Handler是什么

Handler是Android 基于事件驱动的线程之间消息传递处理机制。

独立分配虚拟机的好处

好处在于当自身app发生崩溃时不会影响到手机上的其他app，做好了风险隔离。

Handler中的几个主要类

1.Looper：传递消息入队出队的主要处理者

2.MessageQueue:

一个优先级的消息队列，队列内的消息按照执行时间先后排列

2.Message:队列消息 主要注意 获取Message的复用方式

Handler的整体工作流程

handler机制就是一个传送带的运转机制。
1）MessageQueue就像履带。
2）Thread就像背后的动力，就是我们通信都是基于线程而来的。
3）传送带的滚动需要一个开关给电机通电，那么就相当于我们的loop函数，而这个loop里面的for循环就会带着不断
的滚动，去轮询messageQueue
4）Message就是 我们的货物了。

工作流程.png

• 出队（取出消息）

从 ActivityThread ->main开始看这里启动了整个进程的消息监分发处理

{
    ...
    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);
        //启动loop 开始循环消息
        Looper.loop();

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

接着进入到了Looper的loop方法里,从 Message msg = queue.next(); 中取出消息

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 (;;) {
        //取出消息开始处理
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }
        ...        
          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);
            }
      //消息处理完毕释放回收回消息池里
        msg.recycleUnchecked();
    }
}

从queue.next() 中 msg取出后会计算是否到了执行时间如果没到nativePollOnce继续阻塞等待 ，直到有人调用nativeWakeUp唤醒它。
一般阻塞有2种情况

• 消息未到执行时间 系统调用 nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);重新计算等待时间
• 消息队列为空 nextPollTimeoutMillis = -1

在next()里要退出的loop循环只有一种方式 那就是调用quit的函数将mQuitting置为true

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();
        }
        //休眠等待下个唤醒指令
        nativePollOnce(ptr, nextPollTimeoutMillis);->epoll_wait()

        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            //同步屏障
            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;
            }
   //只有quit才会停下loop
            // 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;
    }
}

//退出loop的办法
void quit(boolean safe) {
    if (!mQuitAllowed) {
        throw new IllegalStateException("Main thread not allowed to quit.");
    }

    synchronized (this) {
        if (mQuitting) {
            return;
        }
        mQuitting = true;

        if (safe) {
            removeAllFutureMessagesLocked();
        } else {
            removeAllMessagesLocked();
        }

          //唤醒
        // We can assume mPtr != 0 because mQuitting was previously false.
        nativeWake(mPtr);
    }
}

• 入队（发送消息）

通常我们使用handler发送消息 都是通过sendMessage 那么就以他为切入点：

sendMessage——>sendMessageDelayed——>sendMessageAtTime

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;
        }
        //这里将消息放入了队列中
        return enqueueMessage(queue, msg, uptimeMillis);
    } 

enqueueMessage：

    boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
        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();
            msg.when = when;
            Message p = mMessages;
            boolean needWake;
            //将消息放入下个节点
            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 {
                needWake = mBlocked && p.target == null && msg.isAsynchronous();
                Message prev;
                for (;;) {
                    prev = p;
                    p = p.next;
                    if (p == null || when < p.when) {
                        break;
                    }
                    //检查当前是否处于同步屏障期间  
                    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) {
                //通知nativePollOnce
                nativeWake(mPtr);
            }
        }
        return true;
    }

入队的流程总体比较简单 主要就是 添加消息到队列，如果当前需要唤醒 发送唤醒

• MessageQueue中的 阻塞方式：

  
  
  nativePollOnce .png

通过调用nativePollOnce 调用到jni的层 再到c++层最终调到epoll_wait 函数等待

nativeWakeUp

nativePollOnce .png

Handler 中的消息同步屏障

当消息队列开启同步屏障的时候（即标识为 msg.target == null ），消息机制在处理消息的时候，优先处理异步消息。这样，同步屏障就起到了一种过滤和优先级的作用。
找到isAsynchronous()为true的异步消息 优先处理
一般系统内比较常用到
Android 系统中
的 UI 更新相关的消息即为异步消息，需要优先处理。
比如，在 View 更新时，draw、requestLayout、invalidate 等很多地方都调用了
注意next方法中

          
           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());
                }

开启同步屏障：
Handler.getLooper().getQueue().postSyncBarrier()
移除同步屏障：
Handler.getLooper().getQueue().removeSyncBarrier();

Message应该怎么用：

一般要创建一个message对象有两种方法 一种是直接new 直接在内存开辟空间 另一种是obtain即 优先使用空的message 如果没有再去new

 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();
    }

HandlerThread

相较于传统的Thread HandlerThread 将looper的准备工作再内部先处理好了

Handler常见QA

1.Q: 一个线程有几个 Handler？

A:看你new几个

2.Q:一个线程有几个 Looper？如何保证？

A：1个 通过threadLocal 来保证 当获取looper时 会先从threadLocal中获取 如果存在即返回，而当你想再次设置新的looper如果旧的已经存在 会抛出异常

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));
    }

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

3.Q：Handler内存泄漏原因？ 为什么其他的内部类没有说过有这个问题？

A：handler持有了外部类的引用 而 message 持有了handler 导致该释放的对象无法释放造成内存泄漏 其他类如adapter因为与外部类生命周期一致 不会存在内存泄漏 随activity的消亡而消亡

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

A：因为主线程的looper从activitythread的main方法就准备好了 而 子线程如果要使用looper应该调用prepare()和loop()方法

5.Q: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？

A:调用quit() 是mQuitting置为true 同时nativeWakeup 唤醒 退出loop循环

6.Q:既然可以存在多个 Handler 往 MessageQueue 中添加数据（发消息时各个 Handler 可能处于不同线程），那它内部是如何确保线程安全的？取消息呢？

A: 通过 synchronized关键字 加锁来确保 无法同时调用入队出队

7.Q:我们使用 Message 时应该如何创建它？

调用 obtain 复用message(享元设计模式）

8.Looper死循环为什么不会导致应用卡死

looper在没消息/未到消息时间处理时 会进入nativePollOnce使自己进入阻塞状态 让出cpu 执行权 供其他线程使用，等到 有消息入队又会唤醒它