Android源码中的jni方法追踪

Android源码中的jni方法通常不是按照Java_路径名_方法名定义的，都是动态注册的，那么Android的JNI_Onload函数在哪里，我们又怎么去查询对应的函数实现呢？

答案就在AndroidRuntime.cpp

这里定义了动态加载的RegJNIRec结构体数组

static const RegJNIRec gRegJNI[] = {
    REG_JNI(register_com_android_internal_os_RuntimeInit),
    REG_JNI(register_android_os_SystemClock),
    REG_JNI(register_android_util_EventLog),
    REG_JNI(register_android_util_Log),
    REG_JNI(register_android_util_MemoryIntArray),
    REG_JNI(register_android_util_PathParser),
    REG_JNI(register_android_app_admin_SecurityLog),
    REG_JNI(register_android_content_AssetManager),
    REG_JNI(register_android_content_StringBlock),
    REG_JNI(register_android_content_XmlBlock),
    REG_JNI(register_android_text_AndroidCharacter),
    REG_JNI(register_android_text_StaticLayout),
    REG_JNI(register_android_text_AndroidBidi),
    REG_JNI(register_android_view_InputDevice),
    REG_JNI(register_android_view_KeyCharacterMap),
    REG_JNI(register_android_os_Process),
    REG_JNI(register_android_os_SystemProperties),
    REG_JNI(register_android_os_Binder),
    REG_JNI(register_android_os_Parcel),
    REG_JNI(register_android_nio_utils),
    REG_JNI(register_android_graphics_Canvas),
    REG_JNI(register_android_graphics_Graphics),
    REG_JNI(register_android_view_DisplayEventReceiver),
    REG_JNI(register_android_view_RenderNode),
    REG_JNI(register_android_view_RenderNodeAnimator),
    REG_JNI(register_android_view_GraphicBuffer),
    REG_JNI(register_android_view_DisplayListCanvas),
    REG_JNI(register_android_view_HardwareLayer),
    REG_JNI(register_android_view_ThreadedRenderer),
    REG_JNI(register_android_view_Surface),
    REG_JNI(register_android_view_SurfaceControl),
    REG_JNI(register_android_view_SurfaceSession),
    REG_JNI(register_android_view_TextureView),
    REG_JNI(register_com_android_internal_view_animation_NativeInterpolatorFactoryHelper),
    REG_JNI(register_com_google_android_gles_jni_EGLImpl),
    REG_JNI(register_com_google_android_gles_jni_GLImpl),
    REG_JNI(register_android_opengl_jni_EGL14),
    REG_JNI(register_android_opengl_jni_EGLExt),
    REG_JNI(register_android_opengl_jni_GLES10),
    REG_JNI(register_android_opengl_jni_GLES10Ext),
    REG_JNI(register_android_opengl_jni_GLES11),
    REG_JNI(register_android_opengl_jni_GLES11Ext),
    REG_JNI(register_android_opengl_jni_GLES20),
    REG_JNI(register_android_opengl_jni_GLES30),
    REG_JNI(register_android_opengl_jni_GLES31),
    REG_JNI(register_android_opengl_jni_GLES31Ext),
    REG_JNI(register_android_opengl_jni_GLES32),

    REG_JNI(register_android_graphics_Bitmap),
    REG_JNI(register_android_graphics_BitmapFactory),
    REG_JNI(register_android_graphics_BitmapRegionDecoder),
    REG_JNI(register_android_graphics_Camera),
    REG_JNI(register_android_graphics_CreateJavaOutputStreamAdaptor),
    REG_JNI(register_android_graphics_CanvasProperty),
    REG_JNI(register_android_graphics_ColorFilter),
    REG_JNI(register_android_graphics_DrawFilter),
    REG_JNI(register_android_graphics_FontFamily),
    REG_JNI(register_android_graphics_Interpolator),
    REG_JNI(register_android_graphics_LayerRasterizer),
    REG_JNI(register_android_graphics_MaskFilter),
    REG_JNI(register_android_graphics_Matrix),
    REG_JNI(register_android_graphics_Movie),
    REG_JNI(register_android_graphics_NinePatch),
    REG_JNI(register_android_graphics_Paint),
    REG_JNI(register_android_graphics_Path),
    REG_JNI(register_android_graphics_PathMeasure),
    REG_JNI(register_android_graphics_PathEffect),
    REG_JNI(register_android_graphics_Picture),
    REG_JNI(register_android_graphics_PorterDuff),
    REG_JNI(register_android_graphics_Rasterizer),
    REG_JNI(register_android_graphics_Region),
    REG_JNI(register_android_graphics_Shader),
    REG_JNI(register_android_graphics_SurfaceTexture),
    REG_JNI(register_android_graphics_Typeface),
    REG_JNI(register_android_graphics_Xfermode),
    REG_JNI(register_android_graphics_YuvImage),
    REG_JNI(register_android_graphics_drawable_AnimatedVectorDrawable),
    REG_JNI(register_android_graphics_drawable_VectorDrawable),
    REG_JNI(register_android_graphics_pdf_PdfDocument),
    REG_JNI(register_android_graphics_pdf_PdfEditor),
    REG_JNI(register_android_graphics_pdf_PdfRenderer),

    REG_JNI(register_android_database_CursorWindow),
    REG_JNI(register_android_database_SQLiteConnection),
    REG_JNI(register_android_database_SQLiteGlobal),
    REG_JNI(register_android_database_SQLiteDebug),
    REG_JNI(register_android_os_Debug),
    REG_JNI(register_android_os_FileObserver),
    REG_JNI(register_android_os_MessageQueue),
    REG_JNI(register_android_os_SELinux),
    REG_JNI(register_android_os_Trace),
    REG_JNI(register_android_os_UEventObserver),
    REG_JNI(register_android_net_LocalSocketImpl),
    REG_JNI(register_android_net_NetworkUtils),
    REG_JNI(register_android_net_TrafficStats),
    REG_JNI(register_android_os_MemoryFile),
    REG_JNI(register_com_android_internal_os_PathClassLoaderFactory),
    REG_JNI(register_com_android_internal_os_Zygote),
    REG_JNI(register_com_android_internal_util_VirtualRefBasePtr),
    REG_JNI(register_android_hardware_Camera),
    REG_JNI(register_android_hardware_camera2_CameraMetadata),
    REG_JNI(register_android_hardware_camera2_legacy_LegacyCameraDevice),
    REG_JNI(register_android_hardware_camera2_legacy_PerfMeasurement),
    REG_JNI(register_android_hardware_camera2_DngCreator),
    REG_JNI(register_android_hardware_Radio),
    REG_JNI(register_android_hardware_SensorManager),
    REG_JNI(register_android_hardware_SerialPort),
    REG_JNI(register_android_hardware_SoundTrigger),
    REG_JNI(register_android_hardware_UsbDevice),
    REG_JNI(register_android_hardware_UsbDeviceConnection),
    REG_JNI(register_android_hardware_UsbRequest),
    REG_JNI(register_android_hardware_location_ActivityRecognitionHardware),
    REG_JNI(register_android_hardware_location_ContextHubService),
    REG_JNI(register_android_media_AudioRecord),
    REG_JNI(register_android_media_AudioSystem),
    REG_JNI(register_android_media_AudioTrack),
    REG_JNI(register_android_media_JetPlayer),
    REG_JNI(register_android_media_RemoteDisplay),
    REG_JNI(register_android_media_ToneGenerator),

    REG_JNI(register_android_opengl_classes),
    REG_JNI(register_android_server_NetworkManagementSocketTagger),
    REG_JNI(register_android_ddm_DdmHandleNativeHeap),
    REG_JNI(register_android_backup_BackupDataInput),
    REG_JNI(register_android_backup_BackupDataOutput),
    REG_JNI(register_android_backup_FileBackupHelperBase),
    REG_JNI(register_android_backup_BackupHelperDispatcher),
    REG_JNI(register_android_app_backup_FullBackup),
    REG_JNI(register_android_app_Activity),
    REG_JNI(register_android_app_ActivityThread),
    REG_JNI(register_android_app_ApplicationLoaders),
    REG_JNI(register_android_app_NativeActivity),
    REG_JNI(register_android_util_jar_StrictJarFile),
    REG_JNI(register_android_view_InputChannel),
    REG_JNI(register_android_view_InputEventReceiver),
    REG_JNI(register_android_view_InputEventSender),
    REG_JNI(register_android_view_InputQueue),
    REG_JNI(register_android_view_KeyEvent),
    REG_JNI(register_android_view_MotionEvent),
    REG_JNI(register_android_view_PointerIcon),
    REG_JNI(register_android_view_VelocityTracker),

    REG_JNI(register_android_content_res_ObbScanner),
    REG_JNI(register_android_content_res_Configuration),

    REG_JNI(register_android_animation_PropertyValuesHolder),
    REG_JNI(register_com_android_internal_content_NativeLibraryHelper),
    REG_JNI(register_com_android_internal_net_NetworkStatsFactory),

};

这个数组存储了140多个register函数指针

#ifdef NDEBUG
    #define REG_JNI(name)      { name }
    struct RegJNIRec {
        int (*mProc)(JNIEnv*);
    };
#else
    #define REG_JNI(name)      { name, #name }
    struct RegJNIRec {
        int (*mProc)(JNIEnv*);
        const char* mName;
    };
#endif

调用是发生在

static int register_jni_procs(const RegJNIRec array[], size_t count, JNIEnv* env)
{
    for (size_t i = 0; i < count; i++) {
        if (array[i].mProc(env) < 0) {
#ifndef NDEBUG
            ALOGD("----------!!! %s failed to load\n", array[i].mName);
#endif
            return -1;
        }
    }
    return 0;
}

由startReg调用register_jni_procs

/*
 * Register android native functions with the VM.
 */
/*static*/ int AndroidRuntime::startReg(JNIEnv* env)
{
    ATRACE_NAME("RegisterAndroidNatives");
    /*
     * This hook causes all future threads created in this process to be
     * attached to the JavaVM.  (This needs to go away in favor of JNI
     * Attach calls.)
     */
    androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);

    ALOGV("--- registering native functions ---\n");

    /*
     * Every "register" function calls one or more things that return
     * a local reference (e.g. FindClass).  Because we haven't really
     * started the VM yet, they're all getting stored in the base frame
     * and never released.  Use Push/Pop to manage the storage.
     */
    env->PushLocalFrame(200);

    if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
        env->PopLocalFrame(NULL);
        return -1;
    }
    env->PopLocalFrame(NULL);

    //createJavaThread("fubar", quickTest, (void*) "hello");

    return 0;
}

源头是由start函数调用的。