android6.0源码分析之AndroidRuntime的建立过程
16lz
2021-12-04
android源码分析之AndroidRuntime的建立过程
android运行时作为android架构的一部分，起着非常重要的作用，它和核心库(Core Libraries)组成了Android运行时库层。本文将依据android源码对AndroidRuntime(android运行时)的建立过程进行分析。
1、Android架构中的AndroidRuntime
android采用的是层次化系统架构，它的架构图如下：
由架构图可知，AndroidRuntime处于Linux内核层以及应用框架层中间，而Java的运行环境依赖于Java虚拟机，所以AndroidRuntime对于android系统非常重要。Init进程是Linux环境下非常重要的一个进程，而Zygote进程是Java环境下的第一个进程，所有其他的Java环境下的进程都是由Zygote进程来进行fork的，而init进程在启动Zygote进程之前，会先进行AndroidRuntime的启动和环境建立，下文将对此过程进行详细分析。
2、init进程分析
init进程作为linux启动过程中的一个重要的进程，它主要负责文件系统的挂载，属性的初始化，各种配置的加载启动以及Action触发，Service的启动，而Zygote作为Java环境下的服务进程，也在Init.rc中进行了配置，而init进程是由main函数进行触发的，其main函数如下：
int main(int argc, char **argv){ ... //创建并挂载了一些基本的系统文件 mkdir("/dev", 0755); mkdir("/proc", 0755); mkdir("/sys", 0755); mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755"); mkdir("/dev/pts", 0755); mkdir("/dev/socket", 0755); mount("devpts", "/dev/pts", "devpts", 0, NULL); mount("proc", "/proc", "proc", 0, NULL); mount("sysfs", "/sys", "sysfs", 0, NULL); ... //初始化了部分属性 property_init(); ... }而Zygote进程在init.rc文件中的配置如下：
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server class main socket zygote stream 660 root system所以，当Linux的Init进程对init.rc文件进行解析加载的时候，会启动此Zygote服务进程。
3、Android运行时的启动分析
Init进程启动Zygote进程的入口为app_main.cpp中的main函数，代码如下：
int main(int argc, char* const argv[]){ ... //AppRuntime继承自AndroidRuntime AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv)); ... if (zygote) {//如果init.rc配置中有--Zygote,则启动ZygoteInit runtime.start("com.android.internal.os.ZygoteInit", args); } else if (className) {//否则RuntimeInit启动 runtime.start("com.android.internal.os.RuntimeInit", args); } ...}首先创建一个AppRuntime对象，接着调用它的start方法，而此start方法在父类AndroidRuntime中进行了定义，并且此处的参数为com.android.internal.os.ZygoteInit，继续追踪start方法。
void AndroidRuntime::start(const char* className, const Vector<String8>& options){ ... //启动虚拟机 if (startVm(&mJavaVM, &env) != 0) { return; } //模板方法模式，只是为了创建VM之后释放slashClassName的内存 onVmCreated(env); //注册Android JNI函数 if (startReg(env) < 0) { ALOGE("Unable to register all android natives\n"); return; } ... //解析类名 char* slashClassName = toSlashClassName(className); //找到需要启动的java类 jclass startClass = env->FindClass(slashClassName); if (startClass == NULL) { ALOGE("JavaVM unable to locate class '%s'\n", slashClassName); } else { //得到指定类中指定方法的ID，这里得到的是ZygoteInit.main()的方法ID jmethodID startMeth = env->GetStaticMethodID(startClass, "main", "([Ljava/lang/String;)V"); if (startMeth == NULL) { ALOGE("JavaVM unable to find main() in '%s'\n", className); } else { //调用上面得到的方法ID和相关参数，即调用Java类ZygoteInit.main(); env->CallStaticVoidMethod(startClass, startMeth, strArray); ... } } ...}首先分析Java虚拟机的启动：由以上代码可知，在start方法中，调用startVm()方法进行虚拟机的启动，startVm()的具体代码如下：
int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote){ ... //读取第一个参数指定的属性操作到buffer，并设置默认的参数 parseRuntimeOption("dalvik.vm.stack-trace-file", stackTraceFileBuf, "-Xstacktracefile:"); ... //添加相应的handler操作 addOption("exit", (void*) runtime_exit); ... //读取作为dex2oat compiler运行时操作 parseCompilerOption(dex2oat_isa_variant_key, dex2oat_isa_variant, "--instruction-set-variant=", "-Xcompiler-option"); // 拷贝特性 sprintf(dex2oat_isa_features_key, "dalvik.vm.isa.%s.features", instruction_set); parseCompilerOption(dex2oat_isa_features_key, dex2oat_isa_features, "--instruction-set-features=", "-Ximage-compiler-option"); parseCompilerOption(dex2oat_isa_features_key, dex2oat_isa_features, "--instruction-set-features=", "-Xcompiler-option"); ... //初始化虚拟机 if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) { ALOGE("JNI_CreateJavaVM failed\n"); return -1; } ...}由代码可知：首先进行一系列的属性读取到buffer的操作，如JIT相关的options,DexClassLoader的相关options等，并将其添加到操作数组容器里面，最后再调用JNI_CreateJavaVM方法进行VM的创建，继续看JNI_CreateJavaVM方法的代码，它的定义出现在DdmConnection的start方法里：
void DdmConnection::start(const char* name) { ... //定义JNI_CreateJavaVM方法 jint (*JNI_CreateJavaVM)(JavaVM** p_vm, JNIEnv** p_env, void* vm_args); //从libart_dso库中解析JNI_CreateJavaVM方法 JNI_CreateJavaVM = reinterpret_cast<decltype(JNI_CreateJavaVM)>( dlsym(libart_dso, "JNI_CreateJavaVM")); //定义registerNatives方法 jint (*registerNatives)(JNIEnv* env, jclass clazz); //从libandroid_runtime_dso库中解析registerNatives方法 registerNatives = reinterpret_cast<decltype(registerNatives)>( dlsym(libandroid_runtime_dso, "Java_com_android_internal_util_WithFramework_registerNatives")); ...}JNI_CreateJavaVM方法是从libart_dso库中解析得到的，同样后面要用到的registerNatives方法从事libandroid_runtime_dso库中解析得到的，而这两个库也是在DdmConnection的start方法中打开的，代码如下：
void* libart_dso = dlopen("libart.so", RTLD_NOW);ALOGE_IF(!libart_dso, "DdmConnection: %s", dlerror());void* libandroid_runtime_dso = dlopen("libandroid_runtime.so", RTLD_NOW);ALOGE_IF(!libandroid_runtime_dso, "DdmConnection: %s", dlerror());而当JNI_CreateJavaVM函数执行成功后，当前本地线程已经把自己的控制权交给JVM，而具体的JNI_CreateJavaVM方法的实现在libart_dso库中，此处不做深究。至此，成功创建了Java虚拟机，而startVm方法也分析结束。
接着分析onVmCreated方法，它使用的是模板方法的设计模式，在androidRuntime中声明并使用，而具体的实现在appRuntime子类中进行，其代码如下：
virtual void onVmCreated(JNIEnv* env){ if(mClassName.isEmpty()){ return;//Zygote.Nothing to do here } char* slashClassName = toSlashClassName(mClassName.string()); mClass = env->FndClass(slashClassName); if(mClass == NULL){ ALOGE("ERROR:could not find class '%s'\n",mClassName.string); } free(slashClassName); mClass = reinterpret<jclass>(env->NewGlobalRef(mClass));}只是简单根据类名获取了类，并释放了类名，所以此处不做过多的分析，接下来分析startReg()方法，方法定义代码如下：
/*static*/ int AndroidRuntime::startReg(JNIEnv* env) /* * 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); ...}startReg方法主要的作用是进行android JNI函数的注册，从代码可知：它调用函数register_jni_procs()方法将gRegJNI中的方法注册到虚拟机，而gRegJNI中的Native方法如下：
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_content_AssetManager), REG_JNI(register_android_content_StringBlock), REG_JNI(register_android_content_XmlBlock), REG_JNI(register_android_emoji_EmojiFactory), 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_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_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_Canvas), 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_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_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_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_ActivityThread), REG_JNI(register_android_app_NativeActivity), 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),};由上可知，在系统建立androidRuntime的时候，创建Java虚拟机，并将系统中一些JNI函数注册到Java虚拟机中，此后只需要通过JNI的方法就可以调用这些Native方法，此外JNI函数的注册还有一种方法，即通过静态代码块System.LoadLibrary(“…”);来加载自定义的JNI方法。
最后，在androidRuntime的start方法中，会通过输入的类名com.android.internal.os.ZygoteInit找到ZygoteInit类，并调用ZygoteInit类的main方法，从而真正由创建虚拟机进入到真正的创建Zygote进程的流程中。至此，androidRuntime的建立过程分析结束，其时序图如下：
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