深入理解Android系统第一弹 - 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;
- }
- 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" : "");
- }
/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);
- }
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。
