用户在登录系统时，出现网络断开，从而系统自动检测到网络状态。

 /** 
 * @author 麦克劳林
 * @功能：持续检测网络是否连通 
 */  
  import java.io.BufferedReader;   
  import java.io.InputStream;  
  import java.io.InputStreamReader;  
  import java.text.SimpleDateFormat;  
  import java.util.Date; 
  public class NetState implements Runnable {     
  public static void main(String[] args) {  
    // TODO Auto-generated method stub  
    NetState ns = new NetState();  
    new Thread(ns).start();//启动线程  
}  

// 判断网络状态  
public void isConnect() {  
    Runtime runtime = Runtime.getRuntime();  
    try {  
        Process process = runtime.exec("ping " + "www.baidu.com");  
        InputStream is = process.getInputStream();  
        InputStreamReader isr = new InputStreamReader(is);  
        BufferedReader br = new BufferedReader(isr);  
        String line = null;  
        StringBuffer sb = new StringBuffer();  
        while ((line = br.readLine()) != null) {  
            sb.append(line);  
            // System.out.println("返回值为:"+line);
        }  
        is.close();  
        isr.close();  
        br.close();  

        if (null != sb && !sb.toString().equals("")) {  
            String logString = "";  
            if (sb.toString().indexOf("TTL") > 0) {  
               // 网络畅通  
              logString = "网络正常，时间 " + this.getCurrentTime();  
              System.out.println("网络正常！时间为："+logString);  
            } else {  
                // 网络不畅通  
              logString = "网络断开，时间 " + this.getCurrentTime();  
              System.out.println("网络断开！时间为"+logString);  
            }    
        }  
    } catch (Exception e) {  
        e.printStackTrace();  
    }  
}    
// 获得当前时间  
public String getCurrentTime() {  
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
    String time = sdf.format(new Date());  
    return time;  
}    
@Override  
public void run() {  
    // TODO Auto-generated method stub  
    while (true) {  
        this.isConnect();  
        try {  
            // 每隔3秒钟测试一次网络是否连通  
            Thread.sleep(3000);  
        } catch (InterruptedException e) {  
            // TODO Auto-generated catch block  
            e.printStackTrace();  
        }  
      }  
  }  
}