当我在Conn.Java类中进行JDBC连接时，我正在尝试将JSP和包中的Java文件链接起来

Question

我正在学习Java Servlet，并在Netbeans中导入MySQL JDBC驱动程序MySQL-connector-Java-5.1.23-bin.jar。我在包mypack 中制作了Conn.java类

package mypack;
import java.sql.*;
import java.sql.DriverManager;
public class Conn{
public Connection c;
public Statement s;
public Conn(){
try{
Class.forName("com.mysql.jdbc.Driver");
c=DriverManager.getConnection("jdbc:mysql:///proj4","root","");
s=c.createStatement();
}
catch(ClassNotFoundException | SQLException e){
}
}
}

然后我编码了modifyser.jsp

<%@page import  ="java.sql.*"%>
<%@page import = "mypack.Conn" %>

<%
String u = "";
mypack.Conn db = new mypack.Conn();
ResultSet rs = db.s.executeQuery("select * from login");  // error!
out.println("<table><th>Username<th>Password<th>type<th>Modify");
while (rs.next()) {
u = rs.getString("username");
out.println("<table><th>Username<th>Password<th>type<th>Modify");
}
%>
<tr>
<td>
<%= u%></td>
<td>
<%= rs.getString("password")%></td>
<td>
<%= rs.getString("type")%></td>
<td>
<a href ="modifyuser1.jsp" user ="<%=u%>">Modify</a></td>
</tr>

然而，在运行此web时(使用服务器Apache Tomcat/8.0.27，Netbeans8.2作为平台(，它会抛出错误-

org.apache.jaster.JasperException：在第7行处理JSP页面/modifyuser.JSP时发生异常

根本原因--java.lang.NullPointerException你能建议我如何消除这个错误吗？

Answer 1

以下代码需要在while内部，因为您当前的代码使其无法迭代Resultset

<tr>
<td>
<%= u%></td>
<td>
<%= rs.getString("password")%></td>
<td>
<%= rs.getString("type")%></td>
<td>
<a href ="modifyuser1.jsp" user ="<%=u%>">Modify</a></td>
</tr>

所以把你的代码改成

<%
String u = "";
mypack.Conn db = new mypack.Conn();
ResultSet rs = db.s.executeQuery("select * from login");  // error!
out.println("<table><th>Username<th>Password<th>type<th>Modify");
out.println("<table><th>Username<th>Password<th>type<th>Modify"); 
while (rs.next()) {
%>
<tr>
<td>
<%= u%></td>
<td>
<%= rs.getString("password")%></td>
<td>
<%= rs.getString("type")%></td>
<td>
<a href ="modifyuser1.jsp" user ="<%=u%>">Modify</a></td>
</tr>
<% } %>