1 實驗環境
本文以 Student (many)——Class (one) 爲例,介紹 MyBatis 中,多對一查詢。
(1)導入 JAR 包
其中,前2個 jar 包下載地址見 → log4j-1.2.17.jar、 mybatis-3.4.1.jar,將 jar 包放入 lib 目錄下,並選中所有 jar 包,右鍵,選擇【Add to Build Path】。
(2) 工作目錄
注意:src 和 conf 目錄下的 com.mapper 包必須同名;ClassMapper.java 和 ClassMapper.xml 僅在2.3節中使用到。
(3)配置文件
log4j.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
<appender name="STDOUT" class="org.apache.log4j.ConsoleAppender">
<param name="Encoding" value="UTF-8" />
<layout class="org.apache.log4j.PatternLayout">
<param name="ConversionPattern" value="%-5p %d{MM-dd HH:mm:ss,SSS} %m (%F:%L)\n" />
</layout>
</appender>
<logger name="java.sql">
<level value="debug" />
</logger>
<logger name="org.apache.ibatis">
<level value="info" />
</logger>
<root>
<priority value="debug" />
<appender-ref ref="STDOUT" />
</root>
</log4j:configuration>
注意:log4j.xml文件名不能隨意更改。
mybatis-config.xml
<?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>
<!-- 設置或引入資源文件 -->
<properties resource="jdbc.properties"></properties>
<!-- 設置連接數據庫的環境,default用於設置默認使用的數據庫環境 -->
<environments default="mysql">
<!-- 設置某個具體的數據庫環境 -->
<environment id="mysql">
<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>
<!-- 引入映射文件 -->
<mappers>
<package name="com.mapper"/>
</mappers>
</configuration>
jdbc.properties
# K = V
jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/users
jdbc.username=root
jdbc.password=0.
2 案例分析
首先在 MySQL 中創建數據庫:users,再在此數據庫中創建表:students 和 classes。
students 包含 sid(int)、sname(varchar)和 cid(int) 3個字段,其中,sid 設置了自增,cid 爲外鍵,指向 classes 表中的 cid,students 表中數據如下:
classes 包含 cid(int)、cname(varchar)2個字段,其中,cid 設置了自增,classes 表中數據如下:
首先介紹下公共的文件,主要包含 Student.java、Class.java、StudentClassMapper.java、Test.java,不同的是 StudentClassMapper.xml,將在各章節分別介紹。
Student.java
package com.bean;
public class Student {
private Integer sid;
private String sname;
private Class cla;
public Integer getSid() {
return sid;
}
public void setSid(Integer sid) {
this.sid = sid;
}
public String getSname() {
return sname;
}
public void setSname(String sname) {
this.sname = sname;
}
public Class getCla() {
return cla;
}
public void setCla(Class cla) {
this.cla = cla;
}
@Override
public String toString() {
return "Student [sid=" + sid + ", sname=" + sname + ", cla=" + cla + "]";
}
}
Class.java
package com.bean;
public class Class {
private Integer cid;
private String cname;
public Integer getCid() {
return cid;
}
public void setCid(Integer cid) {
this.cid = cid;
}
public String getCname() {
return cname;
}
public void setCname(String cname) {
this.cname = cname;
}
@Override
public String toString() {
return "Class [cid=" + cid + ", cname=" + cname + "]";
}
}
StudentClassMapper.java
package com.mapper;
import com.bean.Student;
public interface StudentClassMapper {
public Student getStudentById(Integer sid);
}
Test.java
package com.test;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import com.bean.Student;
import com.mapper.StudentClassMapper;
public class Test {
public static void main(String[] args) throws IOException {
InputStream is=Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(is);
SqlSession sqlSession=sqlSessionFactory.openSession(true); //自動提交事務
//getMapper:會通過動態代理動態生成StudentMapper的代理實現類
StudentClassMapper mapper=sqlSession.getMapper(StudentClassMapper.class);
Student student=mapper.getStudentById(1001);
System.out.println(student);
}
}
2.1 方法一
StudentClassMapper.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.mapper.StudentClassMapper">
<resultMap type="com.bean.Student" id="studentMap">
<id column="sid" property="sid"/>
<result column="sname" property="sname"/>
<result column="cid" property="cla.cid"/>
<result column="cname" property="cla.cname"/>
</resultMap>
<!-- public Student getStudentById(Integer sid); -->
<select id="getStudentById" resultMap="studentMap">
select s.sid,s.sname,s.cid,c.cname from students s left join classes c on s.cid=c.cid where s.sid = #{sid}
</select>
</mapper>
注意:<resultMap> 用於定義從數據庫查詢出來的數據與對象的映射關係,即:記錄屬性→對象屬性, <id> 用於定義主鍵映射關係,<result> 用於定義其他屬性映射關係。
運行結果:
DEBUG 06-13 18:43:11,039 ==> Preparing: select s.sid,s.sname,s.cid,c.cname from students s left join classes c on s.cid=c.cid where s.sid=?
Student [sid=1001, sname=張三, cla=Class [cid=1, cname=一班]]
2.2 方法二(association標籤 )
StudentClassMapper.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.mapper.StudentClassMapper">
<resultMap type="com.bean.Student" id="studentMap">
<id column="sid" property="sid"/>
<result column="sname" property="sname"/>
<result column="cid" property="cla.cid"/>
<association property="cla" javaType="com.bean.Class">
<id column="cid" property="cid"/>
<result column="cname" property="cname"/>
</association>
</resultMap>
<!-- public Student getStudentById(Integer sid); -->
<select id="getStudentById" resultMap="studentMap">
select s.sid,s.sname,s.cid,c.cname from students s left join classes c on s.cid=c.cid where s.sid=#{sid}
</select>
</mapper>
注意:<association> 標籤定義了多對一映射關係。
運行結果:
DEBUG 06-13 18:44:24,441 ==> Preparing: select s.sid,s.sname,s.cid,c.cname from students s left join classes c on s.cid=c.cid where s.sid=?
Student [sid=1001, sname=張三, cla=Class [cid=1, cname=一班]]
2.3 方法三(分步查詢)
分兩步:首先查詢學生信息,再查詢班級信息。
ClassMapper.java
package com.mapper;
import com.bean.Class;
public interface ClassMapper {
public Class getClassById(Integer cid);
}
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.mapper.ClassMapper">
<!-- public Class getClassById(Integer cid); -->
<select id="getClassById" resultType="com.bean.Class">
select cid,cname from classes where cid = #{cid}
</select>
</mapper>
StudentClassMapper.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.mapper.StudentClassMapper">
<resultMap type="com.bean.Student" id="studentMap">
<id column="sid" property="sid"/>
<result column="sname" property="sname"/>
<result column="cid" property="cla.cid"/>
<association property="cla" select="com.mapper.ClassMapper.getClassById" column="cid"/>
</resultMap>
<!-- public Student getStudentById(Integer sid); -->
<select id="getStudentById" resultMap="studentMap">
select sid,sname,cid from students where sid=#{sid}
</select>
</mapper>
注意:<association> 標籤中,select 爲第二步查詢的 SQL id,即接口的全限定名,column 爲第二步查詢的條件,此條件必須是從數據庫中查詢得到。
運行結果:
DEBUG 06-13 18:48:59,783 ==> Preparing: select sid,sname,cid from students where sid=?
DEBUG 06-13 18:48:59,829 ====> Preparing: select cid,cname from classes where cid = ?
Student [sid=1001, sname=張三, cla=Class [cid=1, cname=一班]]
2.4 分步查詢的延遲加載
延遲加載也稱爲懶加載,是指使用時才加載,不使用就不加載,只有分步查詢才能實現延遲加載。爲查看並比較延遲加載的效果,進行了如下3個測試實驗。
測試一(沒有設置延遲加載,使用第一步查詢的數據)
Test.java
package com.test;
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;
import com.bean.Student;
import com.mapper.StudentClassMapper;
public class Test {
public static void main(String[] args) throws IOException {
InputStream is=Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(is);
SqlSession sqlSession=sqlSessionFactory.openSession(true); //自動提交事務
//getMapper:會通過動態代理動態生成StudentMapper的代理實現類
StudentClassMapper mapper=sqlSession.getMapper(StudentClassMapper.class);
Student student=mapper.getStudentById(1001);
System.out.println(student.getSname());
}
}
運行結果:
DEBUG 06-13 19:19:03,149 ==> Preparing: select sid,sname,cid from students where sid=?
DEBUG 06-13 19:19:03,195 ====> Preparing: select cid,cname from classes where cid = ?
張三
可以看到:在沒有設置延遲加載時,只使用第一步查詢的數據,仍然會執行第二步查詢語句。
測試二(設置延遲加載,使用第一步查詢的數據)
本節 Test.java 同測試一。設置延遲加載,需要在 mybatis-config.xml 的<settings>標籤中配置,如下。
mybatis-config.xml
<?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>
<!-- 設置或引入資源文件 -->
<properties resource="jdbc.properties"></properties>
<settings>
<!-- 開啓延遲加載 -->
<setting name="lazyLoadingEnabled" value="true"/>
<!-- 延遲加載中,不查詢所有數據 -->
<setting name="aggressiveLazyLoading" value="flase"/>
</settings>
<!-- 設置連接數據庫的環境,default用於設置默認使用的數據庫環境 -->
<environments default="mysql">
<!-- 設置某個具體的數據庫環境 -->
<environment id="mysql">
<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>
<!-- 引入映射文件 -->
<mappers>
<package name="com.mapper"/>
</mappers>
</configuration>
注意: settings 標籤必須放在 properties 標籤之後,否則會報錯:
The content of element type "configuration" must match
"(properties?,settings?,typeAliases?,typeHandlers?,objectFactory?,objectWrapperFactory?,reflectorFactory?,plugins?,environments?,databaseIdProvider?,mappers?)".
運行結果:
DEBUG 06-13 19:33:21,338 ==> Preparing: select sid,sname,cid from students where sid=?
張三
可以看到:在設置了延遲加載時,只使用第一步查詢的數據,不會執行第二步查詢語句。
測試三(設置延遲加載,使用第二步查詢的數據)
mybatis-config.xml 同測試二。
Test.java
package com.test;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import com.bean.Student;
import com.mapper.StudentClassMapper;
public class Test {
public static void main(String[] args) throws IOException {
InputStream is=Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(is);
SqlSession sqlSession=sqlSessionFactory.openSession(true); //自動提交事務
//getMapper:會通過動態代理動態生成StudentMapper的代理實現類
StudentClassMapper mapper=sqlSession.getMapper(StudentClassMapper.class);
Student student=mapper.getStudentById(1001);
System.out.println("==========================");
System.out.println(student.getCla());
}
}
運行結果:
DEBUG 06-13 19:42:08,299 ==> Preparing: select sid,sname,cid from students where sid=?
==========================
DEBUG 06-13 19:42:08,394 ==> Preparing: select cid,cname from classes where cid = ?
Class [cid=1, cname=一班]
可以看到:在設置了延遲加載時,使用了第二步查詢的數據,纔會執行第二步查詢語句。