接前文[Android] Handler源码解析 (Java层)，接下来对Handler机制在Native层上作解析。
Java层的MessageQueue中有4个native方法：
1 2 3 4 5 6 7 8 9 | // 初始化和销毁 private native static long nativeInit(); private native static void nativeDestroy(long ptr); // 等待和唤醒 private native static void nativePollOnce(long ptr, int timeoutMillis); private native static void nativeWake(long ptr); // 判断native层的状态 private native static boolean nativeIsIdling(long ptr);
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下面分别进行介绍。
NATIVEINIT()和NATIVEDESTROY(LONG PTR)
nativeInit()在MessageQueue初始化时被调用，返回一个long值，保存在mPtr中。
1 2 3 4 5 | MessageQueue(boolean quitAllowed) { mQuitAllowed = quitAllowed; mPtr = nativeInit(); }
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nativeInit()的实现在/frameworks/base/core/jni/android_os_MessageQueue.cpp中：
1 2 3 4 5 6 7 8 9 10 11 | 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); }
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该JNI方法新建了一个NativeMessageQueue对象，然后将其指针用reinterpret_cast为long并返回给java层。同样地：
1 2 3 4 5 | static void android_os_MessageQueue_nativeDestroy(JNIEnv* env, jclass clazz, jlong ptr) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); nativeMessageQueue->decStrong(env); }
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nativeDestory()方法中，将long型的ptr转换为NativeMessageQueue指针，然后再销毁对象。
NativeMessageQueue对象初始化的代码如下所示：
1 2 3 4 5 6 7 8 | NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) { mLooper = Looper::getForThread(); if (mLooper == NULL) { mLooper = new Looper(false); Looper::setForThread(mLooper); } }
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可以看到初始化方法中对mLooper进行了赋值。留意到Looper::getForThread();一句，结合其下的代码，猜想这是类似ThreadLocal模式的应用。接下来看看Looper类。
Looper类的声明在/system/core/include/utils/中，实现在/system/core/libutils/中，先来看一下Looper类的初始化方法：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 | Looper::Looper(bool allowNonCallbacks) : mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) { int wakeFds[2]; // 1. 创建一个匿名管道， // wakeFds[0]代表管道的输出，应用程序读它。 // wakeFds[1]代表管道的输入，应用程序写它。 int result = pipe(wakeFds); LOG_ALWAYS_FATAL_IF(result != 0, "Could not create wake pipe. errno=%d", errno);
mWakeReadPipeFd = wakeFds[0]; mWakeWritePipeFd = wakeFds[1];
// 2. 设置读写管道为non-blocking result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK); LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake read pipe non-blocking. errno=%d", errno);
result = fcntl(mWakeWritePipeFd, F_SETFL, O_NONBLOCK); LOG_ALWAYS_FATAL_IF(result != 0, "Could not make wake write pipe non-blocking. errno=%d", errno);
mIdling = false;
// 3. 新建epoll实体，并将读管道注册到epoll 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 // 表示对应的文件描述符可以读时触发event eventItem.events = EPOLLIN; eventItem.data.fd = mWakeReadPipeFd; result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeReadPipeFd, & eventItem); LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake read pipe to epoll instance. errno=%d", errno); }
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从上面可以看出，Looper对象中维护着两个描述符，分别用于读和写。其中读描述符注册到epoll中。合理猜想looper的夸进程的睡眠和唤醒机制是通过epoll实现的。目标线程在读描述符mWakeReadPipeFd上等待，其他线程往mWakeWritePipeFd写入数据时，即可通过epoll机制将目标线程唤醒。
NATIVEPOLLONCE(LONG PTR, INT TIMEOUTMILLIS)和NATIVEWAKE(LONG PTR)
nativePollOnce和nativeWake方法的实现如下所示：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | void NativeMessageQueue::pollOnce(JNIEnv* env, int timeoutMillis) { mInCallback = true; mLooper->pollOnce(timeoutMillis); mInCallback = false; if (mExceptionObj) { env->Throw(mExceptionObj); env->DeleteLocalRef(mExceptionObj); mExceptionObj = NULL; } }
void NativeMessageQueue::wake() { mLooper->wake(); }
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可见这两个方法只是对Looper类的pollOnce和wake方法的简单封装。先看一下Looper对象的pollOnce方法实现如下所示：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 | inline int pollOnce(int timeoutMillis) { return pollOnce(timeoutMillis, NULL, NULL, NULL); }
...
int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) { int result = 0; for (;;) { while (mResponseIndex < mResponses.size()) { const Response& response = mResponses.itemAt(mResponseIndex++); int ident = response.request.ident; if (ident >= 0) { int fd = response.request.fd; int events = response.events; void* data = response.request.data; if (outFd != NULL) *outFd = fd; if (outEvents != NULL) *outEvents = events; if (outData != NULL) *outData = data; return ident; } }
if (result != 0) { if (outFd != NULL) *outFd = 0; if (outEvents != NULL) *outEvents = 0; if (outData != NULL) *outData = NULL; return result; }
result = pollInner(timeoutMillis); } }
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先不管什么mResponses、outFd、outEvents和outData，我们先来看一下pollInner的实现。pollInner实现比较复杂，这里只看对本文有用的部分：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 | int Looper::pollInner(int timeoutMillis) {
...
// 1. 设置默认result int result = POLL_WAKE;
...
// 2. 开始在mWakeReadPipeFd上等待 mIdling = true;
struct epoll_event eventItems[EPOLL_MAX_EVENTS]; int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
// 3. 等待结束 mIdling = false;
...
// 4. 根据epoll_wait返回的结果设置result if (eventCount < 0) { if (errno == EINTR) { goto Done; } ALOGW("Poll failed with an unexpected error, errno=%d", errno); result = POLL_ERROR; goto Done; }
// Check for poll timeout. if (eventCount == 0) { result = POLL_TIMEOUT; goto Done; }
// 5. 通过awoken()从mWakeReadPipeFd读出标记字符“W”
for (int i = 0; i < eventCount; i++) { int fd = eventItems[i].data.fd; uint32_t epollEvents = eventItems[i].events; if (fd == mWakeReadPipeFd) { if (epollEvents & EPOLLIN) { awoken(); } else { ALOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents); } } else { ... } } Done: ;
...
return result; }
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awoken()的实现代码如下所示：
1 2 3 4 5 6 7 8 | void Looper::awoken() { char buffer[16]; ssize_t nRead; do { nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer)); } while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer)); }
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awoken()只是简单地读出wake()在mWakeWritePipeFd上写入的数据。Looper对象的wake方法实现如下所示：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | void Looper::wake() {
ssize_t nWrite; do { nWrite = write(mWakeWritePipeFd, "W", 1); } while (nWrite == -1 && errno == EINTR);
if (nWrite != 1) { if (errno != EAGAIN) { ALOGW("Could not write wake signal, errno=%d", errno); } } }
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正如前面所述，往mWakeWritePipeFd写数据即可唤醒在mWakeReadPipeFd上等待的线程。
总结
综上，在native层，一次wait/wake过程简述如下：
native层Looper对象初始化时，新建了一个匿名管道，并将读管道（mWakeReadPipeFd）注册到epoll上。
pollOnce方法调用pollInner方法，其中epoll_wait方法在mWakeReadPipeFd上等待读取。（wait）
wake方法被调用，往写管道（mWakeWritePipeFd）上写入字符“W”。
pollInner方法继续执行，调用awoken从mWakeReadPipeFd读出数据。（wake）
可画出框架图如下所示：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 | +------------------+ | Handler | +----^--------+----+ | | dispatch | | send | | | v +----+ <---+ | | | Looper | | | | | +---> +----+ ^ | next | | enqueue | | +--------+------v----------+ | MessageQueue | +--------+------+----------+ | | nativePollOnce | | nativeWake | | +----------------------------------------------+ Native Layer | | pollOnce | | wake | | +--------v------v--------+ | NativeMessageQueue | +--------+------+--------+ | | pollOnce | | wake pollInner| | awoken | | +---v------v---+ | Looper | +-+----------+-+ | | epoll_wait | | wake +-------------v-+ +-v--------------+ |mWakeReadPipeFd| |mWakeWritePipeFd| +-------------^-+ +-+--------------+ | | read | | write | | +-+----------v-+ | Pipe | +--------------+ |
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