Android开机流程-重新梳理

小蚂蚁

最近回顾的一些知识，补充了一下。
源码标准：API : 29「Android 10.0」
android手机是怎么开机的？
android 的底层是 linux kernel「内核」，由 BootLoader「系统启动加载器」 负责加载（类似于计算机的BIOS系统）。

/bootable/recovery/bootloader.h

首先启动 init「父进程，第一个进程」进程，接着运行init.rc脚本，脚本文件有个命令启动了Zygote进程，初始化时会启动虚拟机。

/system/core/rootdir/init.zygote.rc

Zygote进程fork出SystemServer进程，然后会调用SystemServer.main()方法。

/frameworks/base/services/java/com/android/server/SystemService.java

/** The main entry point from zygote.*/
public static void main(String[] args) {
  new SystemServer().run();
}

run方法中，主要是在进程中启动系统的各项服务，比如ActivityManagerService，PackageManagerService，WindowManagerService服务等。

private void run() {
  //创建主线程Looper、ActivityThread、SystemContext
  android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_FOREGROUND);
  Looper.prepareMainLooper();
  // Initialize native services.
  System.loadLibrary("android_servers");  
  // Initialize the system context.
  createSystemContext();
  
  // Create the system service manager.
  mSystemServiceManager = new SystemServiceManager(mSystemContext);
  mSystemServiceManager.setStartInfo(mRuntimeRestart,mRuntimeStartElapsedTime, mRuntimeStartUptime);
  LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
  // 并行线程池
  SystemServerInitThreadPool.get();
  // Start services.
  traceBeginAndSlog("StartServices");
  startBootstrapServices();
  startCoreServices();
  startOtherServices();
  // Loop forever.
  Looper.loop();
}

下面是一些主要的初始化方法。

/**
 * 这些服务具有复杂的相互依赖关系，所以需要放一起全部初始化
 */
private void startBootstrapServices() {
  // Start the watchdog as early as possible so we can crash the system server
  final Watchdog watchdog = Watchdog.getInstance();
  watchdog.start();
  //启动AMS
  ActivityTaskManagerService atm = mSystemServiceManager.startService(
        ActivityTaskManagerService.Lifecycle.class).getService();
  mActivityManagerService = ActivityManagerService.Lifecycle.startService(
        mSystemServiceManager, atm);
  //电源管理器需要提前启动，因为其他服务需要它
  mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);
  // Start the package manager.
  mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
          mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
  //设置Application实例并开始
  mActivityManagerService.setSystemProcess();
  //使用 ActivityManager 实例完成看门狗设置并监听重启
  watchdog.init(context, mActivityManagerService);
}

真正启动是在ActivityManagerService的中systemReady方法，调用resumeTopActivityLocked打开锁屏界面。

/**
 * Starts a miscellaneous grab bag of stuff that has yet to be refactored and organized.
 */
private void startOtherServices() {
  //启动WMS
  wm = WindowManagerService.main();
  mActivityManagerService.setWindowManager(wm);
  
  //WMS 显示默认启动消息
  ActivityManagerNative.getDefault().showBootMessage();
  //开始启动初始应用程序
  mActivityManagerService.systemReady(new Runnable(){
    //SystemUI
    startSystemUi(context, windowManagerF);
  });
}

/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

/** 通过StackSupervisor运行所有 ActivityStacks */
final ActivityStackSupervisor mStackSupervisor;
public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {
  mStackSupervisor.resumeFocusedStackTopActivityLocked();
}

到这里，android的开机流程结束。