SpringBoot-SpringData-懒加载

本节聊一下,懒加载的配置与应用

本工程由SpringBoot-SpringData-ManyToMany为基础修改而来
沿用了之前项目的配置,如有任何疑问请进入查看

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一,目录结构

和SpringBoot-SpringData-ManyToMany项目相同以课程和学生的多对多关系为模型

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二,懒加载的作用和应用场景

预期:     启用懒加载后,对Student表取数,不会自动带出Course集合     当student1.getCourse()使用集合时,再执行Student取数作用:     当不需要使用Course集合引用时,不会执行多余的查询,提升效率

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三,修改domain实现懒加载

主要修改部分:

     @ManyToMany(cascade = {CascadeType.PERSIST}, fetch = FetchType.LAZY)

Student.java

@Entity@Table(name = "student")public class Student {    private String pid;    private String studentName;    private Set<Course> course;    //选修课程    public Student() {    }    public Student(String pid, String studentName, Set<Course> course) {        this.pid = pid;        this.studentName = studentName;        this.course = course;    }    @Id    @Column(name = "pid", unique = true, nullable = false, length = 32)//    @GeneratedValue(generator = "generator")//    @GenericGenerator(name = "generator", strategy = "uuid")    public String getPid() {        return pid;    }    @Column(name = "student_name", unique = true, length = 64)    public String getStudentName() {        return studentName;    }    /**     * Hibernate 会自动创建一张关系表stu_cou， 里边有俩字段stu_id和cou_id分别为两表主键     *     * @return     */    @ManyToMany(cascade = {CascadeType.PERSIST}, fetch = FetchType.LAZY)    @JoinTable(name = "stu_cou", joinColumns = {@JoinColumn(name = "stu_id")}, inverseJoinColumns = {@JoinColumn(name = "cou_id")})    public Set<Course> getCourse() {        return course;    }    public void setPid(String pid) {        this.pid = pid;    }    public void setStudentName(String studentName) {        this.studentName = studentName;    }    public void setCourse(Set<Course> course) {        this.course = course;    }}

Course.java

@Entity@Table(name = "course")public class Course implements java.io.Serializable {    private static final long serialVersionUID = 6398143635533582335L;    private String pid;    private String courseName;// 课程名称    private Set<Student> Student; // 选修课程学生    public Course() {    }    public Course(String pid, String courseName, Set<Student> student) {        this.pid = pid;        this.courseName = courseName;        this.Student = student;    }    @Id    @Column(name = "pid", unique = true, nullable = false, length = 32)//    @GeneratedValue(generator = "generator")//    @GenericGenerator(name = "generator", strategy = "uuid")    public String getPid() {        return pid;    }    @Column(name = "course_name", unique = true, length = 64)    public String getCourseName() {        return courseName;    }    //mappedBy :    //          表示当前所在表和 Student 的关系是定义在 Student 里面的 course 这个成员上面的，    //          他表示此表是一对一关系中的从表，也就是关系是在 Student 表中维护的，    //          Student 表是关系的维护者，有主导权，它有个外键指向 course (Student 中的 getCourse() )    @ManyToMany(fetch = FetchType.LAZY, mappedBy = "course")    //NotFound : 意思是找不到引用的外键数据时忽略，NotFound默认是exception    @NotFound(action = NotFoundAction.IGNORE)    public Set<Student> getStudent() {        return Student;    }    public void setPid(String pid) {        this.pid = pid;    }    public void setCourseName(String courseName) {        this.courseName = courseName;    }    public void setStudent(Set<Student> student) {        Student = student;    }}

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四,application.properties开启懒加载

spring.datasource.url=jdbc:mysql://localhost:3306/testspring.datasource.username=rootspring.datasource.password=123spring.datasource.driver-class-name=com.mysql.jdbc.Driverspring.jpa.properties.hibernate.hbm2ddl.auto=updatespring.jpa.show-sql=true##懒加载spring.jpa.open-in-view=true

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五,添加测试数据

import.sql

INSERT INTO `course` VALUES ('1', 'course1'), ('2', 'course2');INSERT INTO `student` VALUES ('1', 'student1'), ('2', 'student2'), ('3', 'student3');INSERT INTO `stu_cou` VALUES ('1', '1'), ('3', '1'), ('1', '2'), ('3', '2');

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六,单元测试

ManyToManyTest.java

@RunWith(SpringJUnit4ClassRunner.class)@SpringApplicationConfiguration(DemoApplication.class)public class ManyToManyTest {    @Autowired    StudentRepository studentRepository;    @Autowired    CourseRepository courseRepository;    @Before    public void testData(){    }    @Test    @Transactional    public void test() throws Exception {        //取学生-包含课程对象集        Student student1 = studentRepository.findByStudentName("Student1");        //验证懒加载        Set<Course> course = student1.getCourse();        System.out.print("course.size()" + course.iterator().next());    }}

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七,测试懒加载

在测试类,查询Student代码下断点,查看当使用前后,Course集合的引用变化,及SQL输出,验证懒加载

Debug模式运行测试类方法,进入调试模式,先清空无用Log

测试加入测试数据到数据库

INSERT INTO `course` VALUES ('1', 'course1'), ('2', 'course2');INSERT INTO `student` VALUES ('1', 'student1'), ('2', 'student2'), ('3', 'student3');INSERT INTO `stu_cou` VALUES ('1', '1'), ('3', '1'), ('1', '2'), ('3', '2');

F6执行student表取数,查看变量值和LOG输出

此时LOG输出只取了Student表,再F6向下执行student1.getCourse()

LOG输出了取Course集合的SQL语句

至此SpringBoot-SpringData-JPA-懒加载完成

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八,总结

懒加载的配置主要有两点:
1,spring.jpa.open-in-view=true 这个配置默认就是true
2,取数方法使用@Transactional注解–主要

如不使用@Transactional懒加载会报错

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九,代码下载

　　CSDN下载
　　
　　GitHub下载