android6.0源码分析之AndroidRuntime的建立过程
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：
