[Android] Handler源码解析 (Native层)

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：

// 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);

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。

MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}

nativeInit()的实现在/frameworks/base/core/jni/android_os_MessageQueue.cpp中：

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);
}

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：

static void android_os_MessageQueue_nativeDestroy(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->decStrong(env);
}

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：

NativeMessageQueue::NativeMessageQueue() : mInCallback(false), mExceptionObj(NULL) {
mLooper = Looper::getForThread();
if (mLooper == NULL) {
mLooper = new Looper(false);
Looper::setForThread(mLooper);
}
}

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：

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);
}

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：

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();
}

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：

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);
}
}

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：

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;
}

awoken()的实现代码如下所示：

void Looper::awoken() {
char buffer[16];
ssize_t nRead;
do {
nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer));
} while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer));
}

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：

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);
}
}
}

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：

+------------------+
|      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    |
+-+--
ad0d
--------+-+
|          |
epoll_wait |          |  wake
+-------------v-+      +-v--------------+
|mWakeReadPipeFd|      |mWakeWritePipeFd|
+-------------^-+      +-+--------------+
|          |
read  |          | write
|          |
+-+----------v-+
|     Pipe     |
+--------------+

查看原文：http://legendmohe.net/2015/10/26/android-handler%e6%ba%90%e7%a0%81%e8%a7%a3%e6%9e%90-native%e5%b1%82/