Android文件监控FileObserver介绍

在前面的Linux文件系统Inotify机制 中介绍了Linux对文件变更监控过程。Android系统在此基础上封装了一个FileObserver类来方便使用Inotify机制。FileObserver是一个抽象类，需要定义子类实现该类的onEvent抽象方法，当被监控的文件或者目录发生变更事件时，将回调FileObserver的onEvent()函数来处理文件或目录的变更事件。

事件监控过程

在FileObserver类中定义了一个静态内部类ObserverThread，该线程类才是真正实现文件或目录监控过程。各种类型的FileObserver都拥有一个ObserverThread实例：

frameworks\base\core\java\android\os\FileObserver.java

public abstract class FileObserver {
    //可监控的事件类型
	public static final int ALL_EVENTS = ACCESS | MODIFY | ATTRIB | CLOSE_WRITE
            | CLOSE_NOWRITE | OPEN | MOVED_FROM | MOVED_TO | DELETE | CREATE | DELETE_SELF | MOVE_SELF;
	//静态创建并启动一个文件监控线程
	private static ObserverThread s_observerThread;
    static {
        s_observerThread = new ObserverThread();
        s_observerThread.start();
    }
    // instance
    private String m_path;
    private Integer m_descriptor;
    private int m_mask;
}

FileObserver类通过静态方式构造了一个ObserverThread对象：

public ObserverThread() {
	super("FileObserver");
	m_fd = init();//初始化一个inotify实例，Observer线程就是对该inotify实例进行监控
}

frameworks\base\core\jni\android_util_FileObserver.cpp

static jint android_os_fileobserver_init(JNIEnv* env, jobject object)
{
#ifdef HAVE_INOTIFY
    return (jint)inotify_init();//初始化一个inotify实例   
#else // HAVE_INOTIFY
    return -1;
#endif // HAVE_INOTIFY
}

inotify_init()函数实现在Linux文件系统Inotify机制 有详细介绍，然后启动ObserverThread线程，ObserverThread线程运行体：

frameworks\base\core\java\android\os\FileObserve$ObserverThread

public void run() {
	observe(m_fd);//监控inotify实例句柄
}

frameworks\base\core\jni\android_util_FileObserver.cpp

static void android_os_fileobserver_observe(JNIEnv* env, jobject object, jint fd)
{
#ifdef HAVE_INOTIFY
    char event_buf[512];//定义事件数组
    struct inotify_event* event;
    while (1)
    {
        int event_pos = 0;
		//从inotify实例句柄中读取事件
        int num_bytes = read(fd, event_buf, sizeof(event_buf));
        if (num_bytes < (int)sizeof(*event))
        {
            if (errno == EINTR)
                continue;
            ALOGE("***** ERROR! android_os_fileobserver_observe() got a short event!");
            return;
        }
	    //循环处理读取到的事件
        while (num_bytes >= (int)sizeof(*event))
        {
            int event_size;
            event = (struct inotify_event *)(event_buf + event_pos);
            jstring path = NULL;
            if (event->len > 0)
            {
                path = env->NewStringUTF(event->name);
            }
			//调用ObserverThread的onEvent函数通知上层响应
            env->CallVoidMethod(object, method_onEvent, event->wd, event->mask, path);
            if (env->ExceptionCheck()) {
                env->ExceptionDescribe();
                env->ExceptionClear();
            }
            if (path != NULL)
            {
                env->DeleteLocalRef(path);
            }
            event_size = sizeof(*event) + event->len;
            num_bytes -= event_size;
            event_pos += event_size;
        }
    }
#endif // HAVE_INOTIFY
}

ObserverThread线程循环从inotify实例句柄中读取事件，然后回调ObserverThread的onEvent函数来处理事件。

frameworks\base\core\java\android\os\FileObserve$ObserverThread

public void onEvent(int wfd, int mask, String path) {
	// look up our observer, fixing up the map if necessary...
	FileObserver observer = null;
	synchronized (m_observers) {
		//根据wfd句柄从m_observers表中查找出注册的FileObserver对象
		WeakReference weak = m_observers.get(wfd);
		if (weak != null) {  // can happen with lots of events from a dead wfd
			observer = (FileObserver) weak.get();
			if (observer == null) {
				m_observers.remove(wfd);
			}
		}
	}
	// ...then call out to the observer without the sync lock held
	if (observer != null) {
		try {
			//调用对应的FileObserver对象的onEvent函数来处理事件
			observer.onEvent(mask, path);
		} catch (Throwable throwable) {
			Log.wtf(LOG_TAG, "Unhandled exception in FileObserver " + observer, throwable);
		}
	}
}

注册监控watch

FileObserver类提供了startWatching()函数来启动文件监控

frameworks\base\core\java\android\os\FileObserver.java

public void startWatching() {
	if (m_descriptor < 0) {
		m_descriptor = s_observerThread.startWatching(m_path, m_mask, this);
	}
}

由ObserverThread线程对象启动监控

frameworks\base\core\java\android\os\FileObserver$ObserverThread

public int startWatching(String path, int mask, FileObserver observer) {
	//在Inotify实例中添加一个watch对象，并得到一个watch对象句柄
	int wfd = startWatching(m_fd, path, mask);
	Integer i = new Integer(wfd);
	if (wfd >= 0) {
		//将watch对象句柄和响应该watch事件的FileObserver以键值对的形式保存在m_observers成员变量中
		synchronized (m_observers) {
			m_observers.put(i, new WeakReference(observer));
		}
	}
	return i;
}

ObserverThread又调用native方法android_os_fileobserver_startWatching()来添加一个watch

frameworks\base\core\jni\android_util_FileObserver.cpp

static jint android_os_fileobserver_startWatching(JNIEnv* env, jobject object, jint fd, jstring pathString, jint mask)
{
    int res = -1;
#ifdef HAVE_INOTIFY
    if (fd >= 0)
    {
        const char* path = env->GetStringUTFChars(pathString, NULL);
		//在Inotify实例上添加一个watch对象
        res = inotify_add_watch(fd, path, mask);
        env->ReleaseStringUTFChars(pathString, path);
    }
#endif // HAVE_INOTIFY
    return res;
}

注销监控watch

FileObserver类提供了使用stopWatching()函数来停止文件监控。
frameworks\base\core\java\android\os\FileObserver$ObserverThread

public void stopWatching() {
	if (m_descriptor >= 0) {
		s_observerThread.stopWatching(m_descriptor);
		m_descriptor = -1;
	}
}

frameworks\base\core\java\android\os\FileObserve$ObserverThread

public void stopWatching(int descriptor) {
	stopWatching(m_fd, descriptor);
}

frameworks\base\core\jni\android_util_FileObserver.cpp

static void android_os_fileobserver_stopWatching(JNIEnv* env, jobject object, jint fd, jint wfd)
{
#ifdef HAVE_INOTIFY
    inotify_rm_watch((int)fd, (uint32_t)wfd);
#endif // HAVE_INOTIFY
}