【新手求教】用定时器检测超时后关闭socket抛出socket closed 异常

由于是自己自学的java，基础不好，遇到这个问题，研究好久没得出答案，特来请教。希望懂的朋友指点迷津，万谢。
异常提示：
java.net.SocketException: Socket closed
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:150)
at java.net.SocketInputStream.read(SocketInputStream.java:121)
at java.net.SocketInputStream.read(SocketInputStream.java:107)
at my.start.AutoStart$SocketTherad.run(AutoStart.java:108)
java.net.SocketException: Socket is closed
at java.net.Socket.getInputStream(Socket.java:903)
at my.start.AutoStart$SocketTherad.run(AutoStart.java:104)
java.net.SocketException: Socket is closed
at java.net.Socket.getInputStream(Socket.java:903)
at my.start.AutoStart$SocketTherad.run(AutoStart.java:104)

package my.start;





import java.io.IOException;

import java.io.InputStream;

import java.io.OutputStream;

import java.net.ServerSocket;

import java.net.Socket;

import java.util.Timer;

import java.util.TimerTask;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;



//创建AutoStart类，让他实现ServletContextListener（服务程序上下文监听器）接口

//实现ServletContextListener接口需要重写contextDestroyed()和contextInitialized()两个方法

//记得要在web.xml文件中加入

//   <listener>  

//       <listener-class>my.test.AutoStart</listener-class>  

//   </listener>

//这样，tomcat在启动时，就会同时调用该接口的contextInitialized()方法

//在contextInitialized()方法中，开启一个线程，用soket监听一个端口，从而实现TCP通信

public class AutoStart extends  HttpServlet{	



	private static final long serialVersionUID = 1L;

	private int soketPort = 9014;//要监听的服务器端口，可以根据需要进行修改

	@Override

	public void init() throws ServletException {

		// TODO Auto-generated method stub

		super.init();

		new MyThread().start();//创建一个MyThread线程对象，并启动线程

		System.out.println("开启监听线程");

	}

	

	

	//创建MyThread类，继承Thread方法

   class MyThread extends Thread 

   { 

	 //重写Thread类的run()方法，用来实现MyThread线程的功能

           public void run()

           { 

                 //System.out.println("测试开始");

                 try {

						ServerSocket ss = new ServerSocket(soketPort);

						System.out.println("监听到"+(soketPort+"")+"端口");

						while(true){

							//System.out.println("已经创建soket");

							//ss对象的accept()方法可以监听指定端口有没有TCP连接

							//在没有TCP连接之前程序将阻塞在这里

							Socket socket = ss.accept();

							System.out.println("有客户端接入");

							//监听到指定端口有TCP连接后，创建soket对象，程序不再堵塞，往下执行

							//创建一个线程，去监听客户端通过TCP发来的数据

							Thread sockThread=new SocketTherad(socket); 

							//启动线程

							sockThread.start();

					}

				} catch (IOException e) {

					// TODO Auto-generated catch block

					e.printStackTrace();

				}

           } 

   } 

   int timeNum;

	class SocketTherad extends Thread{  //继承Thread类来创建线程

		Socket socket;

		InputStream is;

		OutputStream os;

		boolean run_flag=true;//控制run()函数是否继续运行标志位

		

		Timer heartBeatTimer = new Timer(); //心跳包定时器

		private void startHeartBeatThread() {

			TimerTask heartBeatTask = new TimerTask() {

				public void run() {	                

					timeNum++;   

	                System.out.println("timerNum="+(timeNum+""));	

	                if(timeNum==2){//超时则关闭socket连接及定时器

						try {

							is.close();

							os.close();//关闭输出流

							socket.close();//关闭soket

							run_flag=false;//跳出死循环

							System.out.println("TCP连接断开");	

							heartBeatTimer.cancel();//关闭定时器

						} catch (IOException e) {

							// TODO Auto-generated catch block

							e.printStackTrace();

						}

	                }

				}

			};

			heartBeatTimer.schedule(heartBeatTask, 10*1000, 10*1000);

		}

		

		public  SocketTherad(Socket socket) {

			this.socket = socket;//传递过来的soket参数赋予给socket对象

		}

		

		public void run() {//run()函数用来实现线程要完成的任务

			startHeartBeatThread();

			while(run_flag)

			{

				try {

					String str = null;

					is = socket.getInputStream();//获取输入流

					os=socket.getOutputStream();

					byte[] buffer = new byte[200];//数据缓冲区;

					int length=0;

					length = is.read(buffer);//读取接收到的数据流长度

					

					if(length != (-1)){//不是-1，说明读取到有效的数据

						str = new String(buffer,0,length);//输入流转换成str字符串						

						System.out.print("收到数据：");

						System.out.println(str);

					}

					else if(length == (-1)){//接收到数据长度为-1，说明客户端主动关闭了TCP连接

						is.close();//关闭输入流

						os.close();//关闭输出流

						socket.close();//关闭soket

						run_flag=false;//跳出死循环

						System.out.println("TCP连接断开");	

					}					

					buffer = null;

					System.gc();//垃圾回收

				} catch (IOException e) {

					e.printStackTrace();

				}	

			}

		}

	}



}