MyBatis 级联

ゝ一纸荒年。 2022-10-01 10:55 23阅读 0赞

数据库关系模型中有一对一,一对多,关系, 首先建立如下表 继续用最经典的学生管理系统

# 建表 #

模型图如图所示

学生表为中心 学生证表和其为一对一关系 学生表和课程表为一对多

# 一对一关系 #

因为上图中,学生和学生关系为一对一关系. 由于是数据库的一对一关系,那么对应的bean中,也应该是一对一关系 举栗子的表为学生表和学生证表 其中学生证表的中的学生编号的外键为学生表的主键 那么在学生表的POJO对象中,应该有学生证表的POJO的映射.即

package com.ming.MyBatis.POJO;
    
    /**
     * @author ming
     */
    public class Student {
        private int uid;
        private String studentName;
        private String gender;
        private String studentIdNumber;
        private String remarks;
        private StudentCard studentCard;
    
        public void setStudentCard(StudentCard studentCard) {
            this.studentCard = studentCard;
        }
    
        public StudentCard getStudentCard() {
            return studentCard;
        }
    
        public void setUid(int uid) {
            this.uid = uid;
        }
    
        public void setStudentName(String studentName) {
            this.studentName = studentName;
        }
    
        public void setGender(String gender) {
            this.gender = gender;
        }
    
        public void setStudentIdNumber(String studentIdNumber) {
            this.studentIdNumber = studentIdNumber;
        }
    
        public void setRemarks(String remarks) {
            this.remarks = remarks;
        }
    
        public int getUid() {
            return uid;
        }
    
        public String getStudentName() {
            return studentName;
        }
    
        public String getGender() {
            return gender;
        }
    
        public String getStudentIdNumber() {
            return studentIdNumber;
        }
    
        public String getRemarks() {
            return remarks;
        }
    }
    
    复制代码

对于card的POJO如下所示

package com.ming.MyBatis.POJO;
    
    
    import java.util.Date;
    
    /**
     * @author ming
     */
    public class StudentCard {
        private int uid;
        private String studentNumber;
        private String birthplace;
        private Date dateOfIssue;
        private Date endDate;
        private String remarks;
    
        public void setUid(int uid) {
            this.uid = uid;
        }
    
        public void setStudentNumber(String studentNumber) {
            this.studentNumber = studentNumber;
        }
    
        public void setBirthplace(String birthplace) {
            this.birthplace = birthplace;
        }
    
        public void setDateOfIssue(Date dateOfIssue) {
            this.dateOfIssue = dateOfIssue;
        }
    
        public void setEndDate(Date endDate) {
            this.endDate = endDate;
        }
    
        public void setRemarks(String remarks) {
            this.remarks = remarks;
        }
    
        public int getUid() {
            return uid;
        }
    
        public String getStudentNumber() {
            return studentNumber;
        }
    
        public String getBirthplace() {
            return birthplace;
        }
    
        public Date getDateOfIssue() {
            return dateOfIssue;
        }
    
        public Date getEndDate() {
            return endDate;
        }
    
        public String getRemarks() {
            return remarks;
        }
    }
    
    复制代码

先增加StudentCard的配置

<resultMap id="studentSelfCardMap" type="com.ming.MyBatis.POJO.StudentCard">
    		<id column="uid" property="uid"/>
    		<result column="student_number" property="studentNumber"/>
    		<result column="birthplace" property="birthplace" />
    		<result column="date_of_issue" property="dateOfIssue" jdbcType="DATE" javaType="java.util.Date"/>
    		<result column="end_date" property="endDate" jdbcType="DATE" javaType="java.util.Date"/>
    		<result column="remarks" property="remarks" />
    	</resultMap>
    
    	<select id="findStudentSelfCardByStudentId" parameterType="int" resultMap="studentSelfCardMap">
    		SELECT student_card.uid, student_card.student_number, student_card.remarks,
    		student_card.end_date, student_card.date_of_issue, student_card.birthplace
    		FROM student_card WHERE student_card.uid = #{studentId};
    	</select>
    复制代码

再增加Student的映射

<resultMap id="studentMap" type="com.ming.MyBatis.POJO.Student">
    		<id column="uid" property="uid"/>
    		<result column="student_name" property="studentName"/>
    		<result column="gender" property="gender"/>
    		<result column="student_id_number" property="studentIdNumber"/>
    		<result column="remarks" property="remarks"/>
    		
    		<association property="studentCard" column="uid" select="com.ming.MyBatis.RoleMapper.findStudentSelfCardByStudentId"/>
    	</resultMap>
    	
    	<select id="getStudent" parameterType="int" resultMap="studentMap">
    		SELECT student.uid, student.gender, student.remarks, student.student_id_number,
    		student.student_name
    		FROM student
    		WHERE student.uid = 1;
    	</select>
    复制代码

可以看到当查询到studentCard的时候,将会调用com.ming.MyBatis.RoleMapper.findStudentSelfCardByStudentId sql语句的执行

对于数据展示层来说

<%@ page import="org.apache.ibatis.session.SqlSession" %>
    <%@ page import="com.ming.Util.SqlSessionFactoryUtil" %>
    <%@ page import="com.ming.MyBatis.RoleMapper" %>
    <%@ page import="java.util.List" %>
    <%@ page import="java.util.Iterator" %>
    <%@ page import="com.ming.MyBatis.POJO.Student" %>
    <html>
    <body>
    <h2>Hello World!</h2>
    
    <%
        long startTime = System.currentTimeMillis(); //获取开始时间
        SqlSession sqlSession = null;
        List<Student> students = null;
        for(int i = 0; i < 10; i++) {
            try {
                sqlSession = SqlSessionFactoryUtil.openSqlSesion();
                RoleMapper roleMapper = sqlSession.getMapper(RoleMapper.class);
                students = roleMapper.getStudent(1);
                sqlSession.commit();
            } catch (Exception e) {
                e.printStackTrace();
                sqlSession.rollback();
            } finally {
                if (sqlSession != null) {
                    sqlSession.close();
                }
            }
        }
        long endTime = System.currentTimeMillis(); //获取结束时间
    
    %>
    
    <%
        Iterator iterator = students.iterator();
        while(iterator.hasNext()){
            %>
                <%=((Student)iterator.next()).getStudentCard().getEndDate()%>
    
            <%
        }
    %>
    </body>
    </html>
    
    复制代码

# 一对多 #

这个和上方同理,跳过

# 鉴别器 #

根据学生信息去关联男性或者女性的健康指标

两个类只需要继承同一个类,使用case语句即可. 跳过

# 性能问题 #

在使用级联的时候,会造成多条sql语句的执行,此时,使用延迟加载

一开始不使用sql语句,只有当使用了sql才去发送数据

在setting这里设置,可以降低sql语句的执行问题

<settings>
    		<setting name="lazyLoadingEnabled" value="true"/>
    	</settings>
    复制代码

一般情况下是层级加载,取消层级加载,使用aggressiveLazyLoading值设置为false即可

# 另一种 #