MyBatis 级联

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; }}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; }}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”/> <!–将会调用接口代表的SQL 进行执行查询 –> <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>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>一般情况下是层级加载,取消层级加载,使用aggressiveLazyLoading值设置为false即可另一种写原生sql 这个很简单,跳过