Activity启动流程

 我们启动一个Activity时，一般使用

Intent intent = new Intent(mainActivity.this,otherActivity.class);
context.startActivity(intent);

public abstract void startActivity(Intent intent,Bundle options);
public abstract void startActivity(Intent intent);

我们常用的Context有Application、Activity和Service，其中Application和Service直接继承ContextWrapper，而Activity涉及到View所以通过继承ContextThemeWrapper间接的继承了ContextWrapper，ContextWrapper包装的Context的实现类是ContextImpl。因此我们基本都是调用ContextImple的实现方法去执行startActivity的！

frameworks/base/core/java/android/app/Activity.java

public class Activity extends ContextThemeWrapper{
    ...
    public void startActivity(Intent intent,Bindle options){
        if(options!=null){
            startActivityForResult(intent,-1,options);
        } else startActivityForResult(intent,-1);
    }
}

public void startActivityForResult(Intent intent, int requestCode, Bundle options) {
    if (mParent == null) {
        options = transferSpringboardActivityOptions(options);
        ApplicationThread at = mMainThread.getApplicationThread();
        //
        Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity(
          this, at, mToken, this, intent, requestCode, options);
        if (ar != null) {
            mMainThread.sendActivityResult(
              mToken, mEmbeddedID, requestCode,
              ar.getResultCode(), ar.getResultData());
        }
        if (requestCode >= 0) {
            mStartedActivity = true;
        }

        cancelInputsAndStartExitTransition(options);
    } else {
        if (options != null) {
            mParent.startActivityFromChild(this, intent, requestCode, options);
        } else {
            mParent.startActivityFromChild(this, intent, requestCode);
        }
    }
}

Instrumentation#execStartActivity()

public ActivityResult execStartActivity( Context who, IBinder contextThread,
    IBinder token, Activity target, Intent intent,
    int requestCode, Bundle options) {

    IApplicationThread whoThread = (IApplicationThread) contextThread;

    ...

    try {
        intent.migrateExtraStreamToClipData();
        intent.prepareToLeaveProcess(who);
        int result = ActivityManager.getService().startActivity(whoThread,
            who.getBasePackageName(), intent,
            intent.resolveTypeIfNeeded(who.getContentResolver()),
            token, target != null ? target.mEmbeddedID : null,
            requestCode, 0, null, options);
        checkStartActivityResult(result, intent);
    } catch (RemoteException e) {
        throw new RuntimeException("Failure from system", e);
    }
    return null;
}

最终调用ActivityManager.getService().startActivity()

可以看到这里最终调用了ActivityManager.getService().startActivity()，类似AMS 的启动过程，这里是获取了IActivityManager实例，IActivityManager基于 AIDL 实现 Binder，了解 Binder 的话就会知道，因为 App 进程和 AMS 的进程system_server不是同一个进程，所以这里获取到的是 AMS 的IActivityManger在服务端代理，不过这里还会远程调用到 AMS 中去

frameworks/base/core/java/android/app/ActivityManager.java

public static IActivityManager getService(){
    return IActivityManagerSingleton.get();
}