编写java web 学生选课系统代码

由于该项目比较复杂，无法在短时间内完成。以下是一个简单的Java Web学生选课系统的代码示例：

Student.java:

public class Student {
    private int id;
    private String name;
    private List<Course> courses;

    public Student(int id, String name) {
        this.id = id;
        this.name = name;
        this.courses = new ArrayList<>();
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public List<Course> getCourses() {
        return courses;
    }

    public void addCourse(Course course) {
        courses.add(course);
    }

    public void removeCourse(Course course) {
        courses.remove(course);
    }
}

Course.java:

public class Course {
    private int id;
    private String name;
    private int credit;
    private List<Student> students;

    public Course(int id, String name, int credit) {
        this.id = id;
        this.name = name;
        this.credit = credit;
        this.students = new ArrayList<>();
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getCredit() {
        return credit;
    }

    public List<Student> getStudents() {
        return students;
    }

    public void addStudent(Student student) {
        students.add(student);
    }

    public void removeStudent(Student student) {
        students.remove(student);
    }
}

StudentDAO.java:

public class StudentDAO {
    private List<Student> students;

    public StudentDAO() {
        this.students = new ArrayList<>();
    }

    public List<Student> getAllStudents() {
        return students;
    }

    public Student getStudentById(int id) {
        for (Student student : students) {
            if (student.getId() == id) {
                return student;
            }
        }
        return null;
    }

    public void addStudent(Student student) {
        students.add(student);
    }

    public void removeStudent(Student student) {
        students.remove(student);
    }
}

CourseDAO.java:

public class CourseDAO {
    private List<Course> courses;

    public CourseDAO() {
        this.courses = new ArrayList<>();
    }

    public List<Course> getAllCourses() {
        return courses;
    }

    public Course getCourseById(int id) {
        for (Course course : courses) {
            if (course.getId() == id) {
                return course;
            }
        }
        return null;
    }

    public void addCourse(Course course) {
        courses.add(course);
    }

    public void removeCourse(Course course) {
        courses.remove(course);
    }
}

MainController.java:

@WebServlet("/")
public class MainController extends HttpServlet {
    private StudentDAO studentDAO;
    private CourseDAO courseDAO;

    @Override
    public void init() throws ServletException {
        super.init();
        studentDAO = new StudentDAO();
        courseDAO = new CourseDAO();
        Course course1 = new Course(1, "Java Programming", 3);
        Course course2 = new Course(2, "Web Development", 4);
        courseDAO.addCourse(course1);
        courseDAO.addCourse(course2);
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String action = request.getServletPath();
        switch (action) {
            case "/students":
                listStudents(request, response);
                break;
            case "/courses":
                listCourses(request, response);
                break;
            case "/enroll":
                enrollCourse(request, response);
                break;
            case "/withdraw":
                withdrawCourse(request, response);
                break;
            default:
                response.sendRedirect("/");
                break;
        }
    }

    private void listStudents(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List<Student> students = studentDAO.getAllStudents();
        request.setAttribute("students", students);
        request.getRequestDispatcher("/students.jsp").forward(request, response);
    }

    private void listCourses(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List<Course> courses = courseDAO.getAllCourses();
        request.setAttribute("courses", courses);
        request.getRequestDispatcher("/courses.jsp").forward(request, response);
    }

    private void enrollCourse(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        int studentId = Integer.parseInt(request.getParameter("studentId"));
        int courseId = Integer.parseInt(request.getParameter("courseId"));
        Student student = studentDAO.getStudentById(studentId);
        Course course = courseDAO.getCourseById(courseId);
        if (student != null && course != null) {
            student.addCourse(course);
            course.addStudent(student);
        }
        response.sendRedirect("/students");
    }

    private void withdrawCourse(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        int studentId = Integer.parseInt(request.getParameter("studentId"));
        int courseId = Integer.parseInt(request.getParameter("courseId"));
        Student student = studentDAO.getStudentById(studentId);
        Course course = courseDAO.getCourseById(courseId);
        if (student != null && course != null) {
            student.removeCourse(course);
            course.removeStudent(student);
        }
        response.sendRedirect("/students");
    }
}

students.jsp:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Student List</title>
</head>
<body>
<h1>Student List</h1>
<table>
    <thead>
    <tr>
        <th>ID</th>
        <th>Name</th>
        <th>Courses</th>
    </tr>
    </thead>
    <tbody>
    <c:forEach items="${students}" var="student">
        <tr>
            <td>${student.id}</td>
            <td>${student.name}</td>
            <td>
                <ul>
                    <c:forEach items="${student.courses}" var="course">
                        <li>${course.name}</li>
                    </c:forEach>
                </ul>
                <form method="post" action="enroll">
                    <input type="hidden" name="studentId" value="${student.id}">
                    <select name="courseId">
                        <c:forEach items="${courses}" var="course">
                            <option value="${course.id}">${course.name}</option>
                        </c:forEach>
                    </select>
                    <button type="submit">Enroll</button>
                </form>
                <form method="post" action="withdraw">
                    <input type="hidden" name="studentId" value="${student.id}">
                    <select name="courseId">
                        <c:forEach items="${student.courses}" var="course">
                            <option value="${course.id}">${course.name}</option>
                        </c:forEach>
                    </select>
                    <button type="submit">Withdraw</button>
                </form>
            </td>
        </tr>
    </c:forEach>
    </tbody>
</table>
</body>
</html>

courses.jsp:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Course List</title>
</head>
<body>
<h1>Course List</h1>
<table>
    <thead>
    <tr>
        <th>ID</th>
        <th>Name</th>
        <th>Credit</th>
        <th>Students</th>
    </tr>
    </thead>
    <tbody>
    <c:forEach items="${courses}" var="course">
        <tr>
            <td>${course.id}</td>
            <td>${course.name}</td>
            <td>${course.credit}</td>
            <td>
                <ul>
                    <c:forEach items="${course.students}" var="student">
                        <li>${student.name}</li>
                    </c:forEach>
                </ul>
            </td>
        </tr>
    </c:forEach>
    </tbody>
</table>
</body>
</html>

以上代码只是一个简单的示例，仅供参考。实际开发中，还需要考虑数据持久化、安全性、性能等问题。