我遇到的问题与接收数字作为输入的表单有关，通过jsp页面（调用javabeans）应该连接到db并打印结果。
当我使用执行连接及其查询的类时，在一个简单的java应用程序中一切正常，当我通过jsp页面执行它时（不修改student.java类），我得到以下错误：
org.apache.jasper.jasperexception:java.lang.nullpointerexception
我正在使用glassfish5、netbeans、jdk8和mysql
我将代码的重要部分整合如下：
结果.jsp
<%@page import="TEST.Student"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<jsp:useBean id="RT" scope="application" class="TEST.Student" />
<jsp:setProperty name="RT" property="freshman" param="freshman"/>
</head>
<body>
<h1>The student's name is: <% out.print(RT.getStudentByFreshman()); %></h1>
</body>
</html>学生.java
package test; import java.io.Serializable; import java.sql.Connection; import java.sql.DriverManager; import java.sql.ResultSet; import java.sql.SQLException; import java.sql.Statement;
public class Student implements Serializable{
private int freshman;
private static final String DBNAME="example30_lp";
private static final String USERNAME="root";
private static final String PWD="********";
private static final String JDBC_URL="jdbc:mysql://localhost:3306/"+DBNAME+"?user="+USERNAME+"&password="+PWD+"&serverTimezone=Europe/Rome";
private Connection con;
public Student() {
}
public void setConnection(){
try {
Class.forName("com.mysql.jdbc.Driver").newInstance();
this.con= DriverManager.getConnection(JDBC_URL);
} catch (ClassNotFoundException ex) {
ex.printStackTrace();
} catch (InstantiationException ex) {
ex.printStackTrace();
} catch (IllegalAccessException ex) {
ex.printStackTrace();
} catch (SQLException ex) {
ex.printStackTrace();
}
}
public void setFreshman(int freshman) {
this.freshman= freshman;
}
public String getMessTest(){
return ("The BEAN found (param"+freshman+")"); }
public String getStudentByFreshman(){
setConnection();
String result = null;
try {
Statement stat = con.createStatement();
ResultSet rs = stat.executeQuery(
"SELECT * FROM `students` WHERE `freshman` = '" +
freshman+ "'");
while (rs.next()) {
result=rs.getString("name");
}
} catch (SQLException ex) {
ex.printStackTrace();
}
return result;
}
/*TEST FOR JavaApplication */
/* public static void main(String args[]){
Student RT = new Student();
RT.setFreshman(10120);
System.out.println("Nome "+RT.getStudentByFreshman());
}*/ }索引.html
<!DOCTYPE html>
<html>
<head>
<title>Search Student</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div>Search Student from freshman number</div>
<form action="result.jsp" method="POST">
<input type="text" name="freshman" value="" size="50" />
<input type="submit" value="Search"
style="font-family: Verdana; font-size: large;
font-weight: bold; padding: 8px;">
</form>
</body>
</html>下面是glassfish服务器的stacktrace
[glassfish 5.0] [WARNING] [] [javax.enterprise.web] [tid: _ThreadID=42 _ThreadName=http-listener-1(5)] [timeMillis: 1606757551047] [levelValue: 900] [[
StandardWrapperValve[jsp]: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at TEST.Student.getStudentByFreshman(Student.java:54)
at org.apache.jsp.result_jsp._jspService(result_jsp.java:68)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:750)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:411)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:473)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:377)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:750)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1540)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:217)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:119)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:611)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:550)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:114)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:332)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:199)
at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:463)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:168)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:206)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:242)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:284)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:201)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:133)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:112)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:539)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:112)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:117)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:56)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:137)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:593)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:573)
at java.lang.Thread.run(Thread.java:748)
]]
暂无答案！
目前还没有任何答案，快来回答吧！