利用javabean由servlet向JSP传递数据，HTTP状态 500 - 内部服务器错误，

HTTP状态 500 - 内部服务器错误

类型异常报告

消息在 [23] 行处理 [/index.jsp] 时发生异常

描述服务器遇到一个意外的情况，阻止它完成请求。

例外情况
org.apache.jasper.JasperException: 在 [23] 行处理 [/index.jsp] 时发生异常

20:
21: <%
22: List list = (List)request.getAttribute("ResultSet");
23: for(DepartmentBean item:list){
24: out.println(item.getDepartmentld()+" "+ item.getDepartmentName());
25: }
26: %>

Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:611)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:500)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:378)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:326)
javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
servlet.GetDeptListServlet.doGet(GetDeptListServlet.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:626)
javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

根本原因。
java.lang.NullPointerException: Cannot invoke "java.util.List.iterator()" because "list" is null
org.apache.jsp.index_jsp._jspService(index_jsp.java:146)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:71)
javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:467)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:378)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:326)
javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
servlet.GetDeptListServlet.doGet(GetDeptListServlet.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:626)
javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

Cannot invoke "java.util.List.iterator()" because "list" is null
空指针异常

request获取的ResultSet有问题，需要检查下传输的请求头参数，看下为什么是空的，可以考虑下postman之类的先做接口测试。
空指针异常处理，即使获取参数有问题，代码也要进行规避空指针的情况。修改举例如下:

List list = (List)request.getAttribute("ResultSet");
for(DepartmentBean item:list){
 out.println(item.getDepartmentld()+" "+ item.getDepartmentName());
 }
 修改为：
List list = (List)request.getAttribute("ResultSet");
if (list != null) for(DepartmentBean item:list) {
 out.println(item.getDepartmentld()+" "+ item.getDepartmentName());
 }

package servlet;

import java.io.IOException;
import java.sql.;
import java.util.;
import bean.*;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**

Servlet implementation class GetDeptListServlet
/
@WebServlet("/GetDeptListServlet")
public class GetDeptListServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
/**
- @see HttpServlet#HttpServlet()
- /
  public GetDeptListServlet() {
  super();
  // TODO Auto-generated constructor stub
  }
/**
- @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
- /
  protected void doGet(HttpServletRequest request, HttpServletResponse response)
```
  throws ServletException, IOException {
```
  // TODO Auto-generated method stub
  try {
  Class.forName("com.mysql.jdbc.Driver").newInstance();
  String url = "jdbc:mysql://localhost:3306/CRM";
  String user= "root";
  String pwd ="123456";
  Connection conn = DriverManager.getConnection(url,user,pwd);
  String sql = "SELECT * FROM Department;";
  Statement stmt = conn.createStatement();
  ResultSet rs = stmt.executeQuery(sql);
  List list = new ArrayList();
  while(rs.next()) {
```
  DepartmentBean  model = new DepartmentBean();
  model.setDepartmentld(rs.getString("Departmentld"));
  model.setDepartmentName(rs.getString("DepartmentName"));
  list.add(model);
```
  }
  request.setAttribute("ResultSet",list);
  rs.close();
  stmt.close();
  conn.close();
  }
  catch(Exception ex) {
  }
  request.getRequestDispatcher("index.jsp").forward(request, response);
```
      //rs.close();
      //stmt.close();
      //conn.close();
```
  }
/**
- @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
- /
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  // TODO Auto-generated method stub
  this.doGet(request, response);
  }

<%@ page language="java" contentType="texthtml; charset=UTF-8"
import="java.util.*,java.sql.*,bean.*"
pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath=request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

My JSP 'index.jsp' starting page<%

List list = (List)request.getAttribute("ResultSet");
for(DepartmentBean item:list){
out.println(item.getDepartmentld()+" "+ item.getDepartmentName());
}
%>

package bean;

public class DepartmentBean {
private String departmentld;
private String departmentName;
public String getDepartmentld() {
return departmentld;
}
public void setDepartmentld(String departmentld) {
this.departmentld = departmentld;
}
public String getDepartmentName() {
return departmentName;
}
public void setDepartmentName(String departmentName) {
this.departmentName = departmentName;
}

}

先判断是否为空，如果为空然后new ArrayList即可解决，如果是查询问题则修改底层控制层代码即可

前面已经提到了，就是空指针异常。即你后端查询的数据没有结果，导致变量为null传到jsp，遍历失败！

空指针异常

List list = (List)request.getAttribute("ResultSet");

这个地方list 是null,检查下“ResultSet”这个key.