Handler是Android中最常用的消息机制，具体指示涉及到Looper、MessageQueue、ThreadLocal、binder、epoll的知识。
0、先说自己的总结
- Looper负责从MessageQueue中循环取消息并分发，通过ThreadLocal机制保证每个线程只有一个Looper实例，Looper.getMainLooper()所在的线程就是主线程
- MessageQueue存储维护了一个有序链表，并通过epoll机制实现消息实时处理
- Handler负责发送Message，并在Looper取到消息后进行消息实际回调处理
以下基于android-30的源码进行分析。
1、从Looper说起
Looper中常用的是两个方法：prepare()和loop()
prepare()方法
prepare()方法很简单，将Looper实例加入到ThreadLocal线程变量副本中，如果已经初始化过，会抛出运行时异常Only one Looper may be created per thread：
// 在线程中初始化looper的常用方法
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
// 通过ThreadLocal的机制，保证每个线程只有一个Looper实例
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
...
// 初始化主线程looper的方法，这个方法不需要我们手动调用，在App启动阶段已经有ActivityThread调用过
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
...
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
// 私有的构造方法，在这里初始化了Looper对应的MessageQ对象，保证一一对应
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
可以看到，Looper内通过ThreadLocal机制保证每个线程只能由一个Looper实例和一个MessageQueue实例。
关于ThreadLocal机制，这个不是本文的重点，可以参考文章：Java并发编程：深入剖析ThreadLocal。
loop()方法
loop()就是开启循环的地方，循环读取消息并分发，调用这个方法后当前线程会进入阻塞状态，在它后面的代码不会被执行到：
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
public static void loop() {
// 获取当前线程的looper实例，基于前面的ThreadLocal原理
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;
// 每一个Looper都有唯一的一个MessageQueue
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;
// 开启死循环
for (;;) {
// 从MessageQueue中取下一条消息，next()方法实际是epoll的机制保证没有消息时的线程挂起状态
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
// 如果我们手动给Handler设置logging回调，可以通过监听Dispatching和Finished之间的间隔判断是否发生ANR
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; // app内不能直接设置observer，hide隐藏方法，这里略过不看
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 {
// 将msg交给target的Handler去处理消息
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);
}
// 调用msg的回收方法，将Message内的成员变量置空，并将其加入到缓存链表中，下次调用Message.obtain()方法就可以复用
msg.recycleUnchecked();
}
}
可以看出，Looper的loop方法开启了一个死循环，循环的将MessageQueue中取到的消息分发到target对应的Handler中进行处理。
Message是一个链表结构，通过next变量存储下一个Message，当调用到recycleUnchecked方法时，会将内部变量置空，并加入到Message的静态变量sPool的链表中，供缓存使用。
MainLooper的初始化时机
App在启动时，Zygote进程会反射到ActivityThread类的main方法，在这里进行了MainLooper的初始化。
有关App的启动流程不是本文重点，参考文章：App 启动过程-AndroidOfferKiller
// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#6459
public static void main(String[] args) {
...
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
...
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#6315
private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
...
final IActivityManager mgr = ActivityManager.getService();
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
...
}
...
}
...
}
main方法很简单，loop方法调用后当前线程会进入阻塞状态，可以认为ActivityThread的main方法执行的线程就是主线程。
在attach方法中，和ActivityManagerService(AMS)完成绑定，AMS会在后续中通过binder机制将消息传入到ActivityThread中的内部Handler变量mH中，完成生命周期的回调。
由于binder机制有线程池机制，所以ActivityThread接收到的AMS回调是执行在binder线程中的，再通过handler将消息发送到主线程执行。有关binder线程池机制介绍参考：Binder Driver浅析：Binder线程池
2、Handler源码分析
Handler的源码相对简单一点，它负责发送消息和处理消息。
Handler的构造方法
// 最常用的构造函数是这个
@Deprecated
public Handler() {
this(null, false);
}
// 这个函数是hide的，不能调用，主要是因为async这个变量，async=true表示handler发送的所有消息都是异步的，不受同步屏障影响
// @hide
public Handler(@Nullable Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {// 默认为false，用来检查非static的Handler实例，防止内存泄漏
final Class<? extends Handler> 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对象，通过ThreadLocal机制
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
// 获取和looper一一对应的MessageQueue对象
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
post和postDelayed方法
发送消息最常用的就是post和postDelayed方法：
public final boolean postDelayed(@NonNull Runnable r, long delayMillis) {
return sendMessageDelayed(getPostMessage(r), delayMillis);
}
public final boolean post(@NonNull Runnable r) {
// 可以看到，post内部调用的也是sendMessageDelayed方法，和postDelayed一致
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
// 通过obtain方法复用之前的Message对象
Message m = Message.obtain();
m.callback = r;
return m;
}
public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
if (delayMillis < 0) {
delayMillis = 0;
}
// 转换为确定时间
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
public boolean sendMessageAtTime(@NonNull Message msg, long uptimeMillis) {
// mQueue是在Handler的构造里面初始化的，拿到的是Looper内的queue实例，线程内唯一
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) {
// 将target设置为handler自己，给Looper后面分发消息使用
msg.target = this;
msg.workSourceUid = ThreadLocalWorkSource.getUid();
// 判断是否有异步消息标志，正常开发中这个标志位是不能设置为true的，只有系统函数才能设置，这个可以通过反射或者编译时提供provide jar包的方式调用到
// 异步消息主要和同步屏障相对应，这个后面MessageQueue中会说到
if (mAsynchronous) {
msg.setAsynchronous(true);
}
// 调用MessageQueue的enqueueMessage方法，将消息加入到队列中
return queue.enqueueMessage(msg, uptimeMillis);
}
那Handler是怎么分发消息的呢，通过Looper的loop方法代码可以看到，Looper从MessageQueue中取到消息后，会通过msg.target.dispatchMessage(msg)执行Handler的dispatchMessage方法。
那看一下dispatchMessage方法：
public void dispatchMessage(@NonNull Message msg) {
// 首先判断msg的callback是否为null，这个就是之前post方法内的Runnable对象
// handleCallback方法内部其实调用的就是message.callback.run()，直接执行Runnable内的run方法
if (msg.callback != null) {
handleCallback(msg);
} else {
// 再判断handler本身的callback是否为null，如果不为null，交给callback处理
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
// 最后是handler内可重写的方法handleMessage
handleMessage(msg);
}
}
Handler在子线程中的使用
在子线程中使用Handler机制来发送消息，有两种方式。
第一种就是在子线程中直接初始化Looper，直接用Handler即可：
new Thread(new Runnable() {
@Override
public void run() {
Looper.prepare();
Handler handler = new Handler() {
@Override
public void handleMessage(@NonNull Message msg) {
// 在这里处理消息
}
};
Looper.loop();
}
}).start();
第二种方式是使用HandlerThread，它其实是对上面代码进行的封装，它本身继承自Thread，并在run方法内调用了Looper.prepare()和Looper.loop();：
HandlerThread handlerThread = new HandlerThread("test");
handlerThread.start(); // 启动一个HandlerThread线程
Handler handler = new Handler(handlerThread.getLooper()) {
@Override
public void handleMessage(@NonNull Message msg) {
// 在这里处理消息
}
};
3、MessageQueue源码分析
MessageQueue是消息队列存放处，通过epoll机制提供了消息队列的读取功能。
epoll机制是Linux中kernel常用的IO多路复用机制，参考文章：Android 消息处理以及epoll机制
MessageQueue的构造方法
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit(); // epoll机制的初始化，通过mPtr拿到fd的引用
}
enqueueMessage方法
enqueueMessage方法是放置消息的入口方法，这个方法主要是Handler最终调用，通过前面的Handler可以看到，Message消息自己有target和callback保证回调，另外一个参数when表示确定执行的时间。
boolean enqueueMessage(Message msg, long when) {
// 通过handler发送的消息必须有target
// 没有target的消息是同步屏障消息，这个下面会看到
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
// 同步锁
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
// 对于主线程来说，没有退出机制，主线程queue退出也就代表着App无响应或退出了
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;
}
// 将Message标为已经使用
msg.markInUse();
msg.when = when; // 设置when确定时间
Message p = mMessages; // mMessages是链表的根Message
boolean needWake;
if (p == null || when == 0 || when < p.when) { // 当前消息当做头消息
// 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 (;;) { // 循环，根据when的先后顺序，找到msg应该插入的位置
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;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr); // epoll机制的唤醒操作
}
}
return true;
}
next方法
next方法是取出下一条Message的方法，主要是Looper.loop方法内调用，循环读取下一条消息。
@UnsupportedAppUsage
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; // mPtr是epoll机制的fd标志
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// epoll的阻塞方法，挂起当前线程，直到超时或者被nativeWake(mPtr)唤醒
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;
// msg.target如果是null，表示是同步屏障消息
// 那么，取到第一条异步消息
// 如果没有同步屏障，那么就取当前第一条消息
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) { // 如果当前时间还不到头msg的执行时间，设置下次取消息的休眠时间，进入下一次for循环
// 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 { // 否则，直接返回当前消息
// Got a message.
mBlocked = false; // 将mBlocked置为false，表示不阻塞，和enqueueMessage中对应
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;
}
// 如果当前没有消息需要处理，进入到IdleHandler的处理内
// 通过pendingIdleHandlerCount这个变量表示是否进行过IdleHandler的处理
// 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);
} // end synchronized (this)
// 如果当前没有消息需要处理，进入到IdleHandler的处理内
// 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;
}
}
文档信息
- 本文作者：itlgl
- 本文链接：https://itlgl.com/note/2021/03/12/issues-47/
- 版权声明：自由转载-非商用-非衍生-保持署名（创意共享3.0许可证）