Handler使用简单功能强大,常被用作线程间传递消息的组件,而且还可以用于跨进程。
消息机制背后有包括Looper ,MessageQueue管理和分发消息的实现,同时在Native层也单独实现了一套类似的机制,接收和处理Native层的消息。Java层和Native层的消息循环是独立运行的,彼此的Message并不会互通,Native使用epoll机制来实现监听及触发,并向JAVA层提供了接口。
这里从Java层开始深入探究下Handler和消息机制背后实现的原理。
初探Handler
本章只作为入口,先从宏观上去了解其中的架构,之后再做深入分析。
代码架构
Handler本身只负责发送和处理接收到的消息,其背后有一个消息循环为它管理和提供消息。
MessageQueue是管理着Message链表;而Looper是消息循环的主体,负责循环从MessageQueue中获取需要处理的新消息并向Handler输送。
其中MessageQueue有部分核心实现在native层(后续会讲到)。
MessageQueue.java
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
private native void nativePollOnce(long ptr, int timeoutMillis); /*non-static for callbacks*/
private native static void nativeWake(long ptr);
private native static boolean nativeIsPolling(long ptr);
private native static void nativeSetFileDescriptorEvents(long ptr, int fd, int events);
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native方法实现在JNI层的android_os_MessageQueue.cpp:
android_os_MessageQueue.cpp
static const JNINativeMethod gMessageQueueMethods[] = {
/* name, signature, funcPtr */
{ "nativeInit", "()J", (void*)android_os_MessageQueue_nativeInit },
{ "nativeDestroy", "(J)V", (void*)android_os_MessageQueue_nativeDestroy },
{ "nativePollOnce", "(JI)V", (void*)android_os_MessageQueue_nativePollOnce },
{ "nativeWake", "(J)V", (void*)android_os_MessageQueue_nativeWake },
{ "nativeIsPolling", "(J)Z", (void*)android_os_MessageQueue_nativeIsPolling },
{ "nativeSetFileDescriptorEvents", "(JII)V",
(void*)android_os_MessageQueue_nativeSetFileDescriptorEvents },
};
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什么是Handler?
打开Handler.java,先看下googler留给开发者留的一段说明:
/**
* A Handler allows you to send and process {@link Message} and Runnable
* objects associated with a thread's {@link MessageQueue}. Each Handler
* instance is associated with a single thread and that thread's message
* queue. When you create a new Handler, it is bound to the thread /
* message queue of the thread that is creating it -- from that point on,
* it will deliver messages and runnables to that message queue and execute
* them as they come out of the message queue.
*
* <p>There are two main uses for a Handler: (1) to schedule messages and
* runnables to be executed as some point in the future; and (2) to enqueue
* an action to be performed on a different thread than your own.
*
* <p>Scheduling messages is accomplished with the
* {@link #post}, {@link #postAtTime(Runnable, long)},
* {@link #postDelayed}, {@link #sendEmptyMessage},
* {@link #sendMessage}, {@link #sendMessageAtTime}, and
* {@link #sendMessageDelayed} methods. The <em>post</em> versions allow
* you to enqueue Runnable objects to be called by the message queue when
* they are received; the <em>sendMessage</em> versions allow you to enqueue
* a {@link Message} object containing a bundle of data that will be
* processed by the Handler's {@link #handleMessage} method (requiring that
* you implement a subclass of Handler).
*
* <p>When posting or sending to a Handler, you can either
* allow the item to be processed as soon as the message queue is ready
* to do so, or specify a delay before it gets processed or absolute time for
* it to be processed. The latter two allow you to implement timeouts,
* ticks, and other timing-based behavior.
*
* <p>When a
* process is created for your application, its main thread is dedicated to
* running a message queue that takes care of managing the top-level
* application objects (activities, broadcast receivers, etc) and any windows
* they create. You can create your own threads, and communicate back with
* the main application thread through a Handler. This is done by calling
* the same <em>post</em> or <em>sendMessage</em> methods as before, but from
* your new thread. The given Runnable or Message will then be scheduled
* in the Handler's message queue and processed when appropriate.
*/
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既然决定来了就不能放过每个细节,记录下重点。
运作方式:
每一个Handler实例只与一个单独的Thread和这个Thread的MessageQueue关联;
当我们在Thread中创建一个新的Handler时,会绑定这个Thread和Thread的MessageQueue,之后Handler允许我们向MessageQueue发送Message和Runnable,并在消息出列时处理它们。
Handler的2个主要用途:
1. 让Message和Runnable可以延迟执行;
2. 在另外一个线程中执行处理。
用法:
通过Post开头和sendMessage开头的方法可以发送消息到MessageQueue。
1. post开头的方法可以向队列插入Runnable;
2. sendMessage开头的方法则用于来送Message,Message将在handleMessage方法中被处理。
3. post和send方法既可以让消息“实时”被处理(相对于延时),也可以设置特定的时延,延时去处理。
建议:
应用进程中的的主线程是专门用于管理顶层的数据的,例如activity/广播/窗口等,不宜处理其他我们定义的耗时操作,因此我们应该创建自己的工作线程,通过Handler来向线程的MessageQueue发送要执行的任务。
三个需要理解的问题
看完上面这段话,有3个疑问需要探究:
1. Handler如何与Thread关联?
2. Thread和MessageQueue的关系是?
3. MessageQueue如何运作?它如何管理Runnable和Message?
后面对这些问题一一破解。
从如何使用Handler开始
怎么样才能使Handler正常运作?
例子1——定义在子线程的Handler
public class MainActivity extends AppCompatActivity {
Handler mHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i("handler.demo", "Main Thread:" + Thread.currentThread().toString());
new MyThread().start();
//确保Handler已经在子线程中实例化
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
mHandler.post(new Runnable() {
@Override
public void run() {
Log.i("handler.demo", "Running in Thread:" + Thread.currentThread().toString());
}
});
}
class MyThread extends Thread{
@Override
public void run() {
Looper.prepare();
mHandler = new Handler();
Looper.loop();
}
}
}
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Log输出,runnable的run被调用,而且运行在子线程Thread-4中:
06-10 16:48:51.077 17181-17181/? I/handler.demo: Main Thread:Thread[main,5,main]
06-10 16:48:52.078 17181-17199/com.example.willis.myapplication I/handler.demo: Running in Thread:Thread[Thread-4,5,main]
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例子2——定义在主线程中的Handler
将上面的例子稍作修改,就可以改成子线程向主线程Handler发送消息:
public class MainActivity extends AppCompatActivity {
Handler mHandler = null;
Object mLock = new Object();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i("handler.demo", "Main Thread:" + Thread.currentThread().toString());
mHandler = new Handler();
new MyThread().start();
}
class MyThread extends Thread{
@Override
public void run() {
mHandler.post(new Runnable() {
@Override
public void run() {
Log.i("handler.demo", "Running in Thread:" + Thread.currentThread().toString());
}
});
}
}
}
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Log输出看到Runnable.run()就在主线程执行:
06-10 17:00:24.073 17526-17526/com.example.willis.myapplication I/handler.demo: Main Thread:Thread[main,5,main]
06-10 17:00:24.095 17526-17526/com.example.willis.myapplication I/handler.demo: Running in Thread:Thread[main,5,main]
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例子3——跨进程传递消息
除了在线程间使用外,Handler还可以通过IMessenger和Message来实现进程间的消息传递。
因为Message本身实现了Parcelable接口支持跨进程,Handler中定义了继承IMessenger.Stub的MessengerImpl类作为跨进程传入Message的入口。进程外通过Handler.getIMessenger()
方法获得此Handler的IMessenger即可向它发送消息。
Handler.java
IMessenger mMessenger;
final IMessenger getIMessenger() {
synchronized (mQueue) {
if (mMessenger != null) {
return mMessenger;
}
mMessenger = new MessengerImpl();
return mMessenger;
}
}
private final class MessengerImpl extends IMessenger.Stub {
public void send(Message msg) {
msg.sendingUid = Binder.getCallingUid();
Handler.this.sendMessage(msg);
}
}
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Android framework中实现了一个叫AsyncChannel的类,利用Handler夸进程特性,支持2个进程间的Handler互通消息,有兴趣可以看一下: /frameworks/base/core/java/com/android/internal/util/AsyncChannel.java
第四个问题
例子1中,Looper.prepare()
和Looper.loop()
方法调用顺序是固定的,必须在handler创建前执行Looper.prepare()
,在Handler创建后执行Looper.loop()
,这样Handler才能正常运行在线程中。
但例子2中并没有看到Looper的身影,这是为什么?
第四个问题:
Looper是什么?为什么例子中2个Looper方法要按这样的顺序调用?
Handler和Looper的关系
通过第一个例子可以猜测,Looper应该就是维护消息循环的地方,且Handler的构造方法中一定有某些东西关联到Looper,于是先从Handler的构造方法入手。
Handler构造方法
Handler有6个有参构造方法,另外还有1个个无参构造方法:
这是其中一个Handler的有参构造方法,保存了Looper,MessageQueue等实例,可以说明Handler是直接依赖于Looper的:
Handler.java
final Looper mLooper;
final MessageQueue mQueue;
final Callback mCallback;
final boolean mAsynchronous;
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
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而Handler无参构造方法内部调用了另一个有参构造方法,最后在该方法中还是通过Looper.myLooper()
方法获取到了Looper实例:
Handler.java
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
//实际上还是获取到了Looper
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;
}
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Looper.myLooper()
方法中调用ThreadLocal.get()
返回一个Looper
实例:
Looper.java
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
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既然能get到一个Looper,那么继续寻找sThreadLocal在何处去set这个Looper。
结果找上了Looper.prepare()
方法:
Looper.java
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
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));
}
//唯一构造方法是private的,创建了MessageQueue,并保存了当前的Thread实例
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);//创建MessageQueue,参数指定了这个queue是否能被退出
mThread = Thread.currentThread();//本线程
}
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Looper.prepare()
内部新建了Looper实例,并set到sThreadLocal,那么,就足以解释为什么在new Handler()
之前必须调用Looper.prepare()
。
——因为Handler需要获取到Looper实例,而Looper.prepare()
就是创建Looper的地方。
那么Looper.loop()为什么要在最后执行呢?
Looper.java
public static void loop() {
final Looper me = myLooper();
final MessageQueue queue = me.mQueue;
...
for (;;) {
//获取下一个消息
Message msg = queue.next(); // might block
if (msg == null) {
//退出循环
return;
}
try {
//向Handler分发消息
msg.target.dispatchMessage(msg);
} finally {
...
}
// Make sure that during the course of dispatching the
// identity of the thread wasn't corrupted.
final long newIdent = Binder.clearCallingIdentity();
msg.recycleUnchecked();
}
}
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明显Looper.loop()
方法内部是个死循环,循环从MessageQueue中获取消息并分发给Handler,因此loop()方法必须是最后执行的。
最后,来看为什么第二个例子中,主线程创建Handler时不用显式地初始化Looper?
根据上面2个问题的答案思考下,Handler初始化是必须获得Looper的,而Looper只有在Looper.prepare()
方法中创建。顺藤摸瓜,在Looper.java中找到了一个叫prepareMainLooper的方法,不仅创建了Looper对象,而且将它保存到了sMainLooper变量中。
Looper.java
private static Looper sMainLooper; //主线程的Looper
public static void prepareMainLooper() {
prepare(false);//新建Looper,指定
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
//复值到sMainLooper
sMainLooper = myLooper();
}
}
//quitAllowed参数指定了MessageQueue是否允许退出
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
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搜索下Looper.prepareMainLooper()
的调用者,竟然是ActivityThread.main()
方法。就是说,在Activity创建的时候,主线程创建了自己的main looper,并同样地开启了无限循环模式!这侧面映证了Android应用运行是靠消息驱动的。
ActivityThread.java
public static void main(String[] args) {
...
Process.setArgV0("<pre-initialized>");
//创建主线程Looper
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
//主线程的Event loop开始循环
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
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本章小结
至此,前面提出的4个大问题已经解决了3个:
1. Handler如何与Thread关联?
——这里需要加入Looper的概念。
Handler在线程中创建时获取到looper实例,而Looper在线程中运行消息循环,并分发给Handler。
另外:Looper的消息循环是死循环,因此一个Thread中只能运行一个Looper。而Handler作为消息的发送和处理者,与Looper的关系是多对一的。
因此它们3者的关系是: 1 Looper - 1 Thread - N Handler
2. Thread和MessageQueue的关系是?
——MessageQueue在Thread对应的Looper中创建,用于存储消息,消息最后会分发给Handler处理。
- MessageQueue如何运作?它如何管理Runnable和Message?
——待下一章分析
4. Looper是什么?为什么例子中2个Looper方法要按固定的顺序调用?
分为3个小问题:
当Handler被定义在子线程中时,为什么在new Handler
之前必须先Looper.prepare()
?
——因为Handler需要获取到Looper实例,而Looper.prepare()
就是创建Looper的地方。
为什么Looper.loop()
要最后执行?
——因为内部实现了一个死循环,用作消息的读取和分发,在Looper.quit()
被调用之前,loop循环会一直进行。
在主线程中实现的Handler,为什么无需显式地调用Looper的初始化方法?
——Activity主线程启动时,已经创建好了Looper,我们在Activity中新建的Handler默认绑定这个主线程Looper。
接下来深入分析流程。
消息发送及分发流程
接下来具体讨论消息如何插入,以及如何分发。
Message的组成和消息池
首先来了解下Message是什么。
通过文章开头的类图可以看到,Message是一个数据类,包含用户定义的数据,Runnable实例,关联的Handler。
同时,下一个Message的实例保存在next
变量中,可见Message将以链的形式保存。
//相当于消息的ID,用于在处理时识别消息
public int what;
//arg1和arg2用于存储int类型的数据
public int arg1;
public int arg2;
//保存Object类型的参数,如果在跨进程使用时只支持framework实现的可跨进程的对象
public Object obj;
//保存Bundle形式的数据参数
/*package*/ Bundle data;
//Messenger形式的消息接受者
public Messenger replyTo;
//关联的Handler(消息接收者)
/*package*/ Handler target;
//将被执行的runnable
/*package*/ Runnable callback;
//下一个消息
/*package*/ Message next;
//消息池,Message的重用管理,通过obtain()方法获取可重用的消息
private static Message sPool;
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Message中还实现了消息重用,如果使用Message.obtain()
方法获取Message,将返回可重用的Message。
private static Message sPool;
public static Message obtain() {
synchronized (sPoolSync) {
//如果pool中有可重用的message则直接返回
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();
}
//Looper.loop()处理完一个消息后,会调用此方法去“回收”Message实例
//实际上Message将被重置并放入“消息池”中。
void recycleUnchecked() {
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
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Looper中的消息循环和消息分发
正如前文看到Looper.loop()
开启了一个死循环,从MessageQueue的next方法获取消息后,分发给Handler处理。这里先快速地看下消息如何分发,然后来重点看MessageQueue的next方法。
Looper.java
/**
* 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();
...
final MessageQueue queue = me.mQueue;
...
for (;;) {
//获取下一个Message,没有消息时可能会阻塞
Message msg = queue.next(); // might block
...
try {
//执行runnable,或者分发消息给handleMessage()或callback
msg.target.dispatchMessage(msg);
} finally {
}
...
msg.recycleUnchecked();
}
}
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循环从MessageQueue.next()中获取消息,这里先来看消息的分发:Handler.dispatchMessage()
:
1. 如果通过post(Runnable)发送的Message,那么只执行Runnable.run()。
2. 如果如果实现了Handler.Callback接口,则消息分发给Callback.handleMessage()方法处理,返回ture就不会执行第三步
3. 由Handler.handleMessage()处理
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//执行runnable
handleCallback(msg);
} else {
if (mCallback != null) {
//分发给Handler.Callback处理
//如果实现了Handler.Callback接口,则消息可以在这里被处理
//如果执行完Callback.handleMessage后返回true,则不再分发给Handler.handleMessage()处理
if (mCallback.handleMessage(msg)) {
return;
}
}
//直接在handleMessage中处理
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
public interface Callback {
public boolean handleMessage(Message msg);
}
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
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获取消息:MessageQueue.next()方法
MessageQueue的next方法也是一个循环,主要目的是获取下一个要被处理的Message,其中的几个要点:
1. nativePollOnce是阻塞的,中间执行了epoll_wait等待,通过nativeWake主动唤醒或者到达超时时间后唤醒。
2. 如果插入了SyncBarrier消息(handler为null的消息),则只会处理“异步”的消息(设置了Asynchronous flag的消息,详看后文)
3. 如果当前消息没有到达when设定的时间,则会重新进入nativePollOnce,设置具体的超时时间
4. 到达设定时间的Message会被返回,由Looper分发处理。
5. 如果进入next()时没有消息要被马上处理,则会执行IdleHandler的处理。
Message next() {
final long ptr = mPtr;
...
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
...
//没有消息需要被处理时会阻塞
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 == null则该消息为SyncBarrier消息
//排在SyncBarrier之后的Message中,只有设置了Asynchronous的Message会被处理
//SyncBarrier的概念需要展开来讲。
if (msg != null && msg.target == null) {
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
//消息设置了delay时间(when是将来执行的时间)。
//还没到时间去处理,计算nextPollTimeoutMillis值,由nativePollOnce决定唤醒阻塞的时间
if (now < msg.when) {
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
//返回需要被处理的消息,并删除链节点
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
msg.markInUse();
return msg;
}
} else {
//没有任何消息,nextPollTimeoutMillis置成-1
nextPollTimeoutMillis = -1;
}
//quit()被调用后,退出循环
if (mQuitting) {
dispose();
return null;
}
//获取IdleHandler——列表中没有消息或者正等待超时期间会通知IdleHandler
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);
}
//执行IdleHandler.queueIdle()
//只有当第一次循环没有返回Message时执行,就是说当前所有Message已经处理完
//或者还没到时间处理的时候。
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 {
//通过返回值决定IdleHandler是否保留
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
//不再执行IdleHandler.queueIdle()
pendingIdleHandlerCount = 0;
//执行IdleHandler期间可能有消息插入,因此回头需要马上唤醒nativePollOnce
nextPollTimeoutMillis = 0;
}
}
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忽略IdleHandler和nextPollTimeoutMillis值的影响,大概流程如下:
nativePollOnce
nativePollOnce名字上理解应该是轮询一次的意思,代码如下:
android_os_MessageQueue.cpp在nativePollOnce方法中,调用了Looper的pollOnce方法:
/frameworks/base/core/jni/android_os_MessageQueue.cpp
static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
jlong ptr, jint timeoutMillis) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}
void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
mPollEnv = env;
mPollObj = pollObj;
mLooper->pollOnce(timeoutMillis);
mPollObj = NULL;
mPollEnv = NULL;
if (mExceptionObj) {
env->Throw(mExceptionObj);
env->DeleteLocalRef(mExceptionObj);
mExceptionObj = NULL;
}
}
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流程进入到Looper,那么先来从头了解下它。
Native looper
在java层初始化MessageQueue的时候调用了nativeInit():
MessageQueue.java
MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}
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nativeInit()初始化了NativeMessageQueue和Looper:
android_os_MessageQueue.cpp
static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
//创建NativeMessageQueue
NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
if (!nativeMessageQueue) {
jniThrowRuntimeException(env, "Unable to allocate native queue");
return 0;
}
nativeMessageQueue->incStrong(env);
return reinterpret_cast<jlong>(nativeMessageQueue);
}
NativeMessageQueue::NativeMessageQueue() :
mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
//创建looper
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}
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在Looper的构造函数中调用了rebuildEpollLocked():
1)初始化了epoll实例mEpollFd;
2)注册fd监听——mWakeEventFd。
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),
mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {
mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
AutoMutex _l(mLock);
rebuildEpollLocked();
}
void Looper::rebuildEpollLocked() {
// Close old epoll instance if we have one.
if (mEpollFd >= 0) {
close(mEpollFd);
}
//初始化了epoll实例
mEpollFd = epoll_create(EPOLL_SIZE_HINT);
struct epoll_event eventItem;
memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union
eventItem.events = EPOLLIN;
eventItem.data.fd = mWakeEventFd;
//注册fd用于唤醒——mWakeEventFd
int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);
//注册其他fd
for (size_t i = 0; i < mRequests.size(); i++) {
const Request& request = mRequests.valueAt(i);
struct epoll_event eventItem;
request.initEventItem(&eventItem);
int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);
...
}
}
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马上回到pollOnce函数,它调用的pollInner函数中执行了epoll_wait,等待mWakeEventFd和其他注册的fd被唤醒,然后分发Native消息,等到函数返回后,Java层的MessageQueue.next()才继续执行。
/system/core/libutils/Looper.cpp
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {
...
result = pollInner(timeoutMillis);
}
int Looper::pollInner(int timeoutMillis) {
//调整timeout时间
if (timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
int messageTimeoutMillis = toMillisecondTimeoutDelay(now, mNextMessageUptime);
if (messageTimeoutMillis >= 0
&& (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) {
timeoutMillis = messageTimeoutMillis;
}
}
...
//epoll_wait,等待唤醒或超时
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
...
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeEventFd) {
if (epollEvents & EPOLLIN) {
//清空mWakeEventFd管道
awoken();
} else {
ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);
}
} else {
...
}
}
Done: ;
//Native层消息的分发
...
return result;
}
//清空mWakeEventFd管道
void Looper::awoken() {
uint64_t counter;
TEMP_FAILURE_RETRY(read(mWakeEventFd, &counter, sizeof(uint64_t)));
}
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那么唤醒这个epoll_wait的地方在哪?
nativeWake
android_os_MessageQueue.cpp的nativeWake函数,调用了Looper.cpp的wake()函数,向mWakeEventFd管道写入了数据,epoll_wait被唤醒。
Looper.cpp
void Looper::wake() {
uint64_t inc = 1;
//向mWakeEventFd管道写入数据
ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t)));
...
}
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那么问题又来了,何时去唤醒这个epoll? 答案在java层插入新消息时,调用的MessageQueue.enqueueMessage()
。
插入新的Message
Handler通过post和sendMessage方法向MessageQueue发送Runnable或者Message,实际上最后都会被封装成Message,通过MessageQueue.enqueueMessage()
方法加入到消息链表。
Handler.java
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
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MessageQueue.enqueueMessage
方法接收新的消息,通过消息延迟的时间将其插入到正确的位置。
MessageQueue.java
Message mMessages;
boolean enqueueMessage(Message msg, long when) {
...
synchronized (this) {
if (mQuitting) {
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
//新消息作为链表头
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
//如果找到sync barrier,且当前消息是“异步”的,那么需要重新调整唤醒时间
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
//按照when取值将Message插入对应的位置
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
//下一个是没有设置handler的“异步”消息,无需唤醒native looper
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
//唤醒native looper
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}
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插入消息后,有条件地执行nativeWake去唤醒epoll。needWake的值依赖mBlocked——当进入next()方法时没有待处理的消息,mBlock为true,有消息并返回到looper时,mBlock为false。
结合next()和enqueueMessage()方法,得知nativeWake被调用的条件为:
1. next()方法在等待新消息,且新插入消息的为链表头时。needWake为true
2. 设置了Sync Barrier,且插入的消息是“异步”的。needWake为true
核心流程大概分析完成:
1. java层的looper循环调用MessageQueue.next()获取下一个消息来处理;
2. next()方法进入native层nativePollOnce方法,Looper.cpp进入epoll_wait等待fd被唤醒
3. Handler向MessageQueue插入消息后,有条件地唤醒native looper,使next()方法返回
4. Looper在获取到新消息后分发处理。
关于SyncBarrier
首先,Message中有“同步”和“异步”的概念(貌似实际上只是个状态的区分,主要作用时配合SyncBarrier,并没有同步性上的区别),使用setAsynchronous方法设置:
Message.java
public void setAsynchronous(boolean async) {
if (async) {
flags |= FLAG_ASYNCHRONOUS;
} else {
flags &= ~FLAG_ASYNCHRONOUS;
}
}
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而通过postSyncBarrier()方法,可以发送一个synchronization barrier(就直译称为“同步栏”吧)到Message链表中,用来暂停在同步栏之后发送的“同步”消息的处理,此时只有“异步”的消息能被继续处理。 设置同步栏后必须在条件准备好后移除它(调用removeSyncBarrier()方法)。
这个机制的作用在于可以马上暂停执行“同步”消息,直到条件允许后通过移除同步栏来恢复“同步”消息的处理。例如在View.invalidate需要执行时,将会设置同步栏挂起所有“同步”消息,直到下一帧准备好显示后移除同步栏。
而设置“异步”消息则可以免受同步栏的影响,用于接收输入等需要持续的工作,具体如下面这段注释:
/**
* Asynchronous messages are exempt from synchronization barriers. They typically
* represent interrupts, input events, and other signals that must be handled
* independently even while other work has been suspended.
* */
工作流程大致如图:
设置同步栏就是插入一个不指定handler的Message,通过一个token值来标记:
MessageQueue.java
/**
* @hide
*/
//设置同步栏
//postSyncBarrier不是公开的API,只供系统内部调用
public int postSyncBarrier() {
return postSyncBarrier(SystemClock.uptimeMillis());
}
private int postSyncBarrier(long when) {
synchronized (this) {
final int token = mNextBarrierToken++;
//消息没有设置handler,以此来识别这是个sync barrier
final Message msg = Message.obtain();
msg.markInUse();
msg.when = when;
msg.arg1 = token;
//插入到正确的位置
Message prev = null;
Message p = mMessages;
if (when != 0) {
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
}
if (prev != null) { // invariant: p == prev.next
msg.next = p;
prev.next = msg;
} else {
msg.next = p;
mMessages = msg;
}
return token;
}
}
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移除同步栏时通过token值匹配并删除该Message:
/**
* @hide
*/
//移除同步栏同样是hide的方法
public void removeSyncBarrier(int token) {
// Remove a sync barrier token from the queue.
// If the queue is no longer stalled by a barrier then wake it.
synchronized (this) {
Message prev = null;
Message p = mMessages;
while (p != null && (p.target != null || p.arg1 != token)) {
prev = p;
p = p.next;
}
if (p == null) {
throw new IllegalStateException("The specified message queue synchronization "
+ " barrier token has not been posted or has already been removed.");
}
final boolean needWake;
if (prev != null) {
prev.next = p.next;
needWake = false;
} else {
mMessages = p.next;
needWake = mMessages == null || mMessages.target != null;
}
p.recycleUnchecked();
// If the loop is quitting then it is already awake.
// We can assume mPtr != 0 when mQuitting is false.
if (needWake && !mQuitting) {
nativeWake(mPtr);
}
}
}
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关于IdleHandler
- 只有当前所有Message已经处理完或者待处理的Message还没到时间处理的时候,才会执行IdleHandler处理一次。
- 使用IdleHandler必须定义实现了IdleHandler接口的类,并在queueIdle()定义需要执行的操作(通常是释放资源),返回值决定这个Handler是否一直保留,并在将来空闲时再次执行。
- 通过MessageQueue.addIdleHandler 添加IdleHandler
MessageQueue.java
private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();
private SparseArray<FileDescriptorRecord> mFileDescriptorRecords;
private IdleHandler[] mPendingIdleHandlers;
public static interface IdleHandler {
boolean queueIdle();
}
public void addIdleHandler(@NonNull IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
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一个例子:
ActivityThread中定义了一个IdleHandler,用于执行GC回收垃圾:
ActivityThread.java
final class GcIdler implements MessageQueue.IdleHandler {
@Override
public final boolean queueIdle() {
doGcIfNeeded();
return false;
}
}
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END
写得有点杂乱,以后有时间再来优化下。
在分析过程中,参考了下面几篇文章,写的比较清晰易懂:
《聊一聊Android的消息机制》
《Looper中的睡眠等待与唤醒机制》