MyBatis 級聯

數據庫關係模型中有一對一,一對多,關係,
首先建立如下表 繼續用最經典的學生管理系統

建表

模型圖如圖所示

學生表爲中心
學生證表和其爲一對一關係
學生表和課程表爲一對多

一對一關係

因爲上圖中,學生和學生關係爲一對一關係.
由於是數據庫的一對一關係,那麼對應的bean中,也應該是一對一關係
舉栗子的表爲學生表和學生證表
其中學生證表的中的學生編號的外鍵爲學生表的主鍵
那麼在學生表的POJO對象中,應該有學生證表的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;
    }
}

對於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;
    }
}

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>

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"/>
        <!--將會調用接口代表的SQL 進行執行查詢 -->
        <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>

可以看到當查詢到studentCard的時候,將會調用com.ming.MyBatis.RoleMapper.findStudentSelfCardByStudentId sql語句的執行

對於數據展示層來說

<%@ 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(); //獲取開始時間
    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(); //獲取結束時間

%>

<%
    Iterator iterator = students.iterator();
    while(iterator.hasNext()){
        %>
            <%=((Student)iterator.next()).getStudentCard().getEndDate()%>

        <%
    }
%>
</body>
</html>

一對多

這個和上方同理,跳過

鑑別器

根據學生信息去關聯男性或者女性的健康指標

兩個類只需要繼承同一個類,使用case語句即可.
跳過

性能問題

在使用級聯的時候,會造成多條sql語句的執行,此時,使用延遲加載

一開始不使用sql語句,只有當使用了sql纔去發送數據

在setting這裏設置,可以降低sql語句的執行問題

    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
    </settings>

一般情況下是層級加載,取消層級加載,使用aggressiveLazyLoading值設置爲false即可

另一種

寫原生sql 這個很簡單,跳過