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Android系统启动流程-Native方法的注册
发布于:2019-5-1
最后更新: 2023-6-3
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在使用app_process 创建zygote进程的过程中会执行下面这些操作:
  • 创建ART虚拟机
  • 注册Native方法
  • 找到ZygoteInit,通过反射调用它的main方法进入Java世界
这里说一下注册Native方法的一些细节.

入口

    /*
     * Register android functions.
     */
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\\n");
        return;
    }
源码在frameworks/base/core/jni/AndroidRuntime.cpp中.
/*
 * 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;
}

要注册的方法

static const RegJNIRec gRegJNI[] = {
    REG_JNI(register_com_android_internal_os_RuntimeInit),
    REG_JNI(register_com_android_internal_os_ZygoteInit_nativeZygoteInit),
    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_util_StatsLog),
    REG_JNI(register_android_util_StatsLogInternal),
    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_content_res_ApkAssets),
    REG_JNI(register_android_text_AndroidCharacter),
    REG_JNI(register_android_text_Hyphenator),
    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_os_HidlMemory),
    REG_JNI(register_android_os_HidlSupport),
    REG_JNI(register_android_os_HwBinder),
    REG_JNI(register_android_os_HwBlob),
    REG_JNI(register_android_os_HwParcel),
    REG_JNI(register_android_os_HwRemoteBinder),
    REG_JNI(register_android_os_NativeHandle),
    REG_JNI(register_android_os_VintfObject),
    REG_JNI(register_android_os_VintfRuntimeInfo),
    REG_JNI(register_android_graphics_Canvas),
    // This needs to be before register_android_graphics_Graphics, or the latter
    // will not be able to find the jmethodID for ColorSpace.get().
    REG_JNI(register_android_graphics_ColorSpace),
    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_DisplayListCanvas),
    REG_JNI(register_android_view_InputApplicationHandle),
    REG_JNI(register_android_view_InputWindowHandle),
    REG_JNI(register_android_view_TextureLayer),
    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_CompositionSamplingListener),
    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_EGL15),
    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_ByteBufferStreamAdaptor),
    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_GraphicBuffer),
    REG_JNI(register_android_graphics_ImageDecoder),
    REG_JNI(register_android_graphics_drawable_AnimatedImageDrawable),
    REG_JNI(register_android_graphics_Interpolator),
    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_Region),
    REG_JNI(register_android_graphics_Shader),
    REG_JNI(register_android_graphics_SurfaceTexture),
    REG_JNI(register_android_graphics_Typeface),
    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_fonts_Font),
    REG_JNI(register_android_graphics_fonts_FontFamily),
    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_graphics_text_MeasuredText),
    REG_JNI(register_android_graphics_text_LineBreaker),

    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_GraphicsEnvironment),
    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_os_MemoryFile),
    REG_JNI(register_android_os_SharedMemory),
    REG_JNI(register_com_android_internal_os_ClassLoaderFactory),
    REG_JNI(register_com_android_internal_os_Zygote),
    REG_JNI(register_com_android_internal_os_ZygoteInit),
    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_HardwareBuffer),
    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_media_AudioEffectDescriptor),
    REG_JNI(register_android_media_AudioSystem),
    REG_JNI(register_android_media_AudioRecord),
    REG_JNI(register_android_media_AudioTrack),
    REG_JNI(register_android_media_AudioAttributes),
    REG_JNI(register_android_media_AudioProductStrategies),
    REG_JNI(register_android_media_AudioVolumeGroups),
    REG_JNI(register_android_media_AudioVolumeGroupChangeHandler),
    REG_JNI(register_android_media_MicrophoneInfo),
    REG_JNI(register_android_media_RemoteDisplay),
    REG_JNI(register_android_media_ToneGenerator),
    REG_JNI(register_android_media_midi),

    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_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_android_security_Scrypt),
    REG_JNI(register_com_android_internal_content_NativeLibraryHelper),
    REG_JNI(register_com_android_internal_os_AtomicDirectory),
    REG_JNI(register_com_android_internal_os_FuseAppLoop),
    REG_JNI(register_com_android_internal_os_KernelCpuUidBpfMapReader),
    REG_JNI(register_com_android_internal_os_KernelSingleUidTimeReader),
};
其中register_xx其实是一个函数指针,遍历的时候会依次执行这些方法:
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;
}

int register_com_android_internal_os_RuntimeInit(JNIEnv* env)
{
    const JNINativeMethod methods[] = {
            {"nativeFinishInit", "()V",
             (void*)com_android_internal_os_RuntimeInit_nativeFinishInit},
            {"nativeSetExitWithoutCleanup", "(Z)V",
             (void*)com_android_internal_os_RuntimeInit_nativeSetExitWithoutCleanup},
    };
    return jniRegisterNativeMethods(env, "com/android/internal/os/RuntimeInit",
        methods, NELEM(methods));
}
可以看到需要传入用到JNI方法的Java类名以及用到的Native方法签名.
实际注册操作都是通过jniRegisterNativeMethods方法:
inline int jniRegisterNativeMethods(JNIEnv* env, const char* className, const JNINativeMethod* gMethods, int numMethods) {
    return jniRegisterNativeMethods(&env->functions, className, gMethods, numMethods);
}
实现在libnativehelper/JNIHelp.c中:
int jniRegisterNativeMethods(JNIEnv* env, const char* className,
    const JNINativeMethod* methods, int numMethods)
{
    ALOGV("Registering %s's %d native methods...", className, numMethods);
    jclass clazz = (*env)->FindClass(env, className);
    ALOG_ALWAYS_FATAL_IF(clazz == NULL,
                         "Native registration unable to find class '%s'; aborting...",
                         className);
    int result = (*env)->RegisterNatives(env, clazz, methods, numMethods);
    (*env)->DeleteLocalRef(env, clazz);
    if (result == 0) {
        return 0;
    }

    // Failure to register natives is fatal. Try to report the corresponding exception,
    // otherwise abort with generic failure message.
    jthrowable thrown = (*env)->ExceptionOccurred(env);
    if (thrown != NULL) {
        struct ExpandableString summary;
        ExpandableStringInitialize(&summary);
        if (GetExceptionSummary(env, thrown, &summary)) {
            ALOGF("%s", summary.data);
        }
        ExpandableStringRelease(&summary);
        (*env)->DeleteLocalRef(env, thrown);
    }
    ALOGF("RegisterNatives failed for '%s'; aborting...", className);
    return result;
}
这些要注册的Native方法实现大部分都在frameworks/base/core/jni下.
我门来看一下Canvas中Native方法的注册:
static const JNINativeMethod gMethods[] = {
    {"nGetNativeFinalizer", "()J", (void*) CanvasJNI::getNativeFinalizer},
    {"nFreeCaches", "()V", (void*) CanvasJNI::freeCaches},
    {"nFreeTextLayoutCaches", "()V", (void*) CanvasJNI::freeTextLayoutCaches},
    {"nSetCompatibilityVersion", "(I)V", (void*) CanvasJNI::setCompatibilityVersion},

    // ------------ @FastNative ----------------
    {"nInitRaster", "(J)J", (void*) CanvasJNI::initRaster},
    {"nSetBitmap", "(JJ)V", (void*) CanvasJNI::setBitmap},
    {"nGetClipBounds","(JLandroid/graphics/Rect;)Z", (void*) CanvasJNI::getClipBounds},

    // ------------ @CriticalNative ----------------
    {"nIsOpaque","(J)Z", (void*) CanvasJNI::isOpaque},
    {"nGetWidth","(J)I", (void*) CanvasJNI::getWidth},
    {"nGetHeight","(J)I", (void*) CanvasJNI::getHeight},
    {"nSave","(JI)I", (void*) CanvasJNI::save},
    {"nSaveLayer","(JFFFFJI)I", (void*) CanvasJNI::saveLayer},
    {"nSaveLayerAlpha","(JFFFFII)I", (void*) CanvasJNI::saveLayerAlpha},
    {"nSaveUnclippedLayer","(JIIII)I", (void*) CanvasJNI::saveUnclippedLayer},
    {"nRestoreUnclippedLayer","(JIJ)V", (void*) CanvasJNI::restoreUnclippedLayer},
    {"nGetSaveCount","(J)I", (void*) CanvasJNI::getSaveCount},
    {"nRestore","(J)Z", (void*) CanvasJNI::restore},
    {"nRestoreToCount","(JI)V", (void*) CanvasJNI::restoreToCount},
    {"nGetMatrix", "(JJ)V", (void*)CanvasJNI::getMatrix},
    {"nSetMatrix","(JJ)V", (void*) CanvasJNI::setMatrix},
    {"nConcat","(JJ)V", (void*) CanvasJNI::concat},
    {"nRotate","(JF)V", (void*) CanvasJNI::rotate},
    {"nScale","(JFF)V", (void*) CanvasJNI::scale},
    {"nSkew","(JFF)V", (void*) CanvasJNI::skew},
    {"nTranslate","(JFF)V", (void*) CanvasJNI::translate},
    {"nQuickReject","(JJ)Z", (void*) CanvasJNI::quickRejectPath},
    {"nQuickReject","(JFFFF)Z", (void*)CanvasJNI::quickRejectRect},
    {"nClipRect","(JFFFFI)Z", (void*) CanvasJNI::clipRect},
    {"nClipPath","(JJI)Z", (void*) CanvasJNI::clipPath},
    {"nSetDrawFilter", "(JJ)V", (void*) CanvasJNI::setPaintFilter},
};

// If called from Canvas these are regular JNI
// If called from DisplayListCanvas they are @FastNative
static const JNINativeMethod gDrawMethods[] = {
    {"nDrawColor","(JII)V", (void*) CanvasJNI::drawColor},
    {"nDrawColor","(JJJI)V", (void*) CanvasJNI::drawColorLong},
    {"nDrawPaint","(JJ)V", (void*) CanvasJNI::drawPaint},
    {"nDrawPoint", "(JFFJ)V", (void*) CanvasJNI::drawPoint},
    {"nDrawPoints", "(J[FIIJ)V", (void*) CanvasJNI::drawPoints},
    {"nDrawLine", "(JFFFFJ)V", (void*) CanvasJNI::drawLine},
    {"nDrawLines", "(J[FIIJ)V", (void*) CanvasJNI::drawLines},
    {"nDrawRect","(JFFFFJ)V", (void*) CanvasJNI::drawRect},
    {"nDrawRegion", "(JJJ)V", (void*) CanvasJNI::drawRegion },
    {"nDrawRoundRect","(JFFFFFFJ)V", (void*) CanvasJNI::drawRoundRect},
    {"nDrawDoubleRoundRect", "(JFFFFFFFFFFFFJ)V", (void*) CanvasJNI::drawDoubleRoundRectXY},
    {"nDrawDoubleRoundRect", "(JFFFF[FFFFF[FJ)V", (void*) CanvasJNI::drawDoubleRoundRectRadii},
    {"nDrawCircle","(JFFFJ)V", (void*) CanvasJNI::drawCircle},
    {"nDrawOval","(JFFFFJ)V", (void*) CanvasJNI::drawOval},
    {"nDrawArc","(JFFFFFFZJ)V", (void*) CanvasJNI::drawArc},
    {"nDrawPath","(JJJ)V", (void*) CanvasJNI::drawPath},
    {"nDrawVertices", "(JII[FI[FI[II[SIIJ)V", (void*)CanvasJNI::drawVertices},
    {"nDrawNinePatch", "(JJJFFFFJII)V", (void*)CanvasJNI::drawNinePatch},
    {"nDrawBitmapMatrix", "(JJJJ)V", (void*)CanvasJNI::drawBitmapMatrix},
    {"nDrawBitmapMesh", "(JJII[FI[IIJ)V", (void*)CanvasJNI::drawBitmapMesh},
    {"nDrawBitmap","(JJFFJIII)V", (void*) CanvasJNI::drawBitmap},
    {"nDrawBitmap","(JJFFFFFFFFJII)V", (void*) CanvasJNI::drawBitmapRect},
    {"nDrawBitmap", "(J[IIIFFIIZJ)V", (void*)CanvasJNI::drawBitmapArray},
    {"nDrawText","(J[CIIFFIJ)V", (void*) CanvasJNI::drawTextChars},
    {"nDrawText","(JLjava/lang/String;IIFFIJ)V", (void*) CanvasJNI::drawTextString},
    {"nDrawTextRun","(J[CIIIIFFZJJ)V", (void*) CanvasJNI::drawTextRunChars},
    {"nDrawTextRun","(JLjava/lang/String;IIIIFFZJ)V", (void*) CanvasJNI::drawTextRunString},
    {"nDrawTextOnPath","(J[CIIJFFIJ)V", (void*) CanvasJNI::drawTextOnPathChars},
    {"nDrawTextOnPath","(JLjava/lang/String;JFFIJ)V", (void*) CanvasJNI::drawTextOnPathString},
};

int register_android_graphics_Canvas(JNIEnv* env) {
    int ret = 0;
    ret |= RegisterMethodsOrDie(env, "android/graphics/Canvas", gMethods, NELEM(gMethods));
    ret |= RegisterMethodsOrDie(env, "android/graphics/BaseCanvas", gDrawMethods, NELEM(gDrawMethods));
    ret |= RegisterMethodsOrDie(env, "android/graphics/BaseRecordingCanvas", gDrawMethods, NELEM(gDrawMethods));
    return ret;

}
使用JNIEnv的RegisterNatives方法实际上就是在Java的native方法名与C++中的JNI调用方法签名之间建立一个映射关系,从而避免了在C++中必须写那些丑陋的方法名了.
而RegisterNatives的使用在JNI开发中还是非常常见的,比如我们经常会在JNI_Onload方法中注册Native方法,还是会使用到RegisterNatives方法.

总结

  1. 灵活使用ReigsterNatives方法,不一定必须在JNI_OnLoad()方法中调用,也可以自行调用
  1. JNI中绝大部分方法都在JNIEnv中;
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