图解 | Android 系统的启动

//system/core/rootdir/init.rc
import /init.${ro.zygote}.rc

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
    class main
    priority -20
    user root
    group root readproc reserved_disk
    socket zygote stream 660 root system
    ...

//system/core/init/service.cpp
bool Service::Start() {
    //fork出子进程
    pid = fork();
    if (pid == 0) {//子进程会返回0，父进程会返回子进程的pid
        //strs[0]是执行程序的路径，即execve会运行app_process
        if (execve(strs[0], (char**) &strs[0], (char**) ENV) < 0) {
        }
    }
}

//frameworks/base/cmds/app_process/app_main.cpp
int main(int argc, char* const argv[]){
    if (zygote) {
        //启动Zygote，进入ZygoteInit.main函数
        runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
    }
}

//frameworks/base/core/jni/AndroidRuntime.cpp
void AndroidRuntime::start(...){
    //1. 启动java虚拟机
    if (startVm(&mJavaVM, &env, zygote) != 0) {
        return;
    }
    //2. 为java虚拟机注册JNI方法
    if (startReg(env) < 0) {
        return;
    }
    //根据传入的参数找到ZygoteInit类和他的main函数
    //3. 通过JNI调用ZygoteInit的main函数
    env->CallStaticVoidMethod(startClass, startMeth, strArray);
}

//ZygoteInit.java
public static void main(String argv[]) {
    //是否要创建SystemServer
    boolean startSystemServer = false;
    //默认的socket名字
    String socketName = "zygote";
    //是否要延迟资源的预加载
    boolean enableLazyPreload = false;
    for (int i = 1; i < argv.length; i++) {
        if ("start-system-server".equals(argv[i])) {
            //在init.rc文件中，有--start-system-server参数，表示要创建SystemServer
            startSystemServer = true;
        } else if ("--enable-lazy-preload".equals(argv[i])) {
            //init.rc没有这个参数，资源的预加载不会被延迟
            enableLazyPreload = true;
        } else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
            //init.rc可以通过--socket-name=指定socket名字来覆盖默认值
            socketName = argv[i].substring(SOCKET_NAME_ARG.length());
        }
    }
    //1. 创建服务端socket，名字为socketName即zygote
    zygoteServer.registerServerSocket(socketName);
    if (!enableLazyPreload) {
        //2. 没有被延迟，就预加载资源
        preload(bootTimingsTraceLog);
    }
    if (startSystemServer) {
        //3. fork并启动SystemServer进程
        startSystemServer(abiList, socketName, zygoteServer);
    }
    //4. 等待AMS请求（AMS会通过socket请求Zygote来创建应用程序进程）
    zygoteServer.runSelectLoop(abiList);
}

//ZygoteInit.java
private static boolean startSystemServer(...){
    String args[] = {
        //...
        //启动的类名：
        "com.android.server.SystemServer",
    };
    //fork进程，由native层实现
    pid = Zygote.forkSystemServer();
    //处理SystemServer进程
    handleSystemServerProcess(parsedArgs);
}
private static void handleSystemServerProcess(...){
    ZygoteInit.zygoteInit(...);
}
public static final void zygoteInit(...){
    //启动binder线程池
    ZygoteInit.nativeZygoteInit();
    //内部经过层层调用，找到"com.android.server.SystemServer"类和他的main函数，然后执行
    RuntimeInit.applicationInit(targetSdkVersion, argv, classLoader);
}

//SystemServer.java
public static void main(String[] args) {
    new SystemServer().run();
}
private void run() {
    //创建looper
    Looper.prepareMainLooper();
    //加载动态库libandroid_servers.so
    System.loadLibrary("android_servers");
    //创建系统上下文
    createSystemContext();
    //创建SSM，用于服务的创建、启动和生命周期管理
    mSystemServiceManager = new SystemServiceManager(mSystemContext);
    
    //服务根据优先级被分成3批来启动：
    //启动引导服务，如AMS、PMS等
    startBootstrapServices();
    //启动核心服务
    startCoreServices();
    //启动其他服务
    startOtherServices();
    //开启looper循环
    Looper.loop();
}

//SystemServer.java
private void startBootstrapServices() {
    //由SSM创建启动
    mActivityManagerService = mSystemServiceManager.startService(
        ActivityManagerService.Lifecycle.class).getService();
    mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
}
private void startOtherServices() {
    //AMS准备就绪
    mActivityManagerService.systemReady(...);
}

//SystemServer.java
private void startOtherServices() {
    //AMS准备就绪
    mActivityManagerService.systemReady(...);
}

//ActivityManagerService.java
public void systemReady(...) {
    //经过层层调用来到startHomeActivityLocked
}
boolean startHomeActivityLocked(...) {
    //最终会启动Launcher应用的Activity
    mActivityStarter.startHomeActivityLocked(...);
}