我是新手程序员。谁能看看我的代码，看看有什么问题吗？我的jsp文件（studentList.jsp）没有显示出我所期望的Error Image（应该显示从Servlet的数据库中读取的学生列表）。
我的项目在Netbean的Jave EE / Enterprise Application上运行。该Web应用程序在本地主机上运行。数据库是Java Derby。
我想我没有通过Servlet正确连接到数据库。我在这里显示所有代码。我希望您阅读的时间不会太长。如果有人认为它太长，那么请至少只看两个类的代码：student.jsp和Servlet。我认为问题可能出在这里。
我尝试看了很多教程，但在这种情况下无济于事。请帮我。 预先非常感谢。
学生班
package StudentDB;
public class Student {
public static int studentNumber=0;
private int StID;
private String firstName;
private String lastName;
public Student(int studentID, String firstName, String lastName) {
this.StID = studentID;
if (StID>studentNumber)
{
studentNumber= StID;
}
this.firstName = firstName;
this.lastName = lastName;
}
public Student(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
studentNumber++;
this.setStudentID(studentNumber);
}
@Override
public String toString() {
return "Student{" + "studentID=" + StID + ", firstName=" + firstName + ", lastName=" + lastName + '}';
}
//get and set method
public int getStudentID() {
return StID;
}
public void setStudentID(int studentID) {
this.StID = studentID;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public static int getStudentNumber() {
return studentNumber;
}
public static void setStudentNumber(int studentNumber) {
Student.studentNumber = studentNumber;
}
}
StudentBean类
package StudentDB;
import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import javax.activation.DataSource;
import javax.ejb.Stateless;
import javax.ejb.LocalBean;
@Stateless
@LocalBean
public class StudentBean {
private ArrayList<Student> StudentList;
// The driverURL to contain the Database Driver
private final String driverURL = "org.apache.derby.jdbc.EmbeddedDriver";
// The dbURL to contain the Database URL
private final String dbURL = "jdbc:derby://localhost:1527/DMSDB;" +
"create=true;user=dms;password=dms2018";
private final String tableName="DITMEMAY";
public StudentBean() throws SQLException, ClassNotFoundException
{
//Create StudentList
StudentList= new ArrayList<Student>();
//Create Connection
Connection connection= connectDatabaseSchema();
// Creating the SQL Statement
Statement statement = connection.createStatement();
//if connect sucessfully
//if studentDB table exist
if (isTableExisting(tableName,connection))
{
System.out.println("table existed");
//Reading records from Student Table
ResultSet rs=statement.executeQuery("SELECT * FROM "+tableName);
while(rs.next())
{
int studentID= rs.getInt("StID"); //read studentID
String firstName= rs.getString("firstName"); //read student first Name
String lastName= rs.getString("lastName");// read student last Name
//create Student Object
Student aStudent= new Student(studentID, firstName, lastName);
//add Student to studentList
StudentList.add(aStudent);
System.out.println("one student has been added");
}
}
else //if studentDB table not exist
{
System.out.println("table is not existed");
// Step 4: Creating a new STUDENTDB table in DMSDB
String sqlQuery = "CREATE TABLE "+tableName + " (StID INT PRIMARY KEY," +
" firstName VARCHAR(20), lastName VARCHAR(20))";
int resultDB = statement.executeUpdate(sqlQuery);
if(resultDB == 0)
System.out.println("Student Table is created");
// Step 5: Inserting a record in the Student table in DMSDB
sqlQuery = "INSERT INTO "+ tableName +" VALUES" +
"(1, 'Bob', 'Nilson')," +
"(2, 'Nicholas', 'Jose')," +
"(3, 'Minh', 'Nguyen')," +
"(4, 'Zetting', 'Luo'),"+
"(5, 'Michal', 'Kovac'),"+
"(6, 'Karoline', 'Wang')";
resultDB = statement.executeUpdate(sqlQuery);
if(resultDB == 6)
System.out.println("6 records are insterted in Student Table");
//add data
initialiseStudentList();
}
//close connection
connection.close();
}
// Add business logic below. (Right-click in editor and choose
// "Insert Code > Add Business Method")
public void addStudent(Student aStudent) throws ClassNotFoundException, SQLException {
this.StudentList.add(aStudent);
//Create Connection
Connection connection= connectDatabaseSchema();
// Creating the SQL Statement
Statement statement = connection.createStatement();
String sqlQuery = "INSERT INTO "+ tableName +" VALUES (" +
aStudent.getStudentID()+" ,'"+aStudent.getFirstName()+"','"+aStudent.getLastName()+"')";
statement.executeUpdate(sqlQuery);
System.out.println("Student has been added");
//close connection
connection.close();
}
public void removeStudent(Student aStudent) throws ClassNotFoundException, SQLException
{
this.StudentList.remove(aStudent);
//Create Connection
Connection connection= connectDatabaseSchema();
// Creating the SQL Statement
Statement statement = connection.createStatement();
String sqlQuery = "DELETE FROM "+ tableName +" WHERE StID="+aStudent.getStudentID();
statement.executeUpdate(sqlQuery);
System.out.println("Student has been removed");
//close connection
connection.close();
}
public Student retrieveStudentInformation(int studentID) throws ClassNotFoundException, SQLException
{
//Create Connection
Connection connection= connectDatabaseSchema();
// Creating the SQL Statement
Statement statement = connection.createStatement();
String sqlQuery = "SELECT * FROM "+tableName+" WHERE StID="+studentID;
ResultSet resultSet = statement.executeQuery(sqlQuery);
// Step 7: Reading data from the ResultSet
while(resultSet.next()){
System.out.println(resultSet.getString(1) + "\t"
+ resultSet.getString(2) + "\t"
+ resultSet.getString(3));
}
//close connection
connection.close();
return null;
}
//this method is to check if the table Student already exist in the database
private static boolean isTableExisting(String tableName, Connection theConnection) throws SQLException
{
DatabaseMetaData theMetaData = theConnection.getMetaData();
ResultSet existingTable = theMetaData.getTables(null, null, tableName.toUpperCase(), null);
if(existingTable.next())
{
return true;
}
return false;
}
private Connection connectDatabaseSchema() throws ClassNotFoundException, SQLException
{
// Step 1: Loading the drivers for JAVA DB
Class.forName(driverURL);
// Network Driver both will work with this example.
// You can use any one of them
//Class.forName("org.apache.derby.jdbc.ClientDriver");
// Step 2: Connecting to sample Database in Java DB
Connection connection = DriverManager.getConnection(dbURL);
System.out.println("Database is connected...");
return connection;
}
private void initialiseStudentList()
{
StudentList.add(new Student(1, "Bob", "Nilson"));
StudentList.add(new Student(2, "Nicholas", "Jose"));
StudentList.add(new Student(3, "Minh", "Nguyen"));
StudentList.add(new Student(4, "Zetting", "Luo"));
StudentList.add(new Student(5, "Michal", "Kovac"));
StudentList.add(new Student(6, "Karoline", "Wang"));
}
//getter and setter
public ArrayList<Student> getStudentList() {
return StudentList;
}
public void setStudentList(ArrayList<Student> StudentList) {
this.StudentList = StudentList;
}
}
Servlet
import StudentDB.Student;
import StudentDB.StudentBean;
import java.io.IOException;
import java.util.ArrayList;
import javax.ejb.EJB;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
public class StudentDBServlet extends HttpServlet {
@EJB
private StudentBean studentBean;
@Override
public void init() throws ServletException {
super.init(); //To change body of generated methods, choose Tools | Templates.
try
{
studentBean= new StudentBean();
}
catch (Exception e)
{
e.printStackTrace();
}
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
getStudentListDB(request, response);
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request,response);
}
private void getStudentListDB(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//get StudentList from DB
ArrayList<Student> studentList= studentBean.getStudentList();
//add StudentList to request
request.setAttribute("STUDENT_LIST", studentList);
//send to JSP page
RequestDispatcher dispatcher= request.getRequestDispatcher("/studentList.jsp");
dispatcher.forward(request, response);
}
}
studentList.jsp
<%@page import="java.util.ArrayList"%>
<%@page import="StudentDB.Student"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Student List</title>
</head>
<% ArrayList<Student> studentList= (ArrayList<Student>)request.getAttribute("STUDENT_LIST");%>
<body>
<h2>LIST STUDENTS</h2>
<div>
<table>
<tr>
<th>Student ID</th>
<th>First Name</th>
<th>Last Name</th>
</tr>
<% for (Student aStudent: studentList) {%>
<tr>
<td> <%= aStudent.getStudentID()%> </td>
<td> <%= aStudent.getFirstName() %> </td>
<td> <%= aStudent.getLastName() %> </td>
</tr>
<%}%>
</table>
</div>
</body>
</html>