Service中startForeground(int id, Notification notification)解释（用于进程保活）

官方对该方法的解释

/**
* If your service is started (running through {@link Context#startService(Intent)}), then
* also make this service run in the foreground, supplying the ongoing
* notification to be shown to the user while in this state.
* By default started services are background, meaning that their process won't be given
* foreground CPU scheduling (unless something else in that process is foreground) and,
* if the system needs to kill them to reclaim more memory (such as to display a large page in a
* web browser), they can be killed without too much harm.  You use
* {@link #startForeground} if killing your service would be disruptive to the user, such as
* if your service is performing background music playback, so the user
* would notice if their music stopped playing.
*
* 
Note that calling this method does not put the service in the started state
* itself, even though the name sounds like it.  You must always call
* {@link #startService(Intent)} first to tell the system it should keep the service running,
* and then use this method to tell it to keep it running harder.
*
* @param id The identifier for this notification as per
* {@link NotificationManager#notify(int, Notification)
* NotificationManager.notify(int, Notification)}; must not be 0.
* @param notification The Notification to be displayed.
* 
* @see #stopForeground(boolean)
* 
* 翻译：如果服务(通过{@link Context#startService(Intent)}运行行)已经启动，那么还可以让这个服务运行在前台，并提供给用户的提供一个通知。可以调用这个方法
* 默认情况下，服务启动后是处于后台的，这意味着不会被前台CPU调度（除非该服务的一部分运行在前台）。如果系统需要杀死他们去释放内存（比如，需要在浏览器中展示一个大的界面），他们是很容易被杀死。
* 如果终止该服务会对用户造成破坏，那么可以调用该方法。比如：该服务正在播放一段背景音乐，用户是不希望停止掉的
* 
* 注意：调用该方法并不能启动服务，你必须调用startService方法去通知系统让该服务保持运行，然后调用该方法告诉系统更坚固的运行该服务
*/
public final void startForeground(int id, Notification notification) {
     try {
        mActivityManager.setServiceForeground(
                    new ComponentName(this, mClassName), mToken, id,
                    notification, 0);
     } catch (RemoteException ex) {
     }
}

1. 如果服务(通过{@link Context#startService(Intent)}运行行)已经启动，那么还可以让这个服务运行在前台，并提供给用户的提供一个通知。可以调用这个方法
2. 默认情况下，服务启动后是处于后台的，这意味着不会被前台CPU调度（除非该服务的一部分运行在前台）。如果系统需要杀死他们去释放内存（比如，需要在浏览器中展示一个大的界面），他们是很容易被杀死。

3. 如果终止该服务会对用户造成破坏，那么可以调用该方法。比如：该服务正在播放一段背景音乐，用户是不希望停止掉的

   注：调用该方法并不能启动服务，你必须调用startService方法去通知系统让该服务保持运行，然后调用该方法告诉系统更坚固的运行该服务

作用：使用该方法可以让后台服务编程前台服务，使服务的优先级变低，不容易被回收掉。可以用来做进程保活。可以让当前进程持有这样一个服务，当当前进程进入后台之后，由于持有前台服务，会使得当前进程的优先级变低。一般情况下，若当前进程不持有前台进程，当前进程进入后台之后优先级为5，若持有前台进程，优先级变为2。使得不容易被回收