前言
Handler 的消息传递机制是我们面试中常被问到的问题，它到底如何去实现的呢？这样做的好处是什么呢？接下来我们一起从源码角度探究它的奥秘。
http://androidxref.com/
目录
- Handler 的创建
- Message 的创建
- Handler 发送 Message
- Message 入队 MessageQueue
- Looper 循环获取 Message
- Message 处理
源码解读
1. 创建Handler
new Handler 最终会进入以下构造器，随之获取Looper对象，与Looper中内置的MessageQueue。
public Handler(Callback callback, boolean async) {
// 省略好几行......
// 创建 Handler 会同时创建一个当前线程的 Looper
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
// 绑定 Looper 中的 MessageQueue
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
private Looper(boolean quitAllowed) {
// Looper的构造器中会初始化一个MessageQueue对象。
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
2. 创建Message
推荐调用 Message.obtain() ，Message 的内部其实使用了链表的结构实现了复用机制。如果第一次获取 Message 会 new 一个新的 Message ，之后会复用之前所创建过的 Message 对象，并将其初始化。
public static Message obtain() {
synchronized (sPoolSync) {
// sPool 其实也是一个 Message 对象
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();
}
在 m.next 首先会进入一个 nativePollOnce 的原生函数中，判断如果消息队列中没有消息存在 nativePollOnce 就不会返回，进入休眠状态节省CPU资源，方法返回后就代表next方法就可以获取到消息，循环去获取 Message 。
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();
}
// 当有 Message 添加后该函数才会唤醒，否则会一直阻塞，详细实现请查阅 Looper.cpp
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// 尝试获取下一个 Message
final long now = SystemClock.uptimeMillis();
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;
}
} 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;
}
}
3. Handler 发送 Message
调用发送后最终会进入此函数将 Message 排队放入 MessageQueue 等待处理，其中 uptimeMillis 执行时间是从系统开机时间开始计算的，调用的 SystemClock.uptimeMillis 原生接口。
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
// 这里的 target 所赋的值就是发送它的 Handler
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
在排队时会判断传入的 Message 是否有所属 Handler （即：msg.target，在 Message 创建时赋的值），以及其是否使用过，不符合条件会抛出异常。符合条件时会将其排入 pre.next 。
4. Message 入队 MessageQueue
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;
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 (;;) {
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);
}
}
return true;
}
5. Looper 循环获取 Message
看到这里好像 Message 排好队之后就没有继续执行了？
其实早在应用程序启动时主线程的 Looper.loop() 已经被调用执行了。这个 main() 函数就是我们应用程序的入口，在 ActivityThread 类中。
找不到这个类的小伙伴不要着急，在线查看系统源码网址：http://androidxref.com/。
public static void main(String[] args) {
// 省略了好几号......
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
Message 在我们的 loop() 函数循环的由 msg.target.dispatchMessage(msg) 分发到回主线程。
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();
for (;;) {
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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
final long traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (slowDispatchThresholdMs > 0) {
final long time = end - start;
if (time > slowDispatchThresholdMs) {
Slog.w(TAG, "Dispatch took " + time + "ms on "
+ Thread.currentThread().getName() + ", h=" +
msg.target + " cb=" + msg.callback + " msg=" + msg.what);
}
}
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();
}
}
6. Message 处理
分发 Message 时有三种方式，这是由我们创建 Handler 与 Message 时的方式决定的。最后一种方式就是我们常见的重写 handleMessage() 处理消息。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
总结：
其实在 Java 语言中多线程间数据本来就是可以共享的，通过 Handler 实现线程间通讯主要是使用同步的手段让任务依次执行，保证了多线程访问数据的安全性。
所以只有我们的 Handler 在主线程创建，或者显示传入 new Handler(Looper.getMainLooper()) 时我们发送的消息才是在主线程处理的。
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