1.Handler 简介

Handler 是一套 Android 消息传递机制。在多线程应用场景中，将子线程中需要更新 UI 的操作消息，传递到 UI 主线程，从而实现子线程通知 UI 更新最终实现异步消息处理。说白了是用于线程之间的通信。
Handler主要有4个重要类：Handler、Message、MessageQueue、Looper。

• Handler：负责消息的发送和处理，子线程中使用 sendMessage() 发送消息；在handleMessage()中处理。
• Message：消息载体，里面存储这线程消息。
• MessageQueue：消息队列，遵循先进先出的原则，存储着 sendMessage() 发送来的子线程消息。
• Looper：消息循环器，负责从 MessageQueue 中循环取消息，再将取出的消息分发给handleMessage()，来处理消息。

2.Handler原理

3.Handler 源码

了解 Handler，首先我们要了解 Handler 从消息发送到消息处理这整个流程，下面将分析这一流程，并回答下面几个问题：

1. 一个线程有几个Handler?
2. 一个线程有几个Looper？如何保证？
3. Handler 内存泄漏原因？为什么其他的内部分没有过这个问题？
4. 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？
5. 子线程中的维护的 looper，消息队列无消息的时候处理方案是什么？有什么用？
6. 既然可以存在多个 Handler 往 MessageQueue 中添加数据（发消息时，各个handler 可能处于不同的线程），那么它内部是如何确保线程安全的？
7. 我们使用 Message 时应如何创建它？
8. 使用 Handler 的 postDelay 后，消息队列会有什么变化？
9. Looper 死循环为什么不会导致应用卡死(ANR)?

Handler

Handler 负责消息的发送和处理，该类中通过 sendXXX、postXXX等方法发送消息，共有14个这样的方法。而在 handleMessage() 中处理收到的消息。
这里以 sendMessage(Message msg) 方法为例进行源码分析。
frameworks/base/core/java/android/os/Handler.java

public class Handler {......public final boolean sendMessage(@NonNull Message msg) {return sendMessageDelayed(msg, 0);}public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {if (delayMillis < 0) {delayMillis = 0;}// 第二的参数代表执行的时间，为：系统当前时间+延迟的时间return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}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);}// 不管使用什么方法发送消息，都会调到 Handler#enqueueMessage()private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,long uptimeMillis) {// 把当前对象赋给msg.target，这样 Message 就持有了 Handlermsg.target = this;msg.workSourceUid = ThreadLocalWorkSource.getUid();if (mAsynchronous) {msg.setAsynchronous(true);}// 调用 MessageQueue#enqueueMessage() 往消息队列添加消息。return queue.enqueueMessage(msg, uptimeMillis);}......
}

MessageQueue

enqueueMessage 里面其实是一个优先级队列，将收到消息根据执行的时间 when 进行做排序处理。
frameworks/base/core/java/android/os/MessageQueue.java

public final class MessageQueue {......boolean enqueueMessage(Message msg, long when) {if (msg.target == null) {throw new IllegalArgumentException("Message must have a target.");}synchronized (this) {if (msg.isInUse()) {throw new IllegalStateException(msg + " This message is already in use.");}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;// 如果是0，则放在最前面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 {// 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;// 对单链表轮询，根据 when 进行排序插入消息。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.nextprev.next = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;}......
}

到这里发消息基本完成，后面看如何取消息。
next() 中返回一个 Message。
frameworks/base/core/java/android/os/MessageQueue.java

public final class MessageQueue {......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 iterationint nextPollTimeoutMillis = 0;for (;;) {  // 死循环，if (nextPollTimeoutMillis != 0) {Binder.flushPendingCommands();}// nextPollTimeoutMillis ：-1 表示无限等待，直到有事件为止；0 表示立即执行；其他数字表示等待多时毫秒。// linux 层休眠等待，nativePollOnce(ptr, nextPollTimeoutMillis);synchronized (this) {// Try to retrieve the next message.  Return if found.final long now = SystemClock.uptimeMillis();Message prevMsg = null;// 拿到队列对头消息Message msg = mMessages;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) {// 跟当前时间对比if (now < msg.when) {// 队列中，第一个节点还没到可以执行的时刻，则等待。// 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 {  // 到了可以执行的时间，则把消息 return 出去。// 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;}// 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 handlerboolean 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;}}......
}

到此，消息取出来了，但是谁取的呢？这就涉及到另一个重要的类 Looper

Looper

Looper.loop() 里面会有个for循环，且是个死循环，会不断的调用 MessageQueue#next() 方法。
frameworks/base/core/java/android/os/Looper.java

public final class Looper {......// 初始化Looper public static void prepare() {prepare(true);}private static void prepare(boolean quitAllowed) {if (sThreadLocal.get() != null) {  // 如果该线程有 Looper，则抛出一个异常。throw new RuntimeException("Only one Looper may be created per thread");}// 这里使用了 ThreadLocal，保证了一个线程只有一个Looper。sThreadLocal.set(new Looper(quitAllowed));}public static void loop() {final Looper me = myLooper();if (me == null) {throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");}if (me.mInLoop) {Slog.w(TAG, "Loop again would have the queued messages be executed"+ " before this one completed.");}me.mInLoop = true;// 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);me.mSlowDeliveryDetected = false;for (;;) {  // 死循环// 不断地调用mQueue.next()if (!loopOnce(me, ident, thresholdOverride)) {return;}}}private static boolean loopOnce(final Looper me,final long ident, final int thresholdOverride) {// 这里如果此时队列中没有消息或队列中，第一个节点还没到可以执行的时刻，则会进入等待，block 状态。// 会一直在这等，该等待是Linux层做的，在 mQueue.next()中Message msg = me.mQueue.next(); // might blockif (msg == null) {// No message indicates that the message queue is quitting.return false;}// This must be in a local variable, in case a UI event sets the loggerfinal Printer logging = me.mLogging;// 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 {// 获取到消息后，就会调用 msg.target.dispatchMessage(msg)，即回调 handler#dispatchMessage(msg)// msg.target 为 handler对象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 (me.mSlowDeliveryDetected) {if ((dispatchStart - msg.when) <= 10) {Slog.w(TAG, "Drained");me.mSlowDeliveryDetected = false;}} else {if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",msg)) {// Once we write a slow delivery log, suppress until the queue drains.me.mSlowDeliveryDetected = 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();return true;}......
}

当获取到消息时，回调用 msg.target.dispatchMessage(msg) 即 handler#dispatchMessage(msg)，在 dispatchMessage(msg) 中再回调 handleMessage(msg)，这样就收到消息。至此发消息、收消息整个流程结束。
下面回答上述的问题。

1、一个线程有几个Handler？

答：那个，new 多少就有多少。

2、一个线程有几个Looper？如何保证？

答：一个，在初始化时，使用了 ThreadLocal ，而 ThreadLocal 是一个 <key,value> 这种形式的变量，类似 hashMap。它的 key 是当前线程，value 是 Looper。
下方为对应源码分析：

// 初始化Looper public static void prepare() {prepare(true);}private static void prepare(boolean quitAllowed) {if (sThreadLocal.get() != null) {  // 如果该线程有 Looper，则抛出一个异常。throw new RuntimeException("Only one Looper may be created per thread");}// 这里使用了 ThreadLocal，保证了一个线程只有一个Looper。sThreadLocal.set(new Looper(quitAllowed));}// ThreadLocal 的 set() 方法public void set(T value) {Thread t = Thread.currentThread();  // 当前线程ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);elsecreateMap(t, value);}// ThreadLocal 的 get()方法，public T get() {Thread t = Thread.currentThread();  // 当前线程ThreadLocalMap map = getMap(t);if (map != null) {ThreadLocalMap.Entry e = map.getEntry(this);if (e != null) {@SuppressWarnings("unchecked")T result = (T)e.value;return result;}}return setInitialValue();}

3、Handler 内存泄漏原因？为什么其他的内部分没有过这个问题？

出现内存泄漏的情况：Activity 销毁时，存在待处理的消息。例如：发送一个delay(延迟消息) 2s，在2s内销毁界面。
答：Handler 持有 Activity 的上下文，而 MessageQueue 持有 Message，Message 又持有 Handler；只有当这消息被处理时，才会去销毁对应的 Handler ，Handler 被销毁了，才会去销毁持有的上下文。而其他内部类，例如：RecyclerView 的 ViewHolder，不会产生内存泄漏，因为它没有被其它地方持有该内部类。
frameworks/base/core/java/android/os/Handler.java

public class Handler {......// 不管使用什么方法发送消息，都会调到 Handler#enqueueMessage()private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,long uptimeMillis) {// 把当前对象赋给msg.target，这样 Message 就持有了 Handlermsg.target = this;msg.workSourceUid = ThreadLocalWorkSource.getUid();if (mAsynchronous) {msg.setAsynchronous(true);}// 调用 MessageQueue#enqueueMessage() 往消息队列添加消息。return queue.enqueueMessage(msg, uptimeMillis);}......
}

4、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？

答：主线程在创建时，系统 ActivityThread 就已经创建好了。在子线程中 new Handler 需想初始化 Looper（Looper.prepare()），并启动 loop(Looper.loop())
frameworks/base/core/java/android/app/ActivityThread.java

    public static void main(String[] args) {// 省略部分代码......Looper.prepareMainLooper();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(newLogPrinter(Log.DEBUG, "ActivityThread"));}// End of event ActivityThreadMain.Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);/// M: ANR Debug MechanismmAnrAppManager.setMessageLogger(Looper.myLooper());Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited");}

5、子线程中的维护的 looper，消息队列无消息的时候处理方案是什么？有什么用？

答：在适当地方调用 Looper.quitSafely()；安全地退出 looper。 等所有剩余的消息处理完毕后立即终止。但是，在 loop 循环终止之前，将不会在收到消息。在要求循环程序退出后，任何向队列发送消息的尝试都将失败。

6、既然可以存在多个 Handler 往 MessageQueue 中添加数据（发消息时，各个handler 可能处于不同的线程），那么它内部是如何确保线程安全的？

答：在 MessageQueue#enqueueMessage、MessageQueue#next() 中的代码块使用了 synchronized 修饰。则也会导致 handler 的 delay 消息的时间不完全的准确。

7、我们使用 Message 时应如何创建它？

答：obtain()，避免了每次去 new ，防止了内存抖动。

8、使用 Handler 的 postDelay 后，消息队列会有什么变化？

答：若此时消息队列为空，则不会立马执行(Delay 消息)；当该消息添加进去时，MessageQueue#enqueueMessage 会 调用 nativeWake(mPtr) 唤醒消息队列，就会在 MessageQueue#next() 中，计算等待时间。

9、Looper 死循环为什么不会导致应用卡死(ANR)?

每一个事件都是一个 Message，因为所有事件都在Activity的生命周期里面，而主线程的所有代码都运行在 ActivityThread#main()中的 loop 里面。所以主线程的 loop 不能退出。
主线程唤醒的方式;
1、输入的事件；
2、Looper 添加消息；
输入事件：点击屏幕或按键按下，得到系统响应。
答：ANR是指在5s内没有响应输入事件（例如：按键按下、屏幕触摸），而输入的事件、Looper 添加消息都可以唤醒 Looper 里面的 block。Looper 死循环 与 ANR没有关系。