堆栈跟踪：]与根源显示java.lang.NullPointerException

-3

我在这一点上的代码堆栈跟踪：]与根源显示java.lang.NullPointerException

org.apache.catalina.core.StandardWrapperValve invoke SEVERE: 

Servlet.service() for servlet [jsp] in context with path [/Work] threw exception [An exception occurred processing JSP page /Display.jsp at line 49 
46:<td> 
47: <% 
48: String sql="select * from Patient "; 
49: pstmt=con.prepareStatement(sql); 
50: rs=pstmt.executeQuery(); 
51: while(rs.next()){ 
52: %>

堆栈跟踪收到错误：]与根源显示java.lang.NullPointerException

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN""http://www.w3.org/TR/html4/loose.dtd"> <html> <head> 
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> <title>Insert title here</title> </head> 
<%@page import="java.sql.DriverManager"%> 
<%@page import="java.sql.ResultSet"%> 
<%@page import="java.sql.PreparedStatement"%> 
<%@page import="java.sql.Connection"%> 
<%@page import="com.connection.*"%> <body> 
<%! Connection con=null; 
PreparedStatement pstmt=null; 
ResultSet rs=null; 
public void init() { 
try{ 
    Connection con = ConnectionImplementation.wantConnection(); 
    // Statement stmt= con.createStatement(); 
    } catch (Exception e) { 
    // TODO Auto-generated catch block 
    e.printStackTrace(); 
    } 
} 
%> 
<table> 
<tr> <td> 
<select name="PERSON_TYPE" id="PERSON_TYPE"> 
<option value="">SELECT</option> 
<option value="Doctor">Doctor</option> 
<option value="Nurse">Nurse</option> 
<option value="Patient">Patient</option> 
</select> </td> 
</tr> 
<tr> <td> 
<% String sql="select * from Patient "; 
pstmt=con.prepareStatement(sql); 
rs=pstmt.executeQuery(); 
while(rs.next()){ %> 
<tr><td> <%= rs.getInt(1)%></td> 
<td><%= rs.getString(2) %></td> 
<td><%= rs.getString(3) %></td> 
<td><%= rs.getInt(4) %></td> 
<td><%= rs.getString(5) %></td> 
<td><%= rs.getString(6) %></td> 
<td><%= rs.getString(7) %></td></tr> 
<% } %> </table> </body> </html>

来源

2016-01-13 Munna R Munna

可能重复[什么是空指针异常，以及如何解决它？]（http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how -DO-I-FIX-它） – fractalwrench

你的成员变量con永远不会被分配一个值因此null

在您的init()方法中，您声明一个本地变量con - 因此成员变量Connection con永远不会从null更改为其他任何内容。

来源

2016-01-13 11:28:48 Jan

堆栈跟踪：]与根源显示java.lang.NullPointerException

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