从Android系统启动说起

前言:
看源码也有一段时间了,为了能更深刻的理解Android源码,我准备把看过的源码用自己的思维捋一遍,通过写博客逼迫自己认真去理解.源码太庞大了,尽量能抛开繁琐的源代码把他们的功能讲出来.每次只讲某一段逻辑的源码实现,这样读起来就不会太累.

Android系统启动的第一个进程是init进程,接着init通过解析init.rc依次启动ServiceManager进程,Zygote进程,SystemServer进程.
SystemServer就是AMS,PMS,WMS等我们经常接触到的主要服务的宿主进程.
SystemServer开始启动各种服务,入口函数为main

SystemServer:
public static void main(String[] args) {
  new SystemServer().run();
}

run里面就开启了大部分的Service.
startBootstrapServices();
startCoreServices();
startOtherServices();

SystemServer:
 private void run() {
  ...
  try {
    traceBeginAndSlog("StartServices");
    startBootstrapServices();
    startCoreServices();
    startOtherServices();
    SystemServerInitThreadPool.shutdown();
  } catch (Throwable ex) {
    Slog.e("System", "******************************************");
    Slog.e("System", "************ Failure starting system services", ex);
    throw ex;
  } finally {
    traceEnd();
  }
  ...
}

startBootstrapServices()里就开启了常见的几个Service,比如ActivityManagerService,PowerManagerService等等
startOtherServices()启动了PackageManagerService,VibratorService,AlarmManagerService,WindowManagerService等常见service

SystemServer:
private void startBootstrapServices() {
  ...
  traceBeginAndSlog("StartActivityManager");
  mActivityManagerService = mSystemServiceManager.startService(
                ActivityManagerService.Lifecycle.class).getService();
  mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
  mActivityManagerService.setInstaller(installer);
  traceEnd();
  ...
  // Set up the Application instance for the system process and get started.
  traceBeginAndSlog("SetSystemProcess");
  mActivityManagerService.setSystemProcess();
  traceEnd();
  ...
}

这里是通过SystemServiceManager(一看名字就是SystemServer的管理类API-19新加的,老的源码是通过各个service的main方法去构造自己,比如ActivityManagerService.main())的startService()通过反射去构造各个服务.
AMS启动后还要通过setSystemProcess()去向ServiceManager注册自己

ActivityManagerService:
  public void setSystemProcess() {
        try {
            ServiceManager.addService(Context.ACTIVITY_SERVICE, this, true);
            ServiceManager.addService(ProcessStats.SERVICE_NAME, mProcessStats);
            ServiceManager.addService("meminfo", new MemBinder(this));
            ServiceManager.addService("gfxinfo", new GraphicsBinder(this));
            ServiceManager.addService("dbinfo", new DbBinder(this));
            if (MONITOR_CPU_USAGE) {
                ServiceManager.addService("cpuinfo", new CpuBinder(this));
            }
            ServiceManager.addService("permission", new PermissionController(this));
            ServiceManager.addService("processinfo", new ProcessInfoService(this));

            ApplicationInfo info = mContext.getPackageManager().getApplicationInfo(
                    "android", STOCK_PM_FLAGS | MATCH_SYSTEM_ONLY);
            mSystemThread.installSystemApplicationInfo(info, getClass().getClassLoader());

            synchronized (this) {
                ProcessRecord app = newProcessRecordLocked(info, info.processName, false, 0);
                app.persistent = true;
                app.pid = MY_PID;
                app.maxAdj = ProcessList.SYSTEM_ADJ;
                app.makeActive(mSystemThread.getApplicationThread(), mProcessStats);
                synchronized (mPidsSelfLocked) {
                    mPidsSelfLocked.put(app.pid, app);
                }
                updateLruProcessLocked(app, false, null);
                updateOomAdjLocked();
            }
        } catch (PackageManager.NameNotFoundException e) {
            throw new RuntimeException(
                    "Unable to find android system package", e);
        }
    }

从代码上可以看出他不仅注册的自己的主业务(ACTIVITY_SERVICE),还有很多比如内存管理("meninfo"),cpu管理("cpuinfo"),权限管理("permission")等等诸多副业.
ActivityManagerService启动好之后,我们就可以去启动我们的应用程序了.Launcher程序就是一个特殊的应用程序.
Activity的启动也是个复杂的过程就放到下一节来讲吧.

类的总结:
ServiceManager:Binder机制中的"DNS服务器",负责域名(某Binder服务在ServiceManager注册时提供的名称)到IP地址(Binder驱动分配)的解析.
Zygote进程:字面意思"受精卵",是Android大多数应用和系统进程的孵化器.
SystemServer:大部分服务的宿主Service,SystemServer启动后就进入Launcher主界面了.
ActivityManagerService:寄生在SystemServer里.管理Activity,Service,Broadcast等生命周期.同时负责内存管理,CPU管理,权限管理等功能

源码版本:API-27