警告
本文最后更新于 2022-06-30，文中内容可能已过时。
DAO（Data Access Objects，数据访问对象）是指位于业务逻辑和持久化数据之间实现对持久化数据的访问。通俗来讲，就是将数据库操作都封装起来。
DAO 模式的优势：
隔离了数据访问代码和业务逻辑代码。业务逻辑代码直接调用 DAO 方法即可，完全感觉不到数据库表的存在。分工明确，数据访问层代码变化不影响业务逻辑代码，这符合单一职能原则，降低了耦合性，提高了可复用性。
隔离了不同数据库实现。采用面向接口编程，如果底层数据库变化，如由 MySQL 变成 Oracle 只要增加 DAO 接口的新实现类即可，原有 MySQL 实现不用修改。这符合“开-闭”原则。该原则降低了代码的耦合性，提高了代码扩展性和系统的可移植性。
DAO 模式组成部分：
- DAO 接口。把对数据库的所有操作定义成抽象方法，可以提供多种实现。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
| package org.example.dao;
import org.example.entity.Website;
import java.util.List;
public interface WebsiteDAO {
public void insert(Website website) throws Exception;
public void update(Website website) throws Exception;
public void delete(int websiteId) throws Exception;
public Website queryById(int websiteId) throws Exception;
public List<Website> queryAll() throws Exception;
}
|
- DAO 实现类。针对不同数据库给出 DAO 接口定义方法的具体实现。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
| package org.example.dao.impl;
import org.example.dao.WebsiteDAO;
import org.example.entity.Website;
import org.example.utils.DBConnectionUtil;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList;
import java.util.List;
public class WebsiteDAOImpl implements WebsiteDAO {
@Override
public void insert(Website website) throws Exception {
String sql = "insert into websites (website_name, website_url, website_rank, website_country) values (?, ?, ?, ?)";
PreparedStatement stmt = null;
DBConnectionUtil util = null;
try {
util = new DBConnectionUtil();
stmt = util.getConnection().prepareStatement(sql);
stmt.setString(1, website.getWebsiteName());
stmt.setString(2, website.getWebsiteUrl());
stmt.setInt(3, website.getWebsiteRank());
stmt.setString(4, website.getWebsiteCountry());
stmt.executeUpdate();
stmt.close();
} catch (Exception e) {
throw new Exception("操作出现异常");
} finally {
if (util != null) {
util.close();
}
}
}
@Override
public void update(Website website) throws Exception {
String sql = "update websites set website_name=?, website_url=?, website_rank=?, website_country=? where website_id=?";
PreparedStatement stmt = null;
DBConnectionUtil util = null;
try {
util = new DBConnectionUtil();
stmt = util.getConnection().prepareStatement(sql);
stmt.setString(1, website.getWebsiteName());
stmt.setString(2, website.getWebsiteUrl());
stmt.setInt(3, website.getWebsiteRank());
stmt.setString(4, website.getWebsiteCountry());
stmt.setInt(5, website.getWebsiteId());
stmt.executeUpdate();
stmt.close();
} catch (Exception e) {
throw new Exception("操作出现异常");
} finally {
if (util != null) {
util.close();
}
}
}
@Override
public void delete(int websiteId) throws Exception {
String sql = "delete from websites where website_id=?";
PreparedStatement stmt = null;
DBConnectionUtil util = null;
try {
util = new DBConnectionUtil();
stmt = util.getConnection().prepareStatement(sql);
stmt.setInt(1, websiteId);
stmt.executeUpdate();
stmt.close();
} catch (Exception e) {
throw new Exception("操作出现异常");
} finally {
if (util != null) {
util.close();
}
}
}
@Override
public Website queryById(int websiteId) throws Exception {
String sql = "select website_id, website_name, website_url, website_rank, website_country from websites where website_id=?";
PreparedStatement stmt = null;
DBConnectionUtil util = null;
Website website = null;
try {
util = new DBConnectionUtil();
stmt = util.getConnection().prepareStatement(sql);
stmt.setInt(1, websiteId);
ResultSet rs = stmt.executeQuery();
while (rs.next()) {
website = new Website();
website.setWebsiteId(rs.getInt("website_id"));
website.setWebsiteName(rs.getString("website_name"));
website.setWebsiteUrl(rs.getString("website_url"));
website.setWebsiteRank(rs.getInt("website_rank"));
website.setWebsiteCountry(rs.getString("website_country"));
}
rs.close();
stmt.close();
} catch (Exception e) {
throw new Exception("操作出现异常");
} finally {
if (util != null) {
util.close();
}
}
return website;
}
@Override
public List<Website> queryAll() throws Exception {
String sql = "select website_id, website_name, website_url, website_rank, website_country from websites";
PreparedStatement stmt = null;
DBConnectionUtil util = null;
List<Website> websites = null;
try {
util = new DBConnectionUtil();
stmt = util.getConnection().prepareStatement(sql);
ResultSet rs = stmt.executeQuery();
websites = new ArrayList<>();
while (rs.next()) {
Website website = new Website();
website.setWebsiteId(rs.getInt("website_id"));
website.setWebsiteName(rs.getString("website_name"));
website.setWebsiteUrl(rs.getString("website_url"));
website.setWebsiteRank(rs.getInt("website_rank"));
website.setWebsiteCountry(rs.getString("website_country"));
websites.add(website);
}
rs.close();
stmt.close();
} catch (Exception e) {
throw new Exception("操作出现异常");
} finally {
if (util != null) {
util.close();
}
}
return websites;
}
}
|
- 实体类。用于存放与传输对象数据。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
| package org.example.entity;
public class Website {
public Website(int websiteId, String websiteName, String websiteUrl, int websiteRank, String websiteCountry) {
this.websiteId = websiteId;
this.websiteName = websiteName;
this.websiteUrl = websiteUrl;
this.websiteRank = websiteRank;
this.websiteCountry = websiteCountry;
}
public Website() {
}
@Override
public String toString() {
return "Website{" +
"websiteId=" + websiteId +
", websiteName='" + websiteName + '\'' +
", websiteUrl='" + websiteUrl + '\'' +
", websiteRank=" + websiteRank +
", websiteCountry='" + websiteCountry + '\'' +
'}';
}
public int getWebsiteId() {
return websiteId;
}
public void setWebsiteId(int websiteId) {
this.websiteId = websiteId;
}
public String getWebsiteName() {
return websiteName;
}
public void setWebsiteName(String websiteName) {
this.websiteName = websiteName;
}
public String getWebsiteUrl() {
return websiteUrl;
}
public void setWebsiteUrl(String websiteUrl) {
this.websiteUrl = websiteUrl;
}
public int getWebsiteRank() {
return websiteRank;
}
public void setWebsiteRank(int websiteRank) {
this.websiteRank = websiteRank;
}
public String getWebsiteCountry() {
return websiteCountry;
}
public void setWebsiteCountry(String websiteCountry) {
this.websiteCountry = websiteCountry;
}
private int websiteId;
private String websiteName;
private String websiteUrl;
private int websiteRank;
private String websiteCountry;
}
|
- 数据库连接和关闭工具类。避免了数据库连接和关闭代码的重复使用，方便修改。
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
| package org.example.utils;
import java.sql.Connection;
import java.sql.DriverManager;
public class MySQLConnectionUtil {
private static final String DB_DRIVER = "com.mysql.cj.jdbc.Driver";
private static final String DB_URL = "jdbc:mysql://localhost:3306/demo";
private static final String DB_USER = "root";
private static final String DB_PASSWORD = "12345678";
private Connection connection = null;
public MySQLConnectionUtil() {
try {
Class.forName(DB_DRIVER);
this.connection = DriverManager.getConnection(DB_URL, DB_USER, DB_PASSWORD);
} catch (Exception e) {
System.out.println("加载驱动失败！");
}
}
public Connection getConnection() {
return connection;
}
public void close() {
try {
connection.close();
} catch (Exception e) {
System.out.println("数据库连接关闭失败！");
}
}
}
|
1
2
3
4
5
6
7
8
| <!-- pom.xml -->
<dependencies>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.29</version>
</dependency>
</dependencies>
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
| import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class MySQLConnectionDemo {
// MySQL 8.0 以下版本
// private static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
private static final String JDBC_DRIVER = "com.mysql.cj.jdbc.Driver";
private static final String DB_URL = "jdbc:mysql://localhost:3306/demo";
public static void main(String[] args) {
try {
// 注册 JDBC 驱动
Class.forName(JDBC_DRIVER);
// 建立连接
Connection conn = DriverManager.getConnection(DB_URL, "root", "12345678");
// 执行 SQL
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("select * from websites");
while (rs.next()) {
int websiteId = rs.getInt("website_id");
String websiteName = rs.getString("website_name");
System.out.println(websiteId + " " + websiteName);
}
// 关闭资源
rs.close();
stmt.close();
conn.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
|