Exception in thread "Thread-0" java.lang.NullPointerException

package server;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import publiClass.Datas;
import publiClass.SysConstants;
import publiClass.Tools;
//启动线程
public class Serveryancheng extends Thread{public static void main(String[] args) {//启动服务线程System.out.println("服务器启动成功");new Serveryancheng().start();   }public void run(){//服务器socket初始化ServerSocket serversocket = null;Socket socket =null;ObjectOutputStream oos = null;ObjectInputStream ois= null;try {serversocket =new  ServerSocket(Integer.parseInt(Tools.getVlaue(SysConstants.SYS_LISTENERPORT)));//设置死循环，设置监听while(true){......此处省略很多字......}} catch (NumberFormatException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();     }finally {try {//关闭流oos.close();ois.close();socket.close();} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();      }           }System.out.println("服务器终止");     //处理客户端发送的请求方法    //向客户端发送响应}
}
SysConstants.SYS_LISTENERPORT为监听端口。Tools.getVlaue为取出.properties表中的与SysConstants.SYS_LISTENERPORT对应的port值。Integer.parseInt
运行ServerSocket(Integer.parseInt(Tools.getVlaue(SysConstants.SYS_LISTENERPORT)));之后就出现错误：
服务器启动成功
Exception in thread "Thread-0" java.lang.NullPointerExceptionat server.Serveryancheng.run(Serveryancheng.java:75)

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