JPA学习 —— 第十一课、双向一对一关联关系

骑猪看日落 2022-05-25 03:58 196阅读 0赞

# 双向一对一关联关系 #

一个部门有一个经理，一个经理管一个部门

**Department实体类**

package com.ssj.domain;
    
    import javax.persistence.Column;
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.Id;
    import javax.persistence.JoinColumn;
    import javax.persistence.OneToOne;
    import javax.persistence.Table;
    
    @Table(name="department ")
    @Entity
    public class Department { 
    
        @Column(name="ID")
        @GeneratedValue
        @Id
        private Integer id;
    
        @Column(name="DEPT_NAME")
        private String deptName;
    
        @JoinColumn(name="MANAGER_ID")
        @OneToOne
        private Manager manager;
    
        public Integer getId() {
            return id;
        }
    
        public void setId(Integer id) {
            this.id = id;
        }
    
        public String getDeptName() {
            return deptName;
        }
    
        public void setDeptName(String deptName) {
            this.deptName = deptName;
        }
    
        public Manager getManager() {
            return manager;
        }
    
        public void setManager(Manager manager) {
            this.manager = manager;
        }
    
    }

**Manager实体类**

package com.ssj.domain;
    
    import javax.persistence.Column;
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.Id;
    import javax.persistence.OneToOne;
    import javax.persistence.Table;
    
    @Table(name="manager ")
    @Entity
    public class Manager { 
    
        @Column(name="ID")
        @GeneratedValue
        @Id
        private Integer id;
    
        @Column(name="NAME")
        private String name;
    
        @OneToOne(mappedBy="manager")
        private Department department;
    
        public Integer getId() {
            return id;
        }
    
        public void setId(Integer id) {
            this.id = id;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public Department getDepartment() {
            return department;
        }
    
        public void setDepartment(Department department) {
            this.department = department;
        }
    
    }

**关于mappedBy的使用，在前面的博客有讲到。**

**插入**

Manager manager = new Manager();
    manager.setName("mgr1");
    
    Department department = new Department();
    department.setDeptName("deptName1");
    
    manager.setDepartment(department);
    department.setManager(manager);
    
    entityManager.persist(manager);
    entityManager.persist(department);

**关于插入的顺序，先插入不维护关联关系的一方，即manager（没有外键的一方）**

**查询**

查询维护关联关系的一方（有外键的一方）

Department dept = entityManager.find(Department.class, 1);
    System.out.println(dept.getDeptName());

发现执行了两次sql语句，既查询了department，也查询了manager

但我们此时只想要department，不想要manager。可以用fetch更改为懒加载策略:  
在Department实体类中

@OneToOne(fetch=FetchType.LAZY)
    private Manager manager;

**查询不维护关联关系的一方（没有外键的一方）**

Manager m = entityManager.find(Manager.class, 1);
    System.out.println(m.getName());

运行发现，查询manager时，一定会去查department，即使用fetch改为懒加载策略，结果也是一样的。

不同的是，如果改为懒加载，将会执行两个sql语句查询。默认情况下是执行一句sql语句关联查询。因此不推荐设置fetch

**一定会查询两个对象的原因：**