Notification notification = new Notification(R.drawable.icon, "服务开启", System.currentTimeMillis());
notification.flags|= Notification.FLAG_NO_CLEAR; 
notification.flags=Notification.FLAG_ONGOING_EVENT;
Intent notificationIntent = new Intent(this, MainActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
notification.setLatestEventInfo(this, "service", "防止服务被任务管理器所杀", pendingIntent);
startForeground(ONGOING_NOTIFICATION, notification);

Notification notification = new Notification();
startForeground(1, notification);

public class TestService extends Service {
 private static final Class[] mStartForegroundSignature = new Class[] {
   int.class, Notification.class };
 private static final Class[] mStopForegroundSignature = new Class[] { boolean.class };
 private NotificationManager mNM;
 private Method mStartForeground;
 private Method mStopForeground;
 private Object[] mStartForegroundArgs = new Object[2];
 private Object[] mStopForegroundArgs = new Object[1];
 @Override
 public IBinder onBind(Intent intent) {
  return null;
 }
 @Override
 public void onCreate() {
  super.onCreate();
  mNM = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
  try {
   mStartForeground = TestService.class.getMethod("startForeground",
     mStartForegroundSignature);
   mStopForeground = TestService.class.getMethod("stopForeground",
     mStopForegroundSignature);
  } catch (NoSuchMethodException e) {
   mStartForeground = mStopForeground = null;
  }
  // 我们并不需要为 notification.flags 设置 FLAG_ONGOING_EVENT，因为
  // 前台服务的 notification.flags 总是默认包含了那个标志位
  Notification notification =new Notification();
  // 注意使用 startForeground ，id 为 0 将不会显示 notification
  startForegroundCompat(1, notification);
 }
 @Override
 public void onDestroy() {
  super.onDestroy();
  stopForegroundCompat(1);
 }
 // 以兼容性方式开始前台服务
 private void startForegroundCompat(int id, Notification n) {
  if (mStartForeground != null) {
   mStartForegroundArgs[0] = id;
   mStartForegroundArgs[1] = n;
   try {
    mStartForeground.invoke(this, mStartForegroundArgs);
   } catch (IllegalArgumentException e) {
    e.printStackTrace();
   } catch (IllegalAccessException e) {
    e.printStackTrace();
   } catch (InvocationTargetException e) {
    e.printStackTrace();
   }
   return;
  }
  mNM.notify(id, n);
 }
 // 以兼容性方式停止前台服务
 private void stopForegroundCompat(int id) {
  if (mStopForeground != null) {
   mStopForegroundArgs[0] = Boolean.TRUE;
   try {
    mStopForeground.invoke(this, mStopForegroundArgs);
   } catch (IllegalArgumentException e) {
    e.printStackTrace();
   } catch (IllegalAccessException e) {
    e.printStackTrace();
   } catch (InvocationTargetException e) {
    e.printStackTrace();
   }
   return;
  }
  // 在 setForeground 之前调用 cancel，因为我们有可能在取消前台服务之后
  // 的那一瞬间被kill掉。这个时候 notification 便永远不会从通知一栏移除
  mNM.cancel(id);
 }
}