@Override
 protected void onStop() {
 // TODO Auto-generated method stub
 　　super.onStop();
 　　Log.v("BACKGROUND", "程序进入后台");
 　　showNotification();
 }

 // 点击HOME键时程序进入后台运行
 @Override
 public boolean onKeyDown(int keyCode, KeyEvent event) {
  // TODO Auto-generated method stub
  // 按下HOME键
  if(keyCode == KeyEvent.KEYCODE_HOME){
   // 显示Notification
   notification = new NotificationExtend(this);
   notification.showNotification();
   moveTaskToBack(true);    
 
   return true;
  }
  
  return super.onKeyDown(keyCode, event);
 }

package com.test.background;

import android.app.Activity;
import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Intent;
import android.graphics.Color;

/**
 * Notification扩展类
 * @Description: Notification扩展类

 * @File: NotificationExtend.java

 * @Package com.test.background

 * @Author Hanyonglu

 * @Date 2012-4-13 下午02:00:44

 * @Version V1.0
 */
public class NotificationExtend {
 private Activity context;
 
 public NotificationExtend(Activity context) {
  // TODO Auto-generated constructor stub
  this.context = context;
 }
 
 // 显示Notification
 public void showNotification() {
  // 创建一个NotificationManager的引用
  NotificationManager notificationManager = (
    NotificationManager)context.getSystemService(
      android.content.Context.NOTIFICATION_SERVICE);
  
  // 定义Notification的各种属性
  Notification notification = new Notification(
    R.drawable.icon,"阅读器", 
    System.currentTimeMillis());
  // 将此通知放到通知栏的"Ongoing"即"正在运行"组中
  notification.flags |= Notification.FLAG_ONGOING_EVENT;
  // 表明在点击了通知栏中的"清除通知"后，此通知自动清除。
  notification.flags |= Notification.FLAG_AUTO_CANCEL
  notification.flags |= Notification.FLAG_SHOW_LIGHTS;
  notification.defaults = Notification.DEFAULT_LIGHTS;
  notification.ledARGB = Color.BLUE;
  notification.ledOnMS = 5000;
    
  // 设置通知的事件消息
  CharSequence contentTitle = "阅读器显示信息"; // 通知栏标题
  CharSequence contentText = "推送信息显示，请查看……"; // 通知栏内容
  
  Intent notificationIntent = new Intent(context,context.getClass());
  notificationIntent.setAction(Intent.ACTION_MAIN);
  notificationIntent.addCategory(Intent.CATEGORY_LAUNCHER);
  PendingIntent contentIntent = PendingIntent.getActivity(
  context, 0, notificationIntent,PendingIntent.FLAG_UPDATE_CURRENT);
  notification.setLatestEventInfo(
  context, contentTitle, contentText, contentIntent);
  // 把Notification传递给NotificationManager
  notificationManager.notify(0, notification);
 }
 
 // 取消通知
 public void cancelNotification(){
  NotificationManager notificationManager = (
    NotificationManager) context.getSystemService(
      android.content.Context.NOTIFICATION_SERVICE);
  notificationManager.cancel(0);
 }
}

 <activity android:name="ShowMessageActivity"
    android:launchMode="singleTask"></activity> 

import java.util.List;
import com.test.background.MainActivity;
import com.test.background.NotificationExtend;
import com.test.background.R;
import com.test.util.AppManager;

import android.app.ActivityManager;
import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.ActivityManager.RunningAppProcessInfo;
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.graphics.Color;
import android.os.IBinder;
import android.util.Log;

/**
 * 监听程序是否在前后台运行Service
 * @Description: 监听程序是否在前后台运行Service

 * @FileName: AppStatusService.java 

 * @Package com.test.service

 * @Author Hanyonglu

 * @Date 2012-4-13 下午04:13:47 

 * @Version V1.0
 */
public class AppStatusService extends Service{
 private static final String TAG = "AppStatusService"; 
 private ActivityManager activityManager; 
 private String packageName;
 private boolean isStop = false;
 
 @Override
 public IBinder onBind(Intent intent) {
  // TODO Auto-generated method stub
  return null;
 }
 
 @Override
 public int onStartCommand(Intent intent, int flags, int startId) {
  // TODO Auto-generated method stub
  activityManager = (ActivityManager) this.getSystemService(Context.ACTIVITY_SERVICE); 
  packageName = this.getPackageName(); 
  System.out.println("启动服务");
  
  new Thread() { 
   public void run() { 
    try { 
     while (!isStop) { 
      Thread.sleep(1000); 
      
      if (isAppOnForeground()) { 
       Log.v(TAG, "前台运行");
      } else { 
       Log.v(TAG, "后台运行");
       showNotification();
      } 
     } 
    } catch (Exception e) { 
     e.printStackTrace(); 
    } 
   } 
  }.start(); 
  
  return super.onStartCommand(intent, flags, startId);
 }
 
 /**
  * 程序是否在前台运行
  * @return
  */
 public boolean isAppOnForeground() { 
  // Returns a list of application processes that are running on the device 
  List<RunningAppProcessInfo> appProcesses = activityManager.getRunningAppProcesses(); 
  if (appProcesses == null) return false; 
  
  for (RunningAppProcessInfo appProcess : appProcesses) { 
   // The name of the process that this object is associated with. 
   if (appProcess.processName.equals(packageName) 
     && appProcess.importance == RunningAppProcessInfo.IMPORTANCE_FOREGROUND) { 
    return true; 
   } 
  } 
  
  return false; 
 } 
 
 @Override
 public void onDestroy() {
  // TODO Auto-generated method stub
  super.onDestroy();
  System.out.println("终止服务");
  isStop = true;
 }
 
 // 显示Notification
 public void showNotification() {
  // 创建一个NotificationManager的引用
  NotificationManager notificationManager = (
    NotificationManager)getSystemService(
      android.content.Context.NOTIFICATION_SERVICE);
  
  // 定义Notification的各种属性
  Notification notification = new Notification(
    R.drawable.icon,"阅读器", 
    System.currentTimeMillis());
  // 将此通知放到通知栏的"Ongoing"即"正在运行"组中
  notification.flags |= Notification.FLAG_ONGOING_EVENT;
  // 点击后自动清除Notification
  notification.flags |= Notification.FLAG_AUTO_CANCEL;
  notification.flags |= Notification.FLAG_SHOW_LIGHTS;
  notification.defaults = Notification.DEFAULT_LIGHTS;
  notification.ledARGB = Color.BLUE;
  notification.ledOnMS = 5000;
    
  // 设置通知的事件消息
  CharSequence contentTitle = "阅读器显示信息"; // 通知栏标题
  CharSequence contentText = "推送信息显示，请查看……"; // 通知栏内容
  
  Intent notificationIntent = new Intent(AppManager.context,AppManager.context.getClass());
  notificationIntent.setAction(Intent.ACTION_MAIN);
  notificationIntent.addCategory(Intent.CATEGORY_LAUNCHER);
  PendingIntent contentIntent = PendingIntent.getActivity(
    AppManager.context, 0, notificationIntent,PendingIntent.FLAG_UPDATE_CURRENT);
  notification.setLatestEventInfo(
    AppManager.context, contentTitle, contentText, contentIntent);
  // 把Notification传递给NotificationManager
  notificationManager.notify(0, notification);
 }
}

/**
  *判断当前应用程序处于前台还是后台
  * 
  * @param context
 
  * @return 
  */
 public static boolean isApplicationBroughtToBackground(final Context context) {
  ActivityManager am = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
  List<RunningTaskInfo> tasks = am.getRunningTasks(1);
  if (!tasks.isEmpty()) {
   ComponentName topActivity = tasks.get(0).topActivity;
   if (!topActivity.getPackageName().equals(context.getPackageName())) {
    return true;
   }
  }
  return false;
 
 }

<uses-permission android:name="android.permission.GET_TASKS" />

/**
* 
* @param context
* @return
*/
public static boolean isBackground(Context context) {
 
 
ActivityManager activityManager = (ActivityManager) context
.getSystemService(Context.ACTIVITY_SERVICE);
List<RunningAppProcessInfo> appProcesses = activityManager
.getRunningAppProcesses();
for (RunningAppProcessInfo appProcess : appProcesses) {
if (appProcess.processName.equals(context.getPackageName())) {
if (appProcess.importance == RunningAppProcessInfo.IMPORTANCE_BACKGROUND) {
Log.i(String.format("Background App:", appProcess.processName));
return true;
}else{
Log.i(String.format("Foreground App:", appProcess.processName));
return false;
}
}
}
return false;
}

@Override
 protected void onUserLeaveHint() { //当用户按Home键等操作使程序进入后台时即开始计时
  super.onUserLeaveHint();
  if(!isLeave){
   isLeave=true;
   saveStartTime();
  }
 }