在 Java 应用程序中，三层架构（Three-Tier Architecture）是一种常见的设计模式，用于分离应用程序的表示层、业务逻辑层和数据访问层。这种架构有助于提高代码的可维护性、可扩展性和可重用性。以下是详细解释 Java 应用程序中使用 MySQL 的三层架构：
### 一、三层架构简介
1. **表示层（Presentation Layer）**：
- 负责与用户交互，包括用户界面（UI）的展示和用户输入的处理。
- 通常包含 JSP、Servlet 或其他前端技术（如 Thymeleaf、Angular、React 等）。
2. **业务逻辑层（Business Logic Layer）**：
- 处理应用程序的业务逻辑，执行具体的业务操作。
- 通常包含服务类（Service）、业务对象（BO）等。
3. **数据访问层（Data Access Layer）**：
- 负责与数据库进行交互，执行数据的持久化操作。
- 通常包含数据访问对象（DAO）类。
### 二、示例应用程序
假设我们有一个简单的应用程序，用于管理用户信息，包括用户的 `id` 和 `name`。以下是使用 MySQL 的三层架构示例。
#### 1. 数据库表
首先，创建一个 MySQL 数据库表 `users`：
```sql
CREATE DATABASE testdb;
USE testdb;
CREATE TABLE users (
id INT AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(100) NOT NULL
);
```
#### 2. 数据访问层（DAO）
定义一个数据访问对象（DAO）类，用于与数据库进行交互。
```java
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
public class UserDAO {
private String jdbcURL = "jdbc:mysql://localhost:3306/testdb";
private String jdbcUsername = "root";
private String jdbcPassword = "yourpassword";
private static final String INSERT_USERS_SQL = "INSERT INTO users (name) VALUES (?);";
private static final String SELECT_USER_BY_ID = "SELECT id, name FROM users WHERE id = ?";
private static final String SELECT_ALL_USERS = "SELECT * FROM users";
private static final String DELETE_USER_SQL = "DELETE FROM users WHERE id = ?;";
private static final String UPDATE_USER_SQL = "UPDATE users SET name = ? WHERE id = ?;";
protected Connection getConnection() {
Connection connection = null;
try {
Class.forName("com.mysql.cj.jdbc.Driver");
connection = DriverManager.getConnection(jdbcURL, jdbcUsername, jdbcPassword);
} catch (SQLException | ClassNotFoundException e) {
e.printStackTrace();
}
return connection;
}
public void insertUser(User user) throws SQLException {
try (Connection connection = getConnection();
PreparedStatement preparedStatement = connection.prepareStatement(INSERT_USERS_SQL)) {
preparedStatement.setString(1, user.getName());
preparedStatement.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
}
}
public User selectUser(int id) {
User user = null;
try (Connection connection = getConnection();
PreparedStatement preparedStatement = connection.prepareStatement(SELECT_USER_BY_ID)) {
preparedStatement.setInt(1, id);
ResultSet rs = preparedStatement.executeQuery();
while (rs.next()) {
String name = rs.getString("name");
user = new User(id, name);
}
} catch (SQLException e) {
e.printStackTrace();
}
return user;
}
public List<User> selectAllUsers() {
List<User> users = new ArrayList<>();
try (Connection connection = getConnection();
PreparedStatement preparedStatement = connection.prepareStatement(SELECT_ALL_USERS)) {
ResultSet rs = preparedStatement.executeQuery();
while (rs.next()) {
int id = rs.getInt("id");
String name = rs.getString("name");
users.add(new User(id, name));
}
} catch (SQLException e) {
e.printStackTrace();
}
return users;
}
public boolean deleteUser(int id) throws SQLException {
boolean rowDeleted;
try (Connection connection = getConnection();
PreparedStatement statement = connection.prepareStatement(DELETE_USER_SQL)) {
statement.setInt(1, id);
rowDeleted = statement.executeUpdate() > 0;
}
return rowDeleted;
}
public boolean updateUser(User user) throws SQLException {
boolean rowUpdated;
try (Connection connection = getConnection();
PreparedStatement statement = connection.prepareStatement(UPDATE_USER_SQL)) {
statement.setString(1, user.getName());
statement.setInt(2, user.getId());
rowUpdated = statement.executeUpdate() > 0;
}
return rowUpdated;
}
}
```
#### 3. 业务逻辑层（Service）
定义一个服务类，处理应用程序的业务逻辑。
```java
import java.sql.SQLException;
import java.util.List;
public class UserService {
private UserDAO userDAO;
public UserService() {
userDAO = new UserDAO();
}
public void addUser(User user) {
try {
userDAO.insertUser(user);
} catch (SQLException e) {
e.printStackTrace();
}
}
public User getUser(int id) {
return userDAO.selectUser(id);
}
public List<User> getAllUsers() {
return userDAO.selectAllUsers();
}
public void updateUser(User user) {
try {
userDAO.updateUser(user);
} catch (SQLException e) {
e.printStackTrace();
}
}
public void deleteUser(int id) {
try {
userDAO.deleteUser(id);
} catch (SQLException e) {
e.printStackTrace();
}
}
}
```
#### 4. 表示层（Servlet）
使用 Servlet 处理用户请求和响应。
```java
import java.io.IOException;
import java.util.List;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet("/")
public class UserServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
private UserService userService;
public void init() {
userService = new UserService();
}
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String action = request.getServletPath();
try {
switch (action) {
case "/new":
showNewForm(request, response);
break;
case "/insert":
insertUser(request, response);
break;
case "/delete":
deleteUser(request, response);
break;
case "/edit":
showEditForm(request, response);
break;
case "/update":
updateUser(request, response);
break;
default:
listUser(request, response);
break;
}
} catch (SQLException ex) {
throw new ServletException(ex);
}
}
private void listUser(HttpServletRequest request, HttpServletResponse response)
throws SQLException, IOException, ServletException {
List<User> listUser = userService.getAllUsers();
request.setAttribute("listUser", listUser);
RequestDispatcher dispatcher = request.getRequestDispatcher("user-list.jsp");
dispatcher.forward(request, response);
}
private void showNewForm(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
RequestDispatcher dispatcher = request.getRequestDispatcher("user-form.jsp");
dispatcher.forward(request, response);
}
private void showEditForm(HttpServletRequest request, HttpServletResponse response)
throws SQLException, ServletException, IOException {
int id = Integer.parseInt(request.getParameter("id"));
User existingUser = userService.getUser(id);
RequestDispatcher dispatcher = request.getRequestDispatcher("user-form.jsp");
request.setAttribute("user", existingUser);
dispatcher.forward(request, response);
}
private void insertUser(HttpServletRequest request, HttpServletResponse response)
throws SQLException, IOException {
String name = request.getParameter("name");
User newUser = new User(name);
userService.addUser(newUser);
response.sendRedirect("list");
}
private void updateUser(HttpServletRequest request, HttpServletResponse response)
throws SQLException, IOException {
int id = Integer.parseInt(request.getParameter("id"));
String name = request.getParameter("name");
User user = new User(id, name);
userService.updateUser(user);
response.sendRedirect("list");
}
private void deleteUser(HttpServletRequest request, HttpServletResponse response)
throws SQLException, IOException {
int id = Integer.parseInt(request.getParameter("id"));
userService.deleteUser(id);
response.sendRedirect("list");
}
}
```
#### 5. 用户实体类（User）
定义一个用户实体类，表示数据库表 `users` 中的记录。
```java
public class User {
private int id;
private String name;
public User(int id, String name) {
this.id = id;
this.name = name;
}
public User(String name) {
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
```
#### 6. JSP 页面
定义两个 JSP 页面：一个用于显示用户列表，另一个用于用户表单。
**user-list.jsp**：
```jsp
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>User List</title>
</head>
<body>
<h2>User List</h2>
<a href="new">Add New User</a>
<table border="1">
<tr>
<th>ID</th>
<th>Name</th>
<th>Actions</th>
</tr>
<c:forEach var="user" items="${listUser}">
<tr>
<td>${user.id}</td>
<td>${user.name}</td>
<td>
<a href="edit?id=${user.id}">Edit</a>
<a href="delete?id=${user.id}">Delete</a>
</td>
</tr>
</c:forEach>
</table>
</body>
</html>