0.前言
Android中使用Handler实现消息的传递，相关的类主要有四个，android.os.Handler，android.os.Looper，android.os.Message，android.os.MessageQueue。下面会从源码的角度，来一步步看清整个消息机制的实现原理。
1.使用
先看一下日常开发中，Handler的简单使用都有什么样的。
private static class H extends Handler {
@Override
public void handleMessage(@NonNull Message msg) {
//do something 4处理消息
super.handleMessage(msg);
}
}
public void test() {
//1.发送消息到主线程处理
Handler handler = new Handler(Looper.getMainLooper());
handler.post(new Runnable() {
@Override
public void run() {
//do something
}
});
//2.使用Message接口发送消息，message的callback（一个Runnable实例）处理消息
Message msg = Message.obtain(handler, new Runnable() {
@Override
public void run() {
//do something
}
});
msg.sendToTarget();
//3.使用Handler的Callback处理消息，并决定是否拦截
Handler handler2 = new Handler(new Callback() {
@Override
public boolean handleMessage(@NonNull Message msg) {
if (msg.what == 1) {
//do something
}
return false;//返回是否拦截消息
}
});
handler2.sendEmptyMessage(1);
//4.自定义Handler的子类处理消息
H h = new H();
h.sendMessage(Message.obtain());
}
上述是几种常见的使用方式，不一定很全，只是先大概知道一下怎么用就好。需要说一下，Handler分发消息时候是有优先级的，其中上面的优先级是1=2>3>4，当在2处的message的callback存在时，不会走到3和4，当3处返回true表示拦截消息时，不会走到4。现在只是简单的提一嘴，后面到了具体的源码处，就会知道这里说的是什么意思了。
2.类关键源码
public final class Message implements Parcelable {
//用户定义的消息标识，用来识别区分消息
public int what;
//消息回调
Runnable callback;
//消息触发时间
public long when;
//形成链表结构
Message next;
//用链表作为消息对象缓存池
private static Message sPool;
//静态方法，从池中获取默认消息对象，还有几个重载函数可以传入消息的属性
public static Message obtain() {
}
}
public final class Looper {
//ThreadLocal表示对象是线程隔离的，在一个线程下，只会存在唯一的一个对象，且不和其他线程共享
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
//主线程下的Looper对象
private static Looper sMainLooper;
//消息队列MessageQueue
final MessageQueue mQueue;
//当前Looper所在线程
final Thread mThread;
//Looper初始化
private static void prepare(boolean quitAllowed) {
}
//开启loop循环处理消息
public static void loop() {
}
}
public final class MessageQueue {
//获取队列下一条消息
Message next() {
}
//把消息存入到队列
boolean enqueueMessage(Message msg, long when) {
}
}
public class Handler {
//Handler会持有当前线程的Looper对象，以及Looper的MessageQueue
final Looper mLooper;
final MessageQueue mQueue;
//用户自定义的Handler消息回调
final Handler.Callback mCallback;
//回调接口
public interface Callback {
//返回值表示是否拦截消息
boolean handleMessage(@NonNull android.os.Message msg);
}
//分发消息，把消息交给具体的处理人
public void dispatchMessage(@NonNull Message msg) {
}
//以下方法全是发送消息，最终都会走到下面的enqueueMessage方法，然后会调用MessageQueue的enqueueMessage方法把消息存入到队列
public final boolean post(@NonNull Runnable r) {
}
public final boolean sendMessage(@NonNull Message msg) {
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
}
private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
long uptimeMillis) {
}
}
以上只是简单说明每一个类的关键属性和方法，下面会通过具体的消息发送处理过程，查看源码的方法调用流程，并详细说明。
3.具体流程
先从简单调用说起，新建子线程，发送延迟消息，代码如下：
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();//1初始化Looper
Handler h = new Handler();
h.postDelayed(new Runnable() {//2发送消息
@Override
public void run() {
//do something
}
}, 1000);
Looper.loop();//3开启Looper循环
}
}).start();
为什么在主线程不需要也不能调用1和3处的代码？这是因为系统已经调用了，在ActivityThread中主线程的相关源码如下：
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
//......其他代码
Looper.prepareMainLooper();
//......其他代码
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
初始化Looper
注释1处，在线程中初始化Looper，源码如下：
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));
}
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
以上源码比较简单，初始化做的事就是新建一个Looper实例，给Looper的mQueue和mThread属性赋值，然后把值存到静态变量sThreadLocal中，在同一个线程下，这个方法只能调用一次，否则就会抛出异常。
发送消息
注释2处，调用Handler对象的postDelayed方法，发送延迟消息
先通过Message.obtain()方法获取到消息对象，该方法的源码如下：
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();
}
优先从消息对象缓存池里面拿对象，如果有，返回池的头部对象；如果没有，新建对象返回。
经过几个方法调用，最终会走到enqueueMessage方法，这个方法其实也没有真正处理消息，而是调用MessageQueue的enqueueMessage，下面是MessageQueue的enqueueMessage方法的源码：
boolean enqueueMessage(Message msg, long when) {
//处理消息前的判断，是否有target，是否正在使用
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.");
}
//同步处理
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();//标记消息为正在使用
msg.when = when;//设置消息的触发时间
Message p = mMessages;
boolean needWake;
//当 消息队列为空、消息马上触发，消息触发时间小于队列头部消息触发时间 时，把消息放在消息头部
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;
for (; ; ) {//遍历消息队列，并根据消息的触发时间把消息放在队列的合适位置
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;
}
//如果需要唤醒，调用native唤醒方法
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
逻辑清晰明了，就是根据消息的触发时间，把新的消息放到合适的位置。这样就完成了消息的发送，那么这个消息队列里面的消息什么时候去取呢？就要看下面的内容了。
开启Looper循环
注释3处的代码，开启Looper循环处理消息，源码如下：
public static void loop() {
//通过sThreadLocal属性，获取当前线程下的Looper对象，如果没有抛出异常
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//当前线程下的消息队列
final MessageQueue queue = me.mQueue;
//这是一个native方法，目的如下文说的那样，是为了保此线程的标识是本地进程的标识，并跟踪该标识令牌实际是什么。不做深入了解
// 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();
//这也是native方法，允许通过system prop重写threshold，不做深入了解
// 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;
//1.开启死循环
for (;;) {
//2.获取消息队列的下一条消息，可能会阻塞线程
Message msg = queue.next(); // might block
if (msg == null) {
//无消息表示消息队列退出，退出死循环
// No message indicates that the message queue is quitting.
return;
}
//如果有设置日志打印器，打印日志
// 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 {
//3.通过Handler对象分发消息
msg.target.dispatchMessage(msg);
//给观察者分发消息
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);
}
//4.回收消息，把消息数据清空，如果消息缓存池未满，把消息对象加入到缓存池头部
msg.recycleUnchecked();
}
}
关键的代码有4处，注释1开启死循环，一直到消息队列退出后，这个死循环才会退出，而如果是在主线程，因为主线程的消息队列是不可退出的，所以死循环Looper的死循环也不会退出，除非是发生异常。
注释2是通过消息队列MessageQueue的next()方法获取消息。
注释3处是把消息给到Handler的dispatchMessage()方法处理。
注释4回收消息。
注释2处，消息队列MessageQueue的next()方法源码如下：
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();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
//获取消息队列里的头部消息
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) {//如果当前消息还没有到触发时间，设置唤醒时间，后面会调用nativePollOnce唤醒
// 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 {//如果消息到达触发时间，返回消息
// 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 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;
}
}
主要做的事就是获取队列的头部消息，然后判断这个消息的触发时间是否满足。
这里有一个native方法nativePollOnce()，作用是阻塞当前线程，当指定时间到了，或者有新消息时，调用nativeWake()，就会唤醒线程，这里的阻塞和唤醒，是利用Linux的多路复用机制epoll实现的。
注释3处Handler的dispatchMessage()方法源码如下：
public void dispatchMessage(@NonNull Message msg) {
//1如果消息msg有callback回调，交给msg的callback处理
//2否则交给Handler的mCallback处理消息，如果返回true，表示拦截，不再处理消息
//3如果2返回false，表示不拦截消息，继续交给Handler子类重写的handleMessage方法处理
if (msg.callback != null) {
//1
handleCallback(msg);
} else {
//2
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//3
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
public void handleMessage(@NonNull Message msg) {
}
这里就是文中一开始说到的，消息处理优先级，以及源码实现。
注释4回收消息对象，源码如下：
void recycleUnchecked() {
//清空所有数据
// 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;
//如果消息缓存池未满，把消息加入到缓存池头部
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
同步屏障和IdleHandler
同步屏障可以通过MessageQueue.postSyncBarrier函数来设置。该方法发送了一个没有target的Message到Queue中，在next方法中获取消息时，如果发现没有target的Message，则在一定的时间内跳过同步消息，优先执行异步消息，没有异步消息也不会执行同步消息，而是等待异步消息的到来。换句话说，同步屏障为Handler消息机制增加了一种简单的优先级机制，异步消息的优先级要高于同步消息。在创建Handler时有一个async参数，传true表示此Handler发送的是异步消息。ViewRootImpl.scheduleTraversals方法就使用了同步屏障，保证UI绘优先执行。
相关源码：
Message prevMsg = null;
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) {
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;
}
}
IdleHandler是一个回调接口，可以通过MessageQueue的addIdleHandler添加实现类。当MessageQueue中的任务暂时处理完了（没有新任务或者下一个任务延时在之后），这个时候会回调这个接口，返回false，那么就会移除它，返回true就会在下次message处理完的时候继续回调。当队列中有多个IdleHandler时，一次最多只执行4个。
相关源码：
// 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);
......
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);
}
}
}
对于同步屏障和IdleHandler，可以简单理解成是普通Message优先级不一样的Message，开启同步屏障时，只执行异步消息，所以异步消息优先级最高；普通情况下，按序执行Message；当队列空闲的时候，执行IdleHandler，所以IdleHandler优先级最低。
https://mp.weixin.qq.com/s/HK3SokYCOgVMAu6Uj6c-Xw 同步屏障和IdleHandler详细