Android消息机制:
Android消息机制三剑客之Handler、Looper、Message源码分析(一)
Android消息机制三剑客之Handler、Looper、Message源码分析(二)
消息通信机制的运行原理
上一篇中,单独分析了Handler、Looper、MessageQueue,本篇就分析一下这三者是如何协同工作,实现线程间通信的。我们就以Looper类中官方给出的子线程中经典通信样例来分析分析每步都做了什么。
* class LooperThread extends Thread {
* public Handler mHandler;
*
* public void run() {
* Looper.prepare();
*
* mHandler = new Handler() {
* public void handleMessage(Message msg) {
* // process incoming messages here
* }
* };
*
* Looper.loop();
* }
*
首先,在线程中先调用了Looper.prepare():
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);
mRun = true;
mThread = Thread.currentThread();
}
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
nativeInit();
}
prepare方法为当前线程初始化通信环境,每个线程只能持有一个Looper实例,并且通过set将Looper实例与本地线程绑定。而在Lopper的构造函数中,又创建了MessageQueue的实例,这里也就为当前线程准备好了MessageQueue来接收Message对象,而在MessageQueue的构造函数中调用的是Native层的初始化方法,创建一个单向链表的队列,对Native层有兴趣的朋友可以自行分析,这里就不再深入了。
接下来mHandler = new Handler() {…}创建了Handler的实例,并重写了handleMessage,也就是消息通信的终点,那么我们来看看Handler的构造方法:
public Handler() {
this(null, false);
}
//空构造最终调用了下面的构造方法
/**
* Use the {@link Looper} for the current thread with the specified callback interface
* and set whether the handler should be asynchronous.
*
* Handlers are synchronous by default unless this constructor is used to make
* one that is strictly asynchronous.
*
* Asynchronous messages represent interrupts or events that do not require global ordering
* with represent to synchronous messages. Asynchronous messages are not subject to
* the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
*
* @param callback The callback interface in which to handle messages, or null.
* @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
* each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
*
* @hide
*/
public Handler(Callback callback, boolean async) {
if (FIND_POTENTIAL_LEAKS) {
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());
}
}
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
这里带着注释来看还是因为注释里告诉了我们很多信息,之前就有面试官问道Handler是不是同步的? 当时一下懵住了,没有想过这个问题,但答案其实在注释里已经告诉我们了,这里就不再对注释进行翻译,大家可以自行看一下。在Handler的构造方法里可以看到分别拿到了Looper和Message的实例,mLooper和mQueue,在这里三剑客也就聚齐了。
运行到这里,当前线程下的消息通信环境已经搭建完成,接下来Looper.loop()才是重头戏,为线程开启了消息循环,从这里开始,消息通信开始运行起来:
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop.
*/
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(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.recycle();
}
}
loop()方法中核心就是开启了无限循环,这其中,我们需要仔细研究的是这三个关键方法:
- queue.next():从MessageQueue中取出下一个Message,在MessageQueue中执行一次出队操作;
- msg.target.dispatchMessage(msg) :将取出的Message对象分发给绑定的Handler;
- msg.recycle() : 回收该Message实例,存入global pool,后续可以通过msg.obtain()在global pool中直接获取Message实例,避免了每次都创建新的实例;
首先看一下next()方法的内部实现:
final Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(mPtr, nextPollTimeoutMillis);
synchronized (this) {
if (mQuiting) {
return null;
}
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages; /** mMessage是该链表队列的头结点 **/
if (msg != null && msg.target == null) { /** 对于异步消息单独处理 **/
// 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) { /**队列中按时间排序,因此判断当前消息是否到达发送的时间点**/
// 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 (false) Log.v("MessageQueue", "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// 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;
}
... ...
}//(sync)
}//(for)
}
next()方法中核心逻辑也是一个无限循环,目的就是返回下一个要处理的消息,然后从消息队列中进行出队。其中mMessages保存的是队列的头结点,从该节点开始向下遍历。首先进行判空处理,在msg非空但target为空时表示这是一个异步消息,是否异步是在Handler构造时进行定义的,默认情况下都是同步的,因此这里的逻辑一般不会执行。
接着向下,判断当前时间是否到达msg指定的发送时间,因为消息入队时会根据携带的发送时间进行排序,在未达到发送时间时,msg不出队,而是设置nextPollTimeoutMillis,这个变量有什么用呢,我们回到循环最开始的地方,这里在nextPollTimeoutMillis 非0时,调用flushPendingCommands挂起当前线程,该方法预示系统后续可能会有长时间的线程block到来,也就是最后一部分,在当前线程挂起(两种情况:初始消息队列为空 或 msg需要在未来某时处理)后会取出idelHandler(闲置Handler,如果存在的话)进行处理。如果这时候连idelHandler也不存在的话,线程进入阻塞,continue直接无限循环,直到找到需要处理的msg。
那么找到msg后,因为默认都是同步msg,这里prevMsg为空,走else,将mMessage头结点指针向下移动,然后msg出队,添加已用标记,return返回msg,这样Looper.loop()方法中的Message msg = queue.next()这条语句执行完毕 ,拿到要进行分发的Message对象,那么接下来就是执行msg.target.dispatchMessage(msg); 进行消息分发,这里调用到的是Handler中的方法:
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
dispatchMessage方法很简单,我们在创建Handler实例的时候使用的空构造,因此这里的callback为null,也就是直接调用了handleMessage,也就是在创建时候我们重写的handleMessage方法了,至此消息得到处理,Message对象完成了一次通信,在loop()中最后调用recycle()方法,将Message实例回收,可以通过obtain再次获取到该实例,而不必每次都新建实例了。
至此,我们通过一个官方简单示例分析了整个消息通信的框架,现在我们再根据流程图总体回顾一下:
在本地线程中,首先必须先调用Looper.prepare()方法,该方法会创建号Looper实例和MessageQueue实例,然后再创建Hanlder实例,并重写handlerMessage方法,自定义消息处理,在Handler的构造方法中,会引入Looper和MessageQueue实例。然后调用Looper.loop()开启无限循环,不断的调用next()从消息队列中取出消息,如果消息队列为空或未到消息处理时间时,线程进入阻塞状态。而在外部,调用Handler的send/post系列方法后,最终都调用SendMessageAtTime方法,为msg添加when属性,然后调用Handler内部的enqueueMessage方法,此处为msd绑定target为当前Handler,接着调用到MessageQueue类中定义的enqueueMessage,此方法中将接收的msg进行入队,将mMessages指向队列头部。同时,入队操作还会唤醒已阻塞的线程(如果在阻塞状态的话),接着next()方法就可以取到该入队的msg,并调用msg.target.dispatchMessage()方法,最后就调用到了重写的handleMessage()方法,完成了消息通信。