MyBatis中一对多关联查询

刺骨的言语ヽ痛彻心扉 2022-07-13 01:41 352阅读 0赞

1、一对一关联查询的案例

1.1  需求

根据classId查询对应的班级信息,包括学生,老师

1.2  创建表和数据

在上面的一对一关联查询演示中，我们已经创建了班级表和教师表，因此这里再创建一张学生表

CREATE TABLE student(
        s_id INT PRIMARY KEY AUTO_INCREMENT, 
        s_name VARCHAR(20), 
        class_id INT
    );
    INSERT INTO student(s_name, class_id) VALUES('student_A', 1);
    INSERT INTO student(s_name, class_id) VALUES('student_B', 1);
    INSERT INTO student(s_name, class_id) VALUES('student_C', 1);
    INSERT INTO student(s_name, class_id) VALUES('student_D', 2);
    INSERT INTO student(s_name, class_id) VALUES('student_E', 2);
    INSERT INTO student(s_name, class_id) VALUES('student_F', 2);

2、定义实体类

2.1 Student实体类

public class Student {
    
    	// 定义属性，和student表中的字段对应
    	private int id; // id===>s_id
    	private String name; // name===>s_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;
    	}
    
    	@Override
    	public String toString() {
    		return "Student [id=" + id + ", name=" + name + "]";
    	}
    }

2.2  修改Classes类，添加一个List<Student> students属性，使用一个List<Student>集合属性表示班级拥有的学生，如下：

public class Classes {
    
    	// 定义实体类的属性，与class表中的字段对应
    	private int id; // id===>c_id
    	private String name; // name===>c_name
    
    	/**
    	 * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
    	 * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
    	 */
    	private Teacher teacher;
    
    	// 使用一个List<Student>集合属性表示班级拥有的学生
    	private List<Student> students = new ArrayList<Student>();
    
    	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;
    	}
    
    	public List<Student> getStudents() {
    		return students;
    	}
    
    	public void setStudents(List<Student> students) {
    		this.students = students;
    	}
    
    	public Teacher getTeacher() {
    		return teacher;
    	}
    
    	public void setTeacher(Teacher teacher) {
    		this.teacher = teacher;
    	}
    
    	@Override
    	public String toString() {
    		return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher
    				+ ", students=" + students + "]";
    	}
    
    }

3、定义Mapper对象ClassMapper

import com.baowei.entity.Classes;
    
    public interface ClassMapper {
    	/**
    	 * 测试一对多的非懒加载方式
    	 * 
    	 * @param id
    	 * @return
    	 */
    	public Classes getClass3(int id);
    
    	/**
    	 * 测试一对多的懒加载方式
    	 * 
    	 * @param id
    	 * @return
    	 */
    	public Classes getClass4(int id);
    }

4、定义sql映射文件classMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="com.baowei.mapper.ClassMapper">
    
        
        <select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
            select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and  c.c_id=#{id}
        </select>
        <resultMap type="com.baowei.entity.Classes" id="ClassResultMap3">
            <id property="id" column="c_id"/>
            <result property="name" column="c_name"/>
            <association property="teacher" column="teacher_id" javaType="com.baowei.entity.Teacher">
                <id property="id" column="t_id"/>
                <result property="name" column="t_name"/>
            </association>
            
            <collection property="students" ofType="com.baowei.entity.Student">
                <id property="id" column="s_id"/>
                <result property="name" column="s_name"/>
            </collection>
        </resultMap>
        
        
         <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
            select * from class where c_id=#{id}
         </select>
         <resultMap type="com.baowei.entity.Classes" id="ClassResultMap4">
            <id property="id" column="c_id"/>
            <result property="name" column="c_name"/>
            <association property="teacher" column="teacher_id" javaType="com.baowei.entity.Teacher" select="getTeacher2"></association>
            <collection property="students" ofType="com.baowei.entity.Student" column="c_id" select="getStudent"></collection>
         </resultMap>
         
         <select id="getTeacher2" parameterType="int" resultType="com.baowei.entity.Teacher">
            SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
         </select>
         
         <select id="getStudent" parameterType="int" resultType="com.baowei.entity.Student">
            SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
         </select>
    </mapper

5、Mybatis核心配置文件SqlMapConfig.xml的配置

5.1  打开Mybatis的懒加载功能

5.2  完整的SqlMapConfig.xml配置

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE configuration
    PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-config.dtd">
    <configuration>
    
    	<properties resource="db.properties">
    	</properties>
    	<settings>
    		
    		<setting name="lazyLoadingEnabled" value="true" />
    		
    		<setting name="aggressiveLazyLoading" value="false" />
    	</settings>
    	<environments default="development">
    		<environment id="development">
    			
    			<transactionManager type="JDBC" />
    			
    			<dataSource type="POOLED">
    				<property name="driver" value="${jdbc.driver}" />
    				<property name="url" value="${jdbc.url}" />
    				<property name="username" value="${jdbc.username}" />
    				<property name="password" value="${jdbc.password}" />
    			</dataSource>
    		</environment>
    	</environments>
    	
    	<mappers>
    		<package name="com.baowei.mapper" />
    	</mappers>
    
    </configuration>

6、MyBatisUtil.[Java][]工具的使用

import java.io.IOException;
    import java.io.InputStream;
    
    import org.apache.ibatis.io.Resources;
    import org.apache.ibatis.session.SqlSession;
    import org.apache.ibatis.session.SqlSessionFactory;
    import org.apache.ibatis.session.SqlSessionFactoryBuilder;
    
    public class MyBatisUtil {
    
    	/**
    	 * 获取SqlSessionFactory
    	 * 
    	 * @return SqlSessionFactory
    	 */
    	public static SqlSessionFactory getSqlSessionFactory() {
    		String resource = "SqlMapConfig.xml";
    		InputStream inputStream = null;
    		SqlSessionFactory sqlSessionFactory = null;
    		try {
    			inputStream = Resources.getResourceAsStream(resource);
    			sqlSessionFactory = new SqlSessionFactoryBuilder()
    					.build(inputStream);
    
    		} catch (IOException e) {
    			e.printStackTrace();
    		}
    		return sqlSessionFactory;
    	}
    
    	/**
    	 * 获取SqlSession
    	 * 
    	 * @return SqlSession
    	 */
    	public static SqlSession getSqlSession() {
    		return getSqlSessionFactory().openSession();
    	}
    
    	/**
    	 * 获取SqlSession
    	 * 
    	 * @param isAutoCommit
    	 *            true 表示创建的SqlSession对象在执行完SQL之后会自动提交事务 false
    	 *            表示创建的SqlSession对象在执行完SQL之后不会自动提交事务
    	 *            ，这时就需要我们手动调用sqlSession.commit()提交事务
    	 * @return SqlSession
    	 */
    	public static SqlSession getSqlSession(boolean isAutoCommit) {
    		return getSqlSessionFactory().openSession(isAutoCommit);
    	}
    
    	public static void main(String[] args) {
    		System.out.println(getSqlSessionFactory());
    	}
    }

7、one2many关联查询的[测试][Link 1]代码

import org.apache.ibatis.session.SqlSession;
    import org.junit.Test;
    
    import com.baowei.entity.Classes;
    import com.baowei.mapper.ClassMapper;
    import com.baowei.utils.MyBatisUtil;
    
    public class TestOne2Many {
    
    	/**
    	 * 非懒加载的one2many的测试
    	 */
    	@Test
    	public void testGetClass() {
    		SqlSession sqlSession = MyBatisUtil.getSqlSession();
    
    		ClassMapper mapper = sqlSession.getMapper(ClassMapper.class);
    		Classes clazz = mapper.getClass3(1);
    
    		// 使用SqlSession执行完SQL之后需要关闭SqlSession
    		sqlSession.close();
    		System.out.println(clazz);
    	}
    
    	/**
    	 * 懒加载的one2many的测试
    	 */
    	@Test
    	public void testGetClass2() {
    		SqlSession sqlSession = MyBatisUtil.getSqlSession();
    
    		ClassMapper mapper = sqlSession.getMapper(ClassMapper.class);
    		Classes clazz = mapper.getClass4(1);
    
    		// 使用SqlSession执行完SQL之后需要关闭SqlSession
    		sqlSession.close();
    		System.out.println(clazz.getId());
    		// 可用于测试懒加载(查看日志的sql输出结果，就可以发现使用了懒加载)
    		System.out.println(clazz.getTeacher());
    		System.out.println(clazz.getStudents());
    
    	}
    }

8、Mybatis一对多关联查询的总结

MyBatis中使用collection标签来解决一对多的关联查询，ofType属性指定集合中元素的对象类型。

9、参考的博客地址

http://www.cnblogs.com/xdp-gacl/p/4264440.html

10、[源码下载][Link 2]

[Java]: http://lib.csdn.net/base/javase
[Link 1]: http://lib.csdn.net/base/softwaretest
[Link 2]: http://pan.baidu.com/s/1slpjL5j