每当我尝试从我的 servlet 中的 ajax 参数中读取数据时，我都会收到一个空指针异常。继承人的ajax代码：
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<%@page import="java.sql.*;" %>
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<script>
function findNext(id,label){
alert(id);
alert(label);
var xmlhttp=new XMLHttpRequest();
xmlhttp.onreadystatechange=function(){
if(xmlhttp.readyState===4 && xmlhttp.status===200)
{
var ret=xmlhttp.responseText;
var prob=ret.substring(0,ret.indexOf("///"));
var pdesc=ret.substring(3);
document.getElementById("prob").innerHTML=prob;
document.getElementById("pdesc").innerHTML=pdesc;
}
};
xmlhttp.open("POST","Servlet1",true);
var sendstr="pid="+id+"&labelid="+label;
xmlhttp.send(sendstr);
}
</script>
</head>
<body>
<%
Connection con;
Statement s;
ResultSet rs;
String sql,pid=null,prob=null,pdesc=null,yesL=null,noL=null;
try{
Class.forName("com.mysql.jdbc.Driver");
con=DriverManager.getConnection("jdbc:mysql://localhost:3306/cdt", "root", ".hack%//sign66");
s=con.createStatement();
sql="select * from cdt.problem where pid='1.01'";
rs=s.executeQuery(sql);
rs.next();
pid=rs.getObject("pid").toString();
prob=rs.getObject("problm").toString();
pdesc=rs.getObject("pdesc").toString();
yesL=rs.getObject("yesL").toString();
noL=rs.getObject("noL").toString();
}
catch(Exception e){
e.printStackTrace();
}
%>
<!--STYLE THESE 2 <P> STATEMENTS-->
<p id="prob"><%=prob%></p>
<p id="pdesc"><%=pdesc%></p>
<form>
<input type="button" value="Yes" onclick="findNext(<%=pid%>,<%=yesL%>);"/>
<input type="button" value="No" onclick="findNext(<%=pid%>,<%=noL%>);"/>
</form>
</body>
</html>
我传递给函数的变量可以通过警报很好地读取。
继承人的servlet代码：
package cdt1;
import cdt.FindNextLabelLocal;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.ejb.EJB;
public class Servlet1 extends HttpServlet {
Connection con;
Statement s;
ResultSet rs;
@EJB private FindNextLabelLocal obj;
@Override
public void init(ServletConfig config) throws ServletException{
try{
Class.forName("com.mysql.jdbc.Driver");
con=DriverManager.getConnection("jdbc:mysql://localhost:3306/cdt", "root", ".hack%//sign66");
s=con.createStatement();
}
catch(Exception e){
e.printStackTrace();
}
}
@SuppressWarnings("UnusedAssignment")
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
try {
String pid,label,res;
@SuppressWarnings("UnusedAssignment")
float current,next;
current=next = 0;
pid=request.getParameter("pid").toString(); // pid of the question
label=request.getParameter("labelid").toString(); //label of the next question or solution based on button press
current=Float.parseFloat(pid);
next=Float.parseFloat(label);
res=obj.findNext(current,next).toString();
out.write(res);
}
catch(Exception e){
e.printStackTrace();
}
finally {
out.close();
}
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
@Override
public String getServletInfo() {
return "Short description";
}
}
空异常发生在 cdt1.Servlet1.processRequest(Servlet1.java:66) 的 cdt1.Servlet1.doPost(Servlet1.java:124) 处的 java.lang.NullPointerException 第一个发生在我尝试读取 pid 值第二个时当 processRequest 被调用时。我在网上查找了很多资源，这些资源展示了如何使用 ajax 和 servlet，根据我读过的内容，我没有做错任何事情。如果有人能指出我正确的方向，那就太棒了。提前致谢。我正在使用带有 glassfish 3.1.2 的 netbeans 7.3，如果这会有所不同