###源码位置com.android.server.am.ActivityManagerService.java

public final class ActivityManagerService extends ActivityManagerNative
    implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {}

ActivityManagerService.java继承了ActivityManagerNative,而ActivityManagerNative实现了
IActivityManager的接口

查看startActivity方法

//caller 调用的主线程
//callingPackage 包名
//intent
//resolvedType
//resultTo
//resultWho
//requestCode
//startFlags
//profilerInfo
//bOptions

 @Override
     public final int startActivity(IApplicationThread caller, String callingPackage,
             Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
             int startFlags, ProfilerInfo profilerInfo, Bundle options) {
         return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
             resultWho, requestCode, startFlags, profilerInfo, options,
             UserHandle.getCallingUserId());
     }

接着调用了startActivityAsUser方法,多传入了一个int类型的userId参数
总得来说startActivityAsUser就是做一下调用者的基本权限的检查

@Override
   public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
           Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
           int startFlags, ProfilerInfo profilerInfo, Bundle options, int userId) {
       //检查调用者是否被隔离，如果被隔离会抛出SecurityException。
       enforceNotIsolatedCaller("startActivity");
       //检查调用者是否有权限执行call操作
       userId = handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId,
               false, ALLOW_FULL_ONLY, "startActivity", null);

       // TODO: Switch to user app stacks here.

       return mStackSupervisor.startActivityMayWait(caller, -1, callingPackage, intent,
               resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
               profilerInfo, null, null, options, false, userId, null, null);
   }

####SDK23会走这里 接着又调用了ActivityStackSupervisor-startActivityMayWait方法,先走API23这个流程

####SDK25会走这里 接着又调用了ActivityStarter-startActivityMayWait方法，以后再说

###参考资料

####http://blog.csdn.net/chenkai19920410/article/details/54344295