編輯:關於Android編程
在前面的Linux文件系統Inotify機制 中介紹了Linux對文件變更監控過程。Android系統在此基礎上封裝了一個FileObserver類來方便使用Inotify機制。FileObserver是一個抽象類,需要定義子類實現該類的onEvent抽象方法,當被監控的文件或者目錄發生變更事件時,將回調FileObserver的onEvent()函數來處理文件或目錄的變更事件。
在FileObserver類中定義了一個靜態內部類ObserverThread,該線程類才是真正實現文件或目錄監控過程。各種類型的FileObserver都擁有一個ObserverThread實例:
frameworks\base\core\java\android\os\FileObserver.java
public abstract class FileObserver { //可監控的事件類型 public static final int ALL_EVENTS = ACCESS | MODIFY | ATTRIB | CLOSE_WRITE | CLOSE_NOWRITE | OPEN | MOVED_FROM | MOVED_TO | DELETE | CREATE | DELETE_SELF | MOVE_SELF; //靜態創建並啟動一個文件監控線程 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類通過靜態方式構造了一個ObserverThread對象:
public ObserverThread() { super("FileObserver"); m_fd = init();//初始化一個inotify實例,Observer線程就是對該inotify實例進行監控 }
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();//初始化一個inotify實例 #else // HAVE_INOTIFY return -1; #endif // HAVE_INOTIFY }
inotify_init()函數實現在Linux文件系統Inotify機制 有詳細介紹,然後啟動ObserverThread線程,ObserverThread線程運行體:
frameworks\base\core\java\android\os\FileObserve$ObserverThread
public void run() { observe(m_fd);//監控inotify實例句柄 }
frameworks\base\core\jni\android_util_FileObserver.cpp
static void android_os_fileobserver_observe(JNIEnv* env, jobject object, jint fd) { #ifdef HAVE_INOTIFY char event_buf[512];//定義事件數組 struct inotify_event* event; while (1) { int event_pos = 0; //從inotify實例句柄中讀取事件 int num_bytes = read(fd, event_buf, sizeof(event_buf)); if (num_bytes < (int)sizeof(*event)) { if (errno == EINTR) continue; ALOGE("***** ERROR! android_os_fileobserver_observe() got a short event!"); return; } //循環處理讀取到的事件 while (num_bytes >= (int)sizeof(*event)) { int event_size; event = (struct inotify_event *)(event_buf + event_pos); jstring path = NULL; if (event->len > 0) { path = env->NewStringUTF(event->name); } //調用ObserverThread的onEvent函數通知上層響應 env->CallVoidMethod(object, method_onEvent, event->wd, event->mask, path); if (env->ExceptionCheck()) { env->ExceptionDescribe(); env->ExceptionClear(); } if (path != NULL) { env->DeleteLocalRef(path); } event_size = sizeof(*event) + event->len; num_bytes -= event_size; event_pos += event_size; } } #endif // HAVE_INOTIFY }
ObserverThread線程循環從inotify實例句柄中讀取事件,然後回調ObserverThread的onEvent函數來處理事件。
frameworks\base\core\java\android\os\FileObserve$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) { //根據wfd句柄從m_observers表中查找出注冊的FileObserver對象 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 { //調用對應的FileObserver對象的onEvent函數來處理事件 observer.onEvent(mask, path); } catch (Throwable throwable) { Log.wtf(LOG_TAG, "Unhandled exception in FileObserver " + observer, throwable); } } }
FileObserver類提供了startWatching()函數來啟動文件監控
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線程對象啟動監控
frameworks\base\core\java\android\os\FileObserver$ObserverThread
public int startWatching(String path, int mask, FileObserver observer) { //在Inotify實例中添加一個watch對象,並得到一個watch對象句柄 int wfd = startWatching(m_fd, path, mask); Integer i = new Integer(wfd); if (wfd >= 0) { //將watch對象句柄和響應該watch事件的FileObserver以鍵值對的形式保存在m_observers成員變量中 synchronized (m_observers) { m_observers.put(i, new WeakReference(observer)); } } return i; }
ObserverThread又調用native方法android_os_fileobserver_startWatching()來添加一個watch
frameworks\base\core\jni\android_util_FileObserver.cpp
static jint android_os_fileobserver_startWatching(JNIEnv* env, jobject object, jint fd, jstring pathString, jint mask) { int res = -1; #ifdef HAVE_INOTIFY if (fd >= 0) { const char* path = env->GetStringUTFChars(pathString, NULL); //在Inotify實例上添加一個watch對象 res = inotify_add_watch(fd, path, mask); env->ReleaseStringUTFChars(pathString, path); } #endif // HAVE_INOTIFY return res; }
FileObserver類提供了使用stopWatching()函數來停止文件監控。
frameworks\base\core\java\android\os\FileObserver$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 }
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