JavaWeb三层架构详解

清疚 2023-02-23 12:15 133阅读 0赞

## 什么是三层架构？ ##

三层架构(3-tier architecture) 通常意义上的三层架构就是将整个业务应用划分为：界面层（User Interface layer）、业务逻辑层（Business Logic Layer）、数据访问层（Data access layer）。区分层次的目的即为了“高内聚低耦合”的思想。在软件体系架构设计中，分层式结构是最常见，也是最重要的一种结构。微软推荐的分层式结构一般分为三层，从下至上分别为：数据访问层（又称为持久层）、业务逻辑层（又或称为领域层）、表示层。

### 表示层（UI层）: ###

表示层也称为界面层，位于最外层（最上层），离用户最近。用于显示数据和接收用户输入的数据，为用户提供一种交互式操作的界面。

### 业务逻辑层（BLL层）: ###

负责关键业务的处理和数据的传递。复杂的逻辑判断和涉及到数据库的数据验证都需要在此做出处理。主要是针对具体的问题的操作，也可以理解成对数据层的操作，对数据业务逻辑处理，如果说数据层是积木，那逻辑层就是对这些积木的搭建。

### 数据访问层（DAL层）： ###

主要负责对数据库的直接访问，为业务逻辑层提供数据，根据传入的值来操作数据库，增、删、改、查。

## 为什么要用三层架构呢？ ##

1.团队开发，便于管理

三层架构使得合作开发成为可能，由于各层相互独立，一个小组只需负责一小块就可以。结构化的编程方法面对大型的项目会感到力不从心，因为结构化设计必定会使程序变的错综复杂。逻辑主要在BLL层，就使得UI层也就是客户端不承担太多的职责，即使更新业务逻辑，也无需修改客户端，不用重新部署。

2.解耦

上一层依赖于下一层，如果测试下一层没有问题，那么问题就只有可能发现在本层了，便于发现和改正BUG。体现了“高内聚，低耦合”的思想。比如楼房是分层的，我们要到哪一层楼非常方便，只需在电梯里按下那个楼层的层号即可。而三层架构就好比开发的软件“楼”，哪层出现Bug，哪层有问题，我们作为开发人员能够随时找到，并修正。 各个层次分工明确，将一个复杂问题简单拆分了。

3.代码的复用和劳动成本的减少

分层的根本在于代码的复用和劳动成本的减少。分层的最理想化的结果是实现层与层之间的互不依赖的内部实现，所谓的即插即用！

当然啦，三层架构也是有一定的缺点，但是总的来说，利大于弊。

那么下面写一个小项目来具体地深入了解一下三层架构

项目目录如下:

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZXJyeV92aWNlbnQ_size_16_color_FFFFFF_t_70][]

表示层Model：

package com.gpnu.book.entity;
     
    public class Books {
    	@Override
    	public String toString() {
    		return "Books [book_id=" + book_id + ", book_name=" + book_name + ", isbn=" + isbn + ", author=" + author + "]";
    	}
    	public Books() {
    	}
    	public Books(int book_id, String book_name, String isbn, String author) {
    		super();
    		this.book_id = book_id;
    		this.book_name = book_name;
    		this.isbn = isbn;
    		this.author = author;
    	}
    	private int book_id;
    	private String book_name;
    	private String isbn;
    	private String author;
    	public int getBook_id() {
    		return book_id;
    	}
    	public void setBook_id(int book_id) {
    		this.book_id = book_id;
    	}
    	public String getBook_name() {
    		return book_name;
    	}
    	public void setBook_name(String book_name) {
    		this.book_name = book_name;
    	}
    	public String getIsbn() {
    		return isbn;
    	}
    	public void setIsbn(String isbn) {
    		this.isbn = isbn;
    	}
    	public String getAuthor() {
    		return author;
    	}
    	public void setAuthor(String author) {
    		this.author = author;
    	}
    }

**表示层Controller:**

BooksServlet.java

package com.gpnu.book.servlet;
     
    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;
     
    import com.gpnu.book.entity.Books;
    import com.gpnu.book.service.BooksService;
    import com.gpnu.book.service.impl.BooksServiceImpl;
     
    /**
     * Servlet implementation class Books
     */
    @WebServlet("/BooksServlet")
    public class BooksServlet extends HttpServlet {
    	private static final long serialVersionUID = 1L;
        
    	BooksService booksService = BooksServiceImpl.getInstance();
        /**
         * @see HttpServlet#HttpServlet()
         */
        public BooksServlet() {
            super();
            // TODO Auto-generated constructor stub
        }
     
    	/**
    	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
    	 */
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		// TODO Auto-generated method stub
    		response.getWriter().append("Served at: ").append(request.getContextPath());
    		doPost(request, response);
    	}
     
    	/**
    	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
    	 */
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		request.setCharacterEncoding("UTF-8");
    		String action = request.getParameter("action");
    		System.out.println("action:" + action);
    		if (action.equals("add")) {
    			doAddBooks(request, response);
    			response.sendRedirect("index.jsp");
    		}else if (action.equals("select")) {
    			response.sendRedirect(request.getContextPath() + "/Books/selBooks.jsp");
    		}else if (action.equals("remove")) {
    			doDelBooks(request, response);
    			response.sendRedirect("SelBooksServlet");
    		}
    	}
    	
    	
    	private void doAddBooks(HttpServletRequest request, HttpServletResponse response) throws IOException {
    		Books book = new Books();
    		//book.setBook_id(Integer.parseInt(request.getParameter("book_id")));
    		book.setBook_name(request.getParameter("book_name"));
    		book.setIsbn(request.getParameter("isbn"));
    		book.setAuthor(request.getParameter("author"));
    		int result = booksService.addBooks(book);
    		if (result > 0)
    			System.out.println("添加book成功");
    		else
    			System.out.println("添加book失败");
    	}
    	
    	private void doDelBooks(HttpServletRequest request, HttpServletResponse response) throws IOException {
    		int id = Integer.parseInt(request.getParameter("id"));
    		int result = booksService.delBooks(id);
    		if (result > 0)
    			System.out.println("删除book成功");
    		else
    			System.out.println("删除book失败");
     
    	}
    }

SelBooksServlet.java

package com.gpnu.book.servlet;
     
    import java.io.IOException;
    import java.util.ArrayList;
    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;
     
    import com.gpnu.book.entity.Books;
    import com.gpnu.book.service.BooksService;
    import com.gpnu.book.service.impl.BooksServiceImpl;
     
    /**
     * Servlet implementation class SelBooksServlet
     */
    @WebServlet("/SelBooksServlet")
    public class SelBooksServlet extends HttpServlet {
    	private static final long serialVersionUID = 1L;
    	BooksService booksService = BooksServiceImpl.getInstance();
    	
        /**
         * @see HttpServlet#HttpServlet()
         */
        public SelBooksServlet() {
            super();
            // TODO Auto-generated constructor stub
        }
     
    	/**
    	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
    	 */
    	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		// TODO Auto-generated method stub
    		response.getWriter().append("Served at: ").append(request.getContextPath());
    		doPost(request, response);
    	}
     
    	/**
    	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
    	 */
    	private static String keyword = "";
    	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		request.setCharacterEncoding("UTF-8");
    		keyword = request.getParameter("keyword");
    		System.out.println("keyword:" + keyword);
    		if (keyword == "")
    			doSelAllBooks(request, response);
    		else
    			doSelBooks(request, response);
    	}
     
    	private void doSelAllBooks(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
    		List<Books> list = booksService.selAllBooks();
    		request.setAttribute("books", list);
    		System.out.println("查询所有");
    		RequestDispatcher view = request.getRequestDispatcher("/Books/selResult.jsp");
    		view.forward(request, response);
    	}  
     
    	private void doSelBooks(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
    		Books book = booksService.selBooks(keyword);
    		List list = new ArrayList();
    		list.add(book);
    		request.setAttribute("books", list);
    		System.out.println("条件查询");
    		RequestDispatcher view = request.getRequestDispatcher("/Books/selResult.jsp");
    		view.forward(request, response);
    	}
    	
    }

**表示层View:**

index.jsp

addBooks.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
        pageEncoding="UTF-8"%>
    <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
    <html>
    <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Insert title here</title>
    </head>
    <body>
    <form action="/book/BooksServlet?action=add" method="post">
    <table border="1" cellspacing="0" cellpadding="0" style="align:center;">
        <tr><td>图书名称:</td><td><input type="text" name="book_name"></td></tr>
        <tr><td>国际标准书号:</td><td><input type="text" name="isbn"></td></tr>
        <tr><td>作者:</td><td><input type="text" name="author"></td></tr>
        <tr><td><input type="submit" value="提交"></td><td><input type="button" value="返回"></td>
     	</tr>
     </table>
     </form>
    </body>
    </html>

selBooks.jsp

selResult.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
        pageEncoding="UTF-8"%>
    <%@ page import="com.gpnu.book.entity.Books,java.util.List" %>
    <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
    <html>
    <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Insert title here</title>
    </head>
    <body>
    <a href="Books/addBooks.jsp">新增</a>
    <br>
    <table border="1" cellspacing="0" cellpadding="0" style="align:center;">
        <tr bgcolor="ff9900" style="font-weight:bold;">
        <th>图书编号</th><th>图书名称</th><th>国际标准号</th><th>作者</th></tr>
    <%
    	List<Books> book = (List)request.getAttribute("books");
    	if(book.size()!=0){
        	for(int i=0;i<book.size();i++) {
           		pageContext.setAttribute("book",book.get(i)); 
    %>  
    	<tr>
          <td>${book.book_id }</td>    
          <td>${book.book_name }</td>
          <td>${book.isbn }</td>
          <td>${book.author }</td>
          <td>|<a href="BooksServlet?action=remove&id=${book.book_id }" onclick='return confirm("确定要删除吗?")'
              >删除</a></td>
       </tr> 
         <%
          }
        }else{
         %>
    <tr><td colspan="6">数据库中没有数据！</td></tr>
        <%
        }
        %>
    </table>
    </body>
    </html>

**持久层接口:**

package com.gpnu.book.dao;
     
    import java.util.List;
     
    import com.gpnu.book.entity.Books;
     
    public interface BooksDao {
    	List<Books> selAllBooks();
    	Books selBooks(String keyword);
    	int addBooks(Books book);
    	int delBooks(int book_id);
    	int booksCount();
    }

**持久层实现类:**

package com.gpnu.book.dao.impl;
     
    import java.sql.Connection;
    import java.sql.PreparedStatement;
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import java.util.ArrayList;
    import java.util.List;
     
    import com.gpnu.book.dao.BooksDao;
    import com.gpnu.book.entity.Books;
    import com.gpnu.book.common.DBUtils;
     
    public class BooksDaoImpl implements BooksDao {
     
    	private Connection conn;
    	
    	public BooksDaoImpl(Connection conn) {
    		this.conn = conn;
    	}
    	
    	@Override
    	public List<Books> selAllBooks() {
    		List list = new ArrayList();
    		PreparedStatement pstam = null;
    		ResultSet rs = null;
    		Books book = null;		try {
    			pstam = conn.prepareStatement("select * from tbl_book");
    			rs = pstam.executeQuery();
    			while (rs.next()) {
    				book = new Books();
    				book.setBook_id(rs.getInt("book_id"));
    				book.setBook_name(rs.getString("book_name"));
    				book.setIsbn(rs.getString("isbn"));
    				book.setAuthor(rs.getString("author"));
    				list.add(book);
    			}
    		} catch (SQLException e) {
    			System.out.println("在查询全部book的时候出错了.错误信息是 ：" + e.getMessage());
    		} finally {
    			DBUtils.closeStatement(rs, pstam);
    		}
    		return list;
    	}
     
    	@Override
    	public int booksCount() {
    		// TODO Auto-generated method stub
    		return 0;
    	}
     
    	@Override
    	public Books selBooks(String keyword) {
    		Books book = new Books();
    		PreparedStatement pStatement = null;
    		ResultSet rSet = null;
    		try {
    			pStatement = conn.prepareStatement("select * from tbl_book where book_name like ?");
    			pStatement.setString(1, "%" + keyword + "%");
    			rSet = pStatement.executeQuery();
    			if (rSet.next()) {
    				book.setBook_id(rSet.getInt("book_id"));
    				book.setBook_name(rSet.getString("book_name"));	
    				book.setIsbn(rSet.getString("isbn"));
    				book.setAuthor(rSet.getString("author"));
    			}
    		} catch (SQLException e) {
    			e.printStackTrace();
    		} finally {
    			DBUtils.closeStatement(rSet, pStatement);
    		}
    		return book;
    	}
     
    	@Override
    	public int addBooks(Books book) {
    		int result = 0;
    		PreparedStatement pstam = null;
    		try {
    			pstam = conn.prepareStatement("insert into tbl_book(book_name,isbn,author) values(?,?,?)");
    			//pstam.setInt(1, book.getBook_id());
    			pstam.setString(1, book.getBook_name());
    			pstam.setString(2, book.getIsbn());
    			pstam.setString(3, book.getAuthor());
    			result = pstam.executeUpdate();
    		} catch (SQLException e) {
    			System.out.println("添加book出错.错误信息是 ：" + e.getMessage());
    		} finally {
    			DBUtils.closeStatement(null, pstam);
    		}
    		return result;
    	}
     
    	@Override
    	public int delBooks(int book_id) {
    		int result = 0;
    		PreparedStatement pstam = null;
    		try {
    			pstam = conn.prepareStatement("DELETE FROM tbl_book WHERE book_id =?");
    			pstam.setInt(1, book_id);
    			result = pstam.executeUpdate();
    		} catch (SQLException e) {
    			System.out.println("删除book出错.错误信息是 ：" + e.getMessage());
    		} finally {
    			DBUtils.closeStatement(null, pstam);
    		}
    		return result;
    	}
     
    }

**业务逻辑层接口:**

package com.gpnu.book.service;
     
    import java.util.List;
     
    import com.gpnu.book.entity.Books;
     
    public interface BooksService {
    	List<Books> selAllBooks();
    	Books selBooks(String keyword);
    	int addBooks(Books book);
    	int delBooks(int book_id);
    }

**业务逻辑层实现类**

package com.gpnu.book.service.impl;
     
    import java.sql.Connection;
    import java.util.List;
     
    import com.gpnu.book.entity.Books;
    import com.gpnu.book.service.BooksService;
    import com.gpnu.book.common.DBUtils;
    import com.gpnu.book.dao.BooksDao;
    import com.gpnu.book.dao.impl.BooksDaoImpl;
     
    public class BooksServiceImpl implements BooksService {
     
    	private static final BooksService instance = new BooksServiceImpl();
    	public static BooksService getInstance() {
    		return instance;
    	}
    	
    	@Override
    	public List<Books> selAllBooks() {
    		Connection conn = null;
    		List list = null;
    		try {
    			conn = DBUtils.getConnection();
    			DBUtils.beginTransaction(conn);
    			BooksDao booksDao = new BooksDaoImpl(conn);
    			list = booksDao.selAllBooks();
    			DBUtils.commit(conn);
    		} catch (Exception e) {
    			System.out.println("查询所有books错误" + e.getMessage());
    		} finally {
    			DBUtils.closeConnection(conn);
    		}
    		return list;
    	}
     
    	@Override
    	public Books selBooks(String keyword) {
    		Connection conn = null;
    		Books book = null;
    		try {
    			conn = DBUtils.getConnection();
    			DBUtils.beginTransaction(conn);
    			BooksDao booksDao = new BooksDaoImpl(conn);
    			book = booksDao.selBooks(keyword);
    			DBUtils.commit(conn);
    		} catch (Exception e) {
    			System.out.println("条件查询books错误" + e.getMessage());
    		} finally {
    			DBUtils.closeConnection(conn);
    		}
    		return book;
    	}
     
    	@Override
    	public int addBooks(Books book) {
    		Connection conn = null;
    		int result = 0;
    		try {
    			conn = DBUtils.getConnection();
    			DBUtils.beginTransaction(conn);
    			BooksDao booksDao = new BooksDaoImpl(conn);
    			result = booksDao.addBooks(book);
    			DBUtils.commit(conn);
    		} catch (Exception e) {
    			System.out.println("增加books错误" + e.getMessage());
    		} finally {
    			DBUtils.closeConnection(conn);
    		}
    		return result;
    	}
     
    	@Override
    	public int delBooks(int book_id) {
    		Connection conn = null;
    		int result = 0;
    		try {
    			conn = DBUtils.getConnection();
    			DBUtils.beginTransaction(conn);
    			BooksDao booksDao = new BooksDaoImpl(conn);
    			result = booksDao.delBooks(book_id);
    			DBUtils.commit(conn);
    		} catch (Exception e) {
    			System.out.println("删除books错误" + e.getMessage());
    		} finally {			DBUtils.closeConnection(conn);
    		}
    		return result;
    	}
     
    }

我们启动一下项目，我们作为用户直接看到的就是表示层的视图了

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZXJyeV92aWNlbnQ_size_16_color_FFFFFF_t_70 1][]

而当我们点击查询，会跳转到查询的视图

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZXJyeV92aWNlbnQ_size_16_color_FFFFFF_t_70 2][]

点击查询，我们会跳转到表示层的控制器，也就是Servlet，此时Servle会调用业务逻辑层的方法

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZXJyeV92aWNlbnQ_size_16_color_FFFFFF_t_70 3][]

而业务逻辑层则会调用持久层（DAO层）的方法

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZXJyeV92aWNlbnQ_size_16_color_FFFFFF_t_70 4][]

最后持久层连接到数据库，读取数据库的数据，保存为一个Model类

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZXJyeV92aWNlbnQ_size_16_color_FFFFFF_t_70 5][]

将结果原路返回给表示层的视图View

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZXJyeV92aWNlbnQ_size_16_color_FFFFFF_t_70 6][]

整个运行过程可以浓缩为一张图：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZXJyeV92aWNlbnQ_size_16_color_FFFFFF_t_70 7][]

转自：[https://blog.csdn.net/a549654065/article/details/83064231][https_blog.csdn.net_a549654065_article_details_83064231]

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[https_blog.csdn.net_a549654065_article_details_83064231]: https://blog.csdn.net/a549654065/article/details/83064231