Android启动流程

转载自 古冥 的CSDN博客：http://blog.csdn.net/yangzhihuiguming/article/details/51697801

init进程在启动Zygote进程之后，初始化Zygote进程之前，会先进行AndroidRuntime的启动和环境建立。

1. Android系统架构图
2. Init进程分析
3. Android运行时的启动分析

1、Android系统架构图

Android系统架构图可知，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进程之后，初始化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：

//Init.cppint main(int argc, char **argv){    ...    //清除umask默认属性    umask();    add_environment("PATH",_PATH_DEFPATH);    //判断是不是初始化的第一阶段    bool is_first_stage = (arg == 1)||(strcmp(argv[1],"--second-stage")!=0);    if(is_first_stage){        //进行系统文件系统等的挂载        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);    }    open_devull_stdio();    //klog初始化    klog_init();    klog_set_level(KLOG_NOTICE_LEVEL);    if(!is_first_stage){        //关闭/dev/.booting文件的相关权限        close("/dev/.booting",O_WRONLY|O_WCREAT|O_CLOEXEC,000);        //初始化部分属性        property_init();        //先初始化DT，因为DT的属性集的优先级高于comand line        process_kernel_dt();        //处理内核命令行        process_kernel_cmdline;        //导出内核变量        export_kernel_boot_props();    }    ...    //初始化信号量    signal_handler_init();    property_load_boot_defaults();    //开启属性服务    start_property_service();    //解析/init.rc文件    init_parse_config_file(/init.rc);    ...     }

此处的代码与android 4.4等，在代码上有略微的差别，但是基本的操作还是一样的，首先进行一些属性的初始化，内核命令行等以及信号量的初始化，最后再启动属性服务以及对init.rc文件进行解析，所以，接下来分析Init.rc文件：

//Init.rcimport /init.environ.rcimport /init.usb.rcimport /init.${ro.hardware}.rcimport /init.${ro.zygote}.rcimport /init.trace.rc...

由代码可知，与android4.4等有明显的差别，此处采用导入.rc文件的方式来初始化相应模块，此处分析zygote模块，对于android6.0来说，它提供了多个zygote相关的.rc文件，有Init.zygote32.rc、Init.zygote32_64.rc、Init.zygote64.rc、Init.zygote64_32.rc等，本文分析Init.zygote32.rc，其他流程是一样的，但是在源码中，只发现了Init.zygote32.rc的相关实现。
看Init.zygote32.rc文件：

//Init.zygote32.rcservice zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server    class main    socket zygote stream 660 root system    onrestart write /sys/android_power/request_state wake    onrestart write /sys/power/state on    onrestart restart media    onrestart restart netd      

由代码可知，它将会执行/system/bin/app_process，而后面的是它的参数，请注意–zygote和–start-system-server参数，后面将会分析到，而对于/system/bin/app_process的实现app_main.cpp(frameworks/base/cmds/app_process)，它的配置在frameworks/base/cmds/app_process目录下的Android.mk文件中：

//Android.mk(frameworks/base/cmds/app_process)LOCAL_PATH = $(call my-dir)app_process_common-shared_libs:=\    libwilhelm\include $(CLEAR_VARS)#看，app_process命令绑定的源码就是该目录下的app_main.cppLOCAL_SRC_FILES:=\    app_main.cpp...

所以，app_process命令绑定的源码实现就是app_main.cpp，在解析Init.zygote32.rc文件的时候，会调用到它的main函数，即app_main.cpp的main函数。

3、Android运行时的启动分析

上一节分析了Init进程启动与android运行时的关系，因为对于android运行时的启动来说，它是在Init进程之后，zygote进程启动之后，初始化之前初始化的，所以此时它的入口和zygote的入口一致，即app_main.cpp的main函数，代码如下：

//app_main.cppint 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，参数为com.android.internal.os.ZygoteInit        runtime.start("com.android.internal.os.ZygoteInit", args);    } else if (className) {//否则参数为com.android.internal.os.RuntimeInit        runtime.start("com.android.internal.os.RuntimeInit", args);    }else{        //显示命令的用法说明        app_usage();    }    ...}

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，而对于两种不同的参数而言，只是最后启动的类不一样，如果有–zygote参数，其最后会启动ZygoteInit类，否则就会执行RuntimeInit类，继续追踪start方法。

//AndroidRuntime.cppvoid 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RuntimeInit.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：

//AndroidRuntime.cppint 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,如果此调用发挥成功的话，那么我们就能够济宁JNI调用了，继续看此方法的代码，它的定义出现在DdmConnection的start方法里：

//DdmConnection.cppvoid 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"));    ...}

NI_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。至此，其实Runtime已经初始化完成了。，其时序图如下：