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Looper与MessageQueue的初始化
我们知道每个应用都有一个MainLooper，默认创建的Handler是运行在主线程的，那这一过程是怎么样的呢？原来，在AMS创建Activity的时候，会创建MainLooper并开启消息循环，下面来具体分析下这个过程：
AMS创建MainLooper：
// frameworks\base\core\java\android\app\ActivityThread.java public static void main(String[] args) { ... Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); Looper.loop(); } 经过Looper.prepareMainLooper()->prepare(false)后，Looper创建了mainLooper并保存在sThreadLocal中，同时在执行构造方法的时候创建了MessageQueue
// frameworks\base\core\java\android\os\Looper.java static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); final MessageQueue mQueue; final Thread mThread; public static void prepareMainLooper() { prepare(false); synchronized (Looper.class) { if (sMainLooper != null) { throw new IllegalStateException("The main Looper has already been prepared."); } sMainLooper = myLooper(); } } 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(); } public static @Nullable Looper myLooper() { return sThreadLocal.get(); } java层MessageQueue在构造的时候，执行了nativeInit()并将返回值赋给mPtr，下面我们找到对应的cpp
// frameworks\base\core\java\android\os\MessageQueue.java private final boolean mQuitAllowed; @SuppressWarnings("unused") private long mPtr; // used by native code MessageQueue(boolean quitAllowed) { mQuitAllowed = quitAllowed; mPtr = nativeInit(); } jni层nativeInit()创建了NativeMessageQueue对象，而NativeMessageQueue是c++层MessageQueue的子类，其中在构造NativeMessageQueue()时候创建了c++层的Looper
// frameworks\base\core\jni\android_os_MessageQueue.cpp static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) { NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue(); if (!nativeMessageQueue) { jniThrowRuntimeException(env, "Unable to allocate native queue"); return 0; } nativeMessageQueue->incStrong(env); return reinterpret_cast<jlong>(nativeMessageQueue); }// frameworks\base\core\jni\android_os_MessageQueue.cpp NativeMessageQueue::NativeMessageQueue() : mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) { mLooper = Looper::getForThread(); if (mLooper == NULL) { mLooper = new Looper(false); Looper::setForThread(mLooper); } } 下面我们来分析c++层的Looper
// system\core\libutils\Looper.cpp 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); LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd. errno=%d", errno); AutoMutex _l(mLock); rebuildEpollLocked(); } 上面 rebuildEpollLocked()方法具体如下：其中 mEpollFd = epoll_create(EPOLL_SIZE_HINT)，epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem)，可以看出这里使用pipe系统调用来创建了一个管道用于进程间的通讯
void Looper::rebuildEpollLocked() { // Close old epoll instance if we have one. if (mEpollFd >= 0) { #if DEBUG_CALLBACKS ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this); #endif close(mEpollFd); } // Allocate the new epoll instance and register the wake pipe. mEpollFd = epoll_create(EPOLL_SIZE_HINT); LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance. errno=%d", errno); 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; int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem); LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance. errno=%d", errno); 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); if (epollResult < 0) { ALOGE("Error adding epoll events for fd %d while rebuilding epoll set, errno=%d", request.fd, errno); } } }至此，Java层与c++层Looper和Message均已创建完成，总结一下上面的过程：
- AMS创建activity的时候，创建MainLooper
- java层Looper创建后会构造MessageQueue，创建jni层
- jni层NativeMessageQueue创建后，保存在Java层的消息队列对象mQueue的成员变量mPtr中
- c++层创建Looper后，如果java层消息队列中没有消息，就会使Android应用程序主线程位于等待状态，如果java层消息队列来了消息后，就会唤醒Android应用程序主线程来处理这个消息
2.Looper消息循环
Looper不断地遍历消息队列中的消息进行发送，Handler收到消息进行相应的处理
// frameworks\base\core\java\android\os\Looper.java
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.recycleUnchecked();
}
}
// frameworks\base\core\java\android\os\Handler.java
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
3.消息的发送
创建一个Handler实例后，相应的Looper与MessageQueue都关联了起来
// frameworks\base\core\java\android\os\Handler.java
public Handler() {
this(null, false);
}
public Handler(Callback callback, boolean async) {
...
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread " + Thread.currentThread()
+ " that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
创建一个消息
// frameworks\base\core\java\android\os\Message.java
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();
}
发送消息：将消息放进消息队列，nativeWake(mPtr)来传递到c++层Looper，通过write方式来往管道到写数据，唤醒Android应用程序主线程。
// frameworks\base\core\java\android\os\Message.java
...
public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
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(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
// frameworks\base\core\java\android\os\MessageQueue.java
boolean enqueueMessage(Message msg, long when) {
...
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;
}