jsp 中存在错误 java.lang.NullPointerException。如何修复源代码？

Question

出现此错误。下面的 LogFilter.java 代码中哪一部分很奇怪？我是 jsp 的新手并且使用过 eclipse。我用了 tomcat.

错误如下

java.lang.NullPointerException

filter.LogFilter.getURLPath(LogFilter.java:40)

filter.LogFilter.doFilter(LogFilter.java:18)

代码如下。

package filter;

import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
import java.text.DateFormat;
import java.text.SimpleDateFormat;

public class LogFilter implements Filter  {
    
    public void  init(FilterConfig config) throws ServletException{
        System.out.println("WebMarket 초기화...."); 
    }
    
    public void  doFilter(ServletRequest request,ServletResponse response,FilterChain chain) throws java.io.IOException, ServletException {
        System.out.println(" 접속한 클라이언트 IP : " + request.getRemoteAddr());
        long start = System.currentTimeMillis();
        System.out.println(" 접근한 URL 경로 : " + getURLPath(request));
        System.out.println(" 요청 처리 시작 시각 : " + getCurrentTime());
        chain.doFilter(request,response);
        
        long end = System.currentTimeMillis();      
        System.out.println(" 요청 처리 종료 시각 : " + getCurrentTime());
        System.out.println(" 요청 처리 소요 시간 : " + (end-start)+ "ms ");
        System.out.println("=======================================================");
    }

    public void destroy( ){
      
    }

    private String getURLPath(ServletRequest request) {
        HttpServletRequest req;
        String currentPath="";
        String queryString=""; 
        if(request instanceof HttpServletRequest){
            req = (HttpServletRequest)request;
            currentPath = req.getRequestURI();
            queryString = req.getQueryString();
            queryString = queryString.equals(null) ? "" : "?" + queryString;
        }
        return currentPath+queryString;
    }
    
    private String getCurrentTime() {
        DateFormat formatter = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(System.currentTimeMillis());
        return formatter.format(calendar.getTime());
    }
}

Answer 1

queryString = queryString.equals(null) ? "" : "?" + queryString; 不正确。

建议的替代方案：

 querystring = (queryString == null) ? "" : "?" + queryString;

jsp 中存在错误 java.lang.NullPointerException。如何修复源代码？

There is an error java.lang.NullPointerException in jsp. How do I fix the source code?

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