Android启动流程

1. init进程

2. zygote进程

2.1 AndroidRuntime

2.2 ZygoteInit

3. system_server进程

3.1 init1阶段

3.2 init2阶段

接着Linux启动部分 http://blog.csdn.net/qianjin0703/article/details/6881107

1. init进程

系统在内核空间初始化进程，内存，文件系统，驱动模块之后，调用init_post()函数，开启第一个用户进程init，进入用户空间。

/kernel/init/main.c (仅截取部分重要代码)

 static noinline int init_post(void)
 {

     run_init_process("/sbin/init");
     run_init_process("/etc/init");
     run_init_process("/bin/init");
     run_init_process("/bin/sh");
 }

终端下查看1号进程，

可见，init是系统守护进程和zygote的父进程。

/system/core/init/init.c 

 int main(int argc, char **argv)
 {

     mkdir("/dev", 0755);
     mkdir("/proc", 0755);
     mkdir("/sys", 0755);

     mount("devpts", "/dev/pts", "devpts", 0, NULL);
     mount("proc", "/proc", "proc", 0, NULL);
     mount("sysfs", "/sys", "sysfs", 0, NULL);

     open_devnull_stdio();
     klog_init();
     property_init();

     get_hardware_name(hardware, &revision);
     process_kernel_cmdline();

     init_parse_config_file("/init.rc");
     action_for_each_trigger("early-init", action_add_queue_tail);

     queue_builtin_action(wait_for_coldboot_done_action, "wait_for_coldboot_done");
     queue_builtin_action(keychord_init_action, "keychord_init");
     queue_builtin_action(console_init_action, "console_init");
     action_for_each_trigger("init", action_add_queue_tail);

     for(;;) {

         execute_one_command();
         restart_processes();

         nr = poll(ufds, fd_count, timeout);

         for (i = 0; i < fd_count; i++) {
             if (ufds[i].revents == POLLIN) {
                 if (ufds[i].fd == get_property_set_fd())
                     handle_property_set_fd();
                 else if (ufds[i].fd == get_keychord_fd())
                     handle_keychord();
                 else if (ufds[i].fd == get_signal_fd())
                     handle_signal();
             }
         }
     }

     return 0;
 }

init进程归纳如下

• 解析init.rc和init.*.rc
• 挂载文件系统，建立文件夹和文件，设置权限，为一些文件写入初值
• 初始化property属性
• 启动系统守护进程如adbd, vold, rild等
• 启动zygote进程

2. zygote进程

zygote既是Android系统创建java世界的盘古，又是繁殖system_server和所有应用程序的女娲。

终端下查看zygote进程，

2.1 AndroidRuntime

/frameworks/base/cmds/app_process/app_main.cpp

 int main(int argc, const char* const argv[])
 {

     AppRuntime runtime;

     if (zygote) {
         runtime.start("com.android.internal.os.ZygoteInit",
                 startSystemServer ? "start-system-server" : "");
 }

AppRuntime是从AndroidRuntime类派生的，最后由runtime.start()进入Java世界。

/frameworks/base/core/jni/AndroidRuntime.cpp

 void AndroidRuntime::start(const char* className, const char* options)
 {

     /* start the virtual machine */
     JNIEnv* env;
     if (startVm(&mJavaVM, &env) != 0) {
         return;
     }
     onVmCreated(env);

     /*
      * Register android functions.
      */
     if (startReg(env) < 0) {
         ALOGE("Unable to register all android natives\n");
         return;
     }

     /*
      * Start VM.  This thread becomes the main thread of the VM, and will
      * not return until the VM exits.
      */
     char* slashClassName = toSlashClassName(className);
     jclass startClass = env->FindClass(slashClassName);

     jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
             "([Ljava/lang/String;)V");

     env->CallStaticVoidMethod(startClass, startMeth, strArray);

 }

-> 创建虚拟机

注意这里会设置虚拟机的heapsize，默认16M，一般32M，不能太小，否则在操作大尺寸图片时无法分配所需内存。

-> 注册jni函数

这里注册了本地方法，这些方法最终被编译进动态链接库libandroid_runtime.so，供上层Java调用。

 static const RegJNIRec gRegJNI[] = {
     REG_JNI(register_android_debug_JNITest),
     REG_JNI(register_com_android_internal_os_RuntimeInit),
     REG_JNI(register_android_os_SystemClock),
     REG_JNI(register_android_util_Log),
     REG_JNI(register_android_os_Process),
     REG_JNI(register_android_os_SystemProperties),
     REG_JNI(register_android_os_Binder),
     REG_JNI(register_android_view_Display),
     REG_JNI(register_android_graphics_Bitmap),
     REG_JNI(register_android_graphics_Camera),
     REG_JNI(register_android_database_SQLiteDatabase),
     REG_JNI(register_android_os_Power),
     REG_JNI(register_android_os_StatFs),
     REG_JNI(register_android_net_NetworkUtils),
     REG_JNI(register_android_nfc_NdefMessage),
     REG_JNI(register_com_android_internal_os_ZygoteInit),
     REG_JNI(register_android_hardware_Camera),
     REG_JNI(register_android_hardware_SensorManager),
     REG_JNI(register_android_hardware_UsbDevice),
     REG_JNI(register_android_media_AudioRecord),
     REG_JNI(register_android_media_AudioSystem),
     REG_JNI(register_android_media_AudioTrack),
     REG_JNI(register_android_opengl_classes),
     REG_JNI(register_android_bluetooth_BluetoothSocket),
     REG_JNI(register_android_server_BluetoothService),

     REG_JNI(register_android_app_ActivityThread),
     REG_JNI(register_android_app_NativeActivity),
     REG_JNI(register_android_view_InputChannel),
     REG_JNI(register_android_view_InputQueue),
     REG_JNI(register_android_view_KeyEvent),
     REG_JNI(register_android_view_MotionEvent),
     REG_JNI(register_android_view_PointerIcon),

 };

2.2 ZygoteInit

/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java

 public static void main(String argv[]) {
     try {
         SamplingProfilerIntegration.start();

         registerZygoteSocket();

         preload();

         SamplingProfilerIntegration.writeZygoteSnapshot();

         if (argv[1].equals("start-system-server")) {
             startSystemServer();
         }

             runSelectLoopMode();

         closeServerSocket();
     }
 }

-> 建立服务端Socket

zygote及系统中其他程序的通信没有使用Binder，而是采用了基于AF_UNIX类型的Socket.

-> 预加载类和资源

预加载的类可以在 /frameworks/base/preloaded-classes 查看。预加载的资源生成framewrok-res.apk，主要是一些图标，刷机达人经常修改它。

-> 启动system_server进程

zygote进程fork出system_server子进程，把任务繁重的系统服务全部托管出去，自己负责循环监听所有应用程序的请求。

-> 等待应用程序的请求

zygote通过runSelectLoopMode()处理应用程序客户端的请求，由ZygoteConnection的runOnce()孵化出新的应用程序。

综上所述，zygote进程归纳如下

• 创建虚拟机
• 注册jni函数
• 预加载类和资源
• 启动system_server进程
• 监听应用程序请求

3. system_server进程

/frameworks/base/services/java/com/android/SystemServer.java

 public static void main(String[] args) {

     System.loadLibrary("android_servers");
     init1(args);
 }

3.1 init1阶段
加载libandriod_server.so库，实现init1

/frameworks/base/cmds/system_server/library/system_init.cpp

 extern "C" status_t system_init()
 {

     sp<GrimReaper> grim = new GrimReaper();
     sm->asBinder()->linkToDeath(grim, grim.get(), 0);

     char propBuf[PROPERTY_VALUE_MAX];
     property_get("system_init.startsurfaceflinger", propBuf, "1");
     if (strcmp(propBuf, "1") == 0) {
         // Start the SurfaceFlinger
         SurfaceFlinger::instantiate();
     }

     property_get("system_init.startsensorservice", propBuf, "1");
     if (strcmp(propBuf, "1") == 0) {
         // Start the sensor service
         SensorService::instantiate();
     }

     AndroidRuntime* runtime = AndroidRuntime::getRuntime();

     jclass clazz = env->FindClass("com/android/server/SystemServer");
     jmethodID methodId = env->GetStaticMethodID(clazz, "init2", "()V");
     env->CallStaticVoidMethod(clazz, methodId);

     return NO_ERROR;
 }

3.2 init2阶段

init2回到Java层，启动一个ServerThread线程，启动系统的各项服务。