Android native应用开发简明教程 (2) - 本地应用的原理

本地应用原理

从NativeActivity说起

本地App，本质上是一个Java App调用了一个JNI的库，而应用的逻辑通过native代码来实现。
NativeActivity是继承自Activity的一个类，代码在：/frameworks/base/core/java/android/app/NativeActivity.java中：

public class NativeActivity extends Activity implements SurfaceHolder.Callback2,InputQueue.Callback, OnGlobalLayoutListener{
...

然后我们再看NativeActivity的onCreate方法：

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         String libname = "main";
         String funcname = "ANativeActivity_onCreate";
         ActivityInfo ai;
...

这里我们就看到了一个重要的函数：ANativeActivity_onCreate，这个函数将是本地应用的入口函数。

...
         try {
             ai = getPackageManager().getActivityInfo(
                     getIntent().getComponent(), PackageManager.GET_META_DATA);
             if (ai.metaData != null) {
                 String ln = ai.metaData.getString(META_DATA_LIB_NAME);
                 if (ln != null) libname = ln;
                 ln = ai.metaData.getString(META_DATA_FUNC_NAME);
                 if (ln != null) funcname = ln;
             }
         } catch (PackageManager.NameNotFoundException e) {
             throw new RuntimeException("Error getting activity info", e);
         }
 
         BaseDexClassLoader classLoader = (BaseDexClassLoader) getClassLoader();
         String path = classLoader.findLibrary(libname);
...

大家还记得上一讲我们贴的AndroidManifest.xml吗？记得这一行meta data吗：

我们的本地应用生成的so库的名字，就是通过这里来读出来的。

最后，通用本地代码的入口在这里:

         mNativeHandle = loadNativeCode(path, funcname, Looper.myQueue(),
                 getAbsolutePath(getFilesDir()), getAbsolutePath(getObbDir()),
                 getAbsolutePath(getExternalFilesDir(null)),
                 Build.VERSION.SDK_INT, getAssets(), nativeSavedState,
                 classLoader, classLoader.getLdLibraryPath());

入口点函数是funcname，大家还记得它的定义吧：

String funcname = "ANativeActivity_onCreate";
...
                 ln = ai.metaData.getString(META_DATA_FUNC_NAME);
                 if (ln != null) funcname = ln;

默认情况下，这个入口函数就是ANativeActivity_onCreate.

JNI层的实现

我们继续跟进去看jni中对于loadNativeCode_native的调用：

 loadNativeCode_native(JNIEnv* env, jobject clazz, jstring path, jstring funcName,
         jobject messageQueue, jstring internalDataDir, jstring obbDir,
         jstring externalDataDir, jint sdkVersion, jobject jAssetMgr,
         jbyteArray savedState, jobject classLoader, jstring libraryPath) {
...

下面，根据funcName，就是默认是ANativeActivity_onCreate的这个函数，将变成函数指针，类型为ANativeActivity_createFunc

...
         void* funcPtr = NULL;
         const char* funcStr = env->GetStringUTFChars(funcName, NULL);
         if (needNativeBridge) {
             funcPtr = NativeBridgeGetTrampoline(handle, funcStr, NULL, 0);
         } else {
             funcPtr = dlsym(handle, funcStr);
         }
         code.reset(new NativeCode(handle, (ANativeActivity_createFunc*)funcPtr));
         env->ReleaseStringUTFChars(funcName, funcStr);
 
         if (code->createActivityFunc == NULL) {
             ALOGW("ANativeActivity_onCreate not found");
             return 0;
         }
...

android_native_app_glue的实现

我们再来看看NDK中的android_native_app_glue中，入口函数ANativeActivity_onCreate都做了些什么：

void ANativeActivity_onCreate(ANativeActivity* activity,
        void* savedState, size_t savedStateSize) {
    LOGV("Creating: %p\n", activity);
    activity->callbacks->onDestroy = onDestroy;
    activity->callbacks->onStart = onStart;
    activity->callbacks->onResume = onResume;
    activity->callbacks->onSaveInstanceState = onSaveInstanceState;
    activity->callbacks->onPause = onPause;
    activity->callbacks->onStop = onStop;
    activity->callbacks->onConfigurationChanged = onConfigurationChanged;
    activity->callbacks->onLowMemory = onLowMemory;
    activity->callbacks->onWindowFocusChanged = onWindowFocusChanged;
    activity->callbacks->onNativeWindowCreated = onNativeWindowCreated;
    activity->callbacks->onNativeWindowDestroyed = onNativeWindowDestroyed;
    activity->callbacks->onInputQueueCreated = onInputQueueCreated;
    activity->callbacks->onInputQueueDestroyed = onInputQueueDestroyed;

    activity->instance = android_app_create(activity, savedState, savedStateSize);
}

可以看到，这个封装中，除了设置了若干个callback函数以外，就是封装成了android_app_create函数。

我们再来看android_app_create的实现：

static struct android_app* android_app_create(ANativeActivity* activity,
        void* savedState, size_t savedStateSize) {
    struct android_app* android_app = (struct android_app*)malloc(sizeof(struct android_app));
    memset(android_app, 0, sizeof(struct android_app));
    android_app->activity = activity;

    pthread_mutex_init(&android_app->mutex, NULL);
    pthread_cond_init(&android_app->cond, NULL);

    if (savedState != NULL) {
        android_app->savedState = malloc(savedStateSize);
        android_app->savedStateSize = savedStateSize;
        memcpy(android_app->savedState, savedState, savedStateSize);
    }

    int msgpipe[2];
    if (pipe(msgpipe)) {
        LOGE("could not create pipe: %s", strerror(errno));
        return NULL;
    }
    android_app->msgread = msgpipe[0];
    android_app->msgwrite = msgpipe[1];

    pthread_attr_t attr; 
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
    pthread_create(&android_app->thread, &attr, android_app_entry, android_app);

    // Wait for thread to start.
    pthread_mutex_lock(&android_app->mutex);
    while (!android_app->running) {
        pthread_cond_wait(&android_app->cond, &android_app->mutex);
    }
    pthread_mutex_unlock(&android_app->mutex);

    return android_app;
}

这个封装处理了线程安全相关的问题，创新主线程来执行android_app_entry函数。

static void* android_app_entry(void* param) {
    struct android_app* android_app = (struct android_app*)param;

    android_app->config = AConfiguration_new();
    AConfiguration_fromAssetManager(android_app->config, android_app->activity->assetManager);

    print_cur_config(android_app);

    android_app->cmdPollSource.id = LOOPER_ID_MAIN;
    android_app->cmdPollSource.app = android_app;
    android_app->cmdPollSource.process = process_cmd;
    android_app->inputPollSource.id = LOOPER_ID_INPUT;
    android_app->inputPollSource.app = android_app;
    android_app->inputPollSource.process = process_input;

    ALooper* looper = ALooper_prepare(ALOOPER_PREPARE_ALLOW_NON_CALLBACKS);
    ALooper_addFd(looper, android_app->msgread, LOOPER_ID_MAIN, ALOOPER_EVENT_INPUT, NULL,
            &android_app->cmdPollSource);
    android_app->looper = looper;

    pthread_mutex_lock(&android_app->mutex);
    android_app->running = 1;
    pthread_cond_broadcast(&android_app->cond);
    pthread_mutex_unlock(&android_app->mutex);

    android_main(android_app);

    android_app_destroy(android_app);
    return NULL;
}

这一层的封装，将处理AConfiguration。但是最主要的是要处理ALooper，构建一个消息队列。
最终才会调用android_main，这一层才是android_native_app_glue封装好的对外接口。

小结

• Android支持通过本质上是个so的本地代码来写应用
• 骨架代码是Android framework中的NativeActivity，该类通过装载so库的方式来调用本地应用代码
• 默认的入口点在ANativeActivity_onCreate，函数名和库名都可以在AndroidManifest.xml中配置

最后，我们来看一张图来复习一下：

ANativeActivity procedure