue is no longer stalled by a barrier then wake it.
        synchronized (this) {
            Message prev = null;
            Message p = mMessages;
            //定义同步栅栏Message
            while (p != null && (p.target != null || p.arg1 != token)) {
                prev = p;
                p = p.next;
            }
            if (p == null) {
                throw new IllegalStateException("The specified message queue synchronization "
                        + " barrier token has not been posted or has already been removed.");
            }
            final boolean needWake;
            //删除同步栅栏Message
            if (prev != null) {
                prev.next = p.next;
                needWake = false;
            } else {
                mMessages = p.next;
                needWake = mMessages == null || mMessages.target != null;
            }
            p.recycleUnchecked();
  
            // If the loop is quitting then it is already awake.
            // We can assume mPtr != 0 when mQuitting is false.
            //唤醒
            if (needWake && !mQuitting) {
                nativeWake(mPtr);
            }
        }
    }
  

  • 同步栅栏Message，用于拖延同步Message；代码逻辑在next方法中；
  • 添加同步栅栏，将同步栅栏Message插入到合适的位置，该位置之后的Message都被拖延，只有异步消息可以执行，直到该同步栅栏消息被移除；
  • 删除同步栅栏，根据token删除对应的同步栅栏，如果需要，唤醒线程；

扩展

IdleHandler

• IdleHandler用于在线程空闲时（没有Message要执行时）执行任务；
• IdleHandler的应用场景
  • UI更新后，获取对应的属性，通过IdleHandler在空闲时执行，空闲意味着表示UI测量/布局/绘制已经结束；
  • Activity中初始化以及加载数据可以在IdleHandler中执行；
  • 用于App启动时间优化；
  • 预加载/预处理；

HandlerThread

@Override
  public void run() {
      mTid = Process.myTid();
      Looper.prepare();
      synchronized (this) {
          mLooper = Looper.myLooper();
          notifyAll();
      }
      Process.setThreadPriority(mPriority);
      onLooperPrepared();
      Looper.loop();
      mTid = -1;
  }

• HandlerThread继承于Thread，但是多了消息队列；
• 在 run 方法中调用 Looper.prepare() 初始化消息队列，然后进入循环队列；

Messager

• 跨进程通信，待补充；

ANR

• 待补充

总结

• Message是消息的载体（包括Runnable）；Handler是消息的发送者（MessageQueue入列）和处理者；Looper负责循环分发消息（MessageQueue出列）；MessageQueue负责消息的入列，出列，同步栅栏的处理，IdleHandler的处理；
• MessageQueue是Java层和Native的纽带；Java层和Native层都有消息队列，优先处理Native层消息，再处理Java层消息，如果Java层没有可执行Message，则Native层进入休眠（超时休眠或者一直休眠）；入列或者删除栅栏可唤醒休眠；

存疑

• Messager跨进程通信；
• Native与Java层的消息队列的交互；