JPA

@Author:hanguixian

@Email:hn_hanguixian@163.com

五 映射关联关系

映射单向多对一的关联关系

• Customer类

package com.ifox.hgx.jpa.entity;

import javax.persistence.*;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;

@Table(name = "JPA_CUSTOMTERS")
@Entity
public class Customer {


    private Integer id ;
    private String lastName ;

    private String email ;
    private Integer age ;

    private Date createTime ;
    private Date birth ;

    @Temporal(TemporalType.TIMESTAMP)
    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    @Temporal(TemporalType.DATE)
    public Date getBirth() {
        return birth;
    }

    public void setBirth(Date birth) {
        this.birth = birth;
    }

    // pkColumnName = "PK_NAME",pkColumnValue = "CUSTOMER_ID"确定行
    // valueColumnName = "PK_VALUE"确定列
    //allocationSize 每次增加多少
//    @TableGenerator(name = "ID_GENERATOR",table = "jap_id_generators",
//            pkColumnName = "PK_NAME",pkColumnValue = "CUSTOMER_ID",
//            valueColumnName = "PK_VALUE",initialValue = 1,allocationSize = 100
//
//    )
//    @GeneratedValue(strategy = GenerationType.TABLE,generator = "ID_GENERATOR")

//    GeneratedValue 生成方式:策略为 GenerationType.AUTO 自动选择
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name = "LAST_NAME")
    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @Column(length = 50)
    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    //不是需要映射的字段的 需要加上注解Transient
    @Transient
    public String getInfo(){
        return "lastName:"+lastName+" Email:" +email ;
    }


}

• Order类

package com.ifox.hgx.jpa.entity;

import javax.persistence.*;

@Table(name = "JPA_ORDER")
@Entity
public class Order {
    private Integer id ;
    private String orderName ;
    private Customer customer ;

    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name = "ORDER_NAME")
    public String getOrderName() {
        return orderName;
    }

    public void setOrderName(String orderName) {
        this.orderName = orderName;
    }

//  @ManyToOne(fetch = FetchType.LAZY) 懒加载

    @JoinColumn(name = "CUSTOMER_ID")
    @ManyToOne(fetch = FetchType.LAZY)
    public Customer getCustomer() {
        return customer;
    }
    public void setCustomer(Customer customer) {
        this.customer = customer;
    }
}

• 测试示例:

package com.ifox.hgx.jpa.test;

import com.ifox.hgx.jpa.entity.Customer;
import com.ifox.hgx.jpa.entity.Order;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import java.util.Date;

public class TestMappingRelations {

    private EntityManagerFactory entityManagerFactory ;
    private EntityManager entityManager ;
    private EntityTransaction transaction ;

    @Before
    public void init(){
        entityManagerFactory = Persistence.createEntityManagerFactory("jpa_m1") ;
        entityManager = entityManagerFactory.createEntityManager() ;
        transaction = entityManager.getTransaction() ;
        transaction.begin();
    }

    @After
    public void distroy(){
        transaction.commit();
        entityManager.close();
        entityManagerFactory.close();
    }

//    保存多对一时, 建议先保存 1 的一端, 后保存 n 的一端, 这样不会多出额外的 UPDATE 语句.
    @Test
    public void testManyToOnePersist(){
        Customer customer = new Customer();
        customer.setAge(19);
        customer.setBirth(new Date());
        customer.setCreateTime(new Date());
        customer.setEmail("GG@163.com");
        customer.setLastName("GG");

        Order order1 = new Order() ;
        Order order2 = new Order() ;
        order1.setOrderName("GG-0-1");
        order2.setOrderName("GG-0-2");

        order1.setCustomer(customer);
        order2.setCustomer(customer);

        //3条insert sql语句
//        entityManager.persist(customer);
//        entityManager.persist(order1);
//        entityManager.persist(order2);

        //3条insert 2条update
        entityManager.persist(order1);
        entityManager.persist(order2);
        entityManager.persist(customer);
    }
    //默认情况下, 使用左外连接的方式来获取 n 的一端的对象和其关联的 1 的一端的对象. 
    //可使用 @ManyToOne 的 fetch 属性来修改默认的关联属性的加载策略
    //默认不使用懒加载
    @Test
    public void testManyToOneFind(){
        Order order = entityManager.find(Order.class,1) ;
        System.out.println(order.getOrderName());
        System.out.println(order.getCustomer().getLastName());
    }



    @Test
    public void testManyToOneUpdate(){
        Order order = entityManager.find(Order.class,1) ;
        order.getCustomer().setLastName("FF");

    }

    //不能直接删除 1 的一端, 因为有外键约束.
    @Test
    public void testManyToOneRemove(){
//        Order order = entityManager.find(Order.class,1) ;
//        entityManager.remove(order);

        Customer customer = entityManager.find(Customer.class,1) ;
        entityManager.remove(customer);
    }


}

• persistence.xml中加入:

<!--添加持久化类-->
        <class>com.ifox.hgx.jpa.entity.Customer</class>
        <class>com.ifox.hgx.jpa.entity.Order</class>

映射单向一对多的关联关系

• Customer类中:

package com.ifox.hgx.jpa.entity;

import javax.persistence.*;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;

@Table(name = "JPA_CUSTOMTERS")
@Entity
public class Customer {


    private Integer id ;
    private String lastName ;

    private String email ;
    private Integer age ;

    private Date createTime ;
    private Date birth ;

    private Set<Order> orders = new HashSet<>() ;

    //1-n 添加时,需要cascade = CascadeType.PERSIST 级联添加
    //1-n 级联删除,需要CascadeType.REMOVE
    @JoinColumn(name = "CUSTOMER_ID")
    @OneToMany(cascade = {CascadeType.PERSIST,CascadeType.REMOVE})
    public Set<Order> getOrders() {
        return orders;
    }

    public void setOrders(Set<Order> orders) {
        this.orders = orders;
    }

    @Temporal(TemporalType.TIMESTAMP)
    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    @Temporal(TemporalType.DATE)
    public Date getBirth() {
        return birth;
    }

    public void setBirth(Date birth) {
        this.birth = birth;
    }

    // pkColumnName = "PK_NAME",pkColumnValue = "CUSTOMER_ID"确定行
    // valueColumnName = "PK_VALUE"确定列
    //allocationSize 每次增加多少
//    @TableGenerator(name = "ID_GENERATOR",table = "jap_id_generators",
//            pkColumnName = "PK_NAME",pkColumnValue = "CUSTOMER_ID",
//            valueColumnName = "PK_VALUE",initialValue = 1,allocationSize = 100
//
//    )
//    @GeneratedValue(strategy = GenerationType.TABLE,generator = "ID_GENERATOR")

//    GeneratedValue 生成方式:策略为 GenerationType.AUTO 自动选择
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name = "LAST_NAME")
    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @Column(length = 50)
    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    //不是需要映射的字段的 需要加上注解Transient
    @Transient
    public String getInfo(){
        return "lastName:"+lastName+" Email:" +email ;
    }

    @Override
    public String toString() {
        return "Customer{" +
                "id=" + id +
                ", lastName='" + lastName + '\'' +
                ", email='" + email + '\'' +
                ", age=" + age +
                ", createTime=" + createTime +
                ", birth=" + birth +
                ", orders=" + orders +
                '}';
    }
}

• Order类:

package com.ifox.hgx.jpa.entity;

import javax.persistence.*;

@Table(name = "JPA_ORDER")
@Entity
public class Order {
    private Integer id ;
    private String orderName ;
//    private Customer customer ;

    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name = "ORDER_NAME")
    public String getOrderName() {
        return orderName;
    }

    public void setOrderName(String orderName) {
        this.orderName = orderName;
    }

////  @ManyToOne(fetch = FetchType.LAZY) 懒加载
//    @JoinColumn(name = "CUSTOMER_ID")
//    @ManyToOne(fetch = FetchType.LAZY)
//    public Customer getCustomer() {
//        return customer;
//    }
//
//    public void setCustomer(Customer customer) {
//        this.customer = customer;
//    }
}

测试示例:

package com.ifox.hgx.jpa.test;

import com.ifox.hgx.jpa.entity.Customer;
import com.ifox.hgx.jpa.entity.Order;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import java.util.Date;

public class TestMapping_OneToMany {

    private EntityManagerFactory entityManagerFactory ;
    private EntityManager entityManager ;
    private EntityTransaction transaction ;

    @Before
    public void init(){
        entityManagerFactory = Persistence.createEntityManagerFactory("jpa_m1") ;
        entityManager = entityManagerFactory.createEntityManager() ;
        transaction = entityManager.getTransaction() ;
        transaction.begin();
    }

    @After
    public void destroy(){
        transaction.commit();
        entityManager.close();
        entityManagerFactory.close();
    }

    //单向 1-n 关联关系执行保存时, 一定会多出 UPDATE 语句.
    //因为 n 的一端在插入时不会同时插入外键列.
    //
    @Test
    public void testOneToManyPersist(){
        Customer customer = new Customer() ;
        customer.setLastName("KKK");
        customer.setCreateTime(new Date());
        customer.setBirth(new Date());
        customer.setEmail("KKK@qq.com");
        customer.setAge(34);

        Order order = new Order() ;
        order.setOrderName("KK-01-1");

        Order order1 = new Order() ;
        order1.setOrderName("KK-02-2");

        customer.getOrders().add(order);
        customer.getOrders().add(order1) ;

        entityManager.persist(order);
        entityManager.persist(order);

        entityManager.persist(customer);


    }

    //默认对关联的多的一方使用懒加载的加载策略.
    //可以使用 @OneToMany 的 fetch 属性来修改默认的加载策略
    @Test
    public void testOneToManyFind(){
        Customer customer = entityManager.find(Customer.class,1) ;
        System.out.println(customer.getLastName());
        System.out.println(customer.getOrders());
    }

    //默认情况下, 若删除 1 的一端, 则会先把关联的 n 的一端的外键置空, 然后进行删除.
    //可以通过 @OneToMany 的 cascade 属性来修改默认的删除策略.
    @Test
    public void testOneToManyRemove(){
        Customer customer  = entityManager.find(Customer.class,1) ;
        entityManager.remove(customer);
    }

    @Test
    public void testOneToManyUpdate(){
        Customer customer  = entityManager.find(Customer.class,1) ;
        customer.getOrders().iterator().next().setOrderName("0-xxx-x");
    }
}

映射双向多对一的关联关系

• 双向一对多关系中，必须存在一个关系维护端，在 JPA 规范中，要求 many 的一方作为关系的维护端(owner side), one 的一方作为被维护端(inverse side).

• 可以在 one 方指定 @OneToMany 注释并设置 mappedBy 属性，以指定它是这一关联中的被维护端，many 为维护端.

• 在 many 方指定 @ManyToOne 注释，并使用 @JoinColumn 指定外键名称.

• Customer类

  package com.ifox.hgx.jpa.entity;
  

import javax.persistence.*;
import java.util.Date;
import java.util.HashSet;
import java.util.Set;

@Table(name = "JPA_CUSTOMTERS")
@Entity
public class Customer {

private Integer id ;
private String lastName ;

private String email ;
private Integer age ;

private Date createTime ;
private Date birth ;

private Set<Order> orders = new HashSet<>() ;

//1-n 添加时,需要cascade = CascadeType.PERSIST 级联添加
//1-n 级联删除,需要CascadeType.REMOVE
//@JoinColumn(name = "CUSTOMER_ID")
@OneToMany(cascade = {CascadeType.PERSIST,CascadeType.REMOVE},mappedBy = "customer",fetch = FetchType.EAGER)
public Set<Order> getOrders() {
    return orders;
}

public void setOrders(Set<Order> orders) {
    this.orders = orders;
}

@Temporal(TemporalType.TIMESTAMP)
public Date getCreateTime() {
    return createTime;
}

public void setCreateTime(Date createTime) {
    this.createTime = createTime;
}

@Temporal(TemporalType.DATE)
public Date getBirth() {
    return birth;
}

public void setBirth(Date birth) {
    this.birth = birth;
}

// pkColumnName = "PK_NAME",pkColumnValue = "CUSTOMER_ID"确定行
// valueColumnName = "PK_VALUE"确定列
//allocationSize 每次增加多少

// @TableGenerator(name = "ID_GENERATOR",table = "jap_id_generators",
// pkColumnName = "PK_NAME",pkColumnValue = "CUSTOMER_ID",
// valueColumnName = "PK_VALUE",initialValue = 1,allocationSize = 100
//
// )
// @GeneratedValue(strategy = GenerationType.TABLE,generator = "ID_GENERATOR")

// GeneratedValue 生成方式:策略为 GenerationType.AUTO 自动选择
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Id
public Integer getId() {
return id;
}

public void setId(Integer id) {
    this.id = id;
}

@Column(name = "LAST_NAME")
public String getLastName() {
    return lastName;
}

public void setLastName(String lastName) {
    this.lastName = lastName;
}

@Column(length = 50)
public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public Integer getAge() {
    return age;
}

public void setAge(Integer age) {
    this.age = age;
}

//不是需要映射的字段的 需要加上注解Transient
@Transient
public String getInfo(){
    return "lastName:"+lastName+" Email:" +email ;
}

@Override
public String toString() {
    return "Customer{" +
            "id=" + id +
            ", lastName='" + lastName + '\'' +
            ", email='" + email + '\'' +
            ", age=" + age +
            ", createTime=" + createTime +
            ", birth=" + birth +
            ", orders=" + orders +
            '}';
}

}

- Order类

```java
package com.ifox.hgx.jpa.entity;

import javax.persistence.*;

@Table(name = "JPA_ORDER")
@Entity
public class Order {
    private Integer id ;
    private String orderName ;
    private Customer customer ;



    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name = "ORDER_NAME")
    public String getOrderName() {
        return orderName;
    }

    public void setOrderName(String orderName) {
        this.orderName = orderName;
    }

//  @ManyToOne(fetch = FetchType.LAZY) 懒加载
    @JoinColumn(name = "CUSTOMER_ID")
    @ManyToOne(fetch = FetchType.LAZY)
    public Customer getCustomer() {
        return customer;
    }

    public void setCustomer(Customer customer) {
        this.customer = customer;
    }
}

• 测试示例:

package com.ifox.hgx.jpa.test;

import com.ifox.hgx.jpa.entity.Customer;
import com.ifox.hgx.jpa.entity.Order;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import java.util.Date;

public class TestMapping_TowWayOneToMany {

    private EntityManagerFactory entityManagerFactory;
    private EntityManager entityManager;
    private EntityTransaction transaction;

    @Before
    public void init() {
        entityManagerFactory = Persistence.createEntityManagerFactory("jpa_m1");
        entityManager = entityManagerFactory.createEntityManager();
        transaction = entityManager.getTransaction();
        transaction.begin();
    }

    @After
    public void destroy() {
        transaction.commit();
        entityManager.close();
        entityManagerFactory.close();
    }

    //若是双向 1-n 的关联关系, 执行保存时
    //若先保存 n 的一端, 再保存 1 的一端, 默认情况下, 会多出 n 条 UPDATE 语句.
    //若先保存 1 的一端, 则会多出 n 条 UPDATE 语句
    //在进行双向 1-n 关联关系时, 建议使用 n 的一方来维护关联关系, 而 1 的一方不维护关联系, 这样会有效的减少 SQL 语句.
    //注意: 若在 1 的一端的 @OneToMany 中使用 mappedBy 属性, 则 @OneToMany 端就不能再使用 @JoinColumn 属性了.
    @Test
    public void testTowWayOneToManyPersist() {
        Customer customer = new Customer();
        customer.setLastName("CCC");
        customer.setCreateTime(new Date());
        customer.setBirth(new Date());
        customer.setEmail("CCC@qq.com");
        customer.setAge(34);

        Order order = new Order();
        order.setOrderName("CC-01-1");

        Order order1 = new Order();
        order1.setOrderName("CC-02-2");

        customer.getOrders().add(order);
        customer.getOrders().add(order1);

        order.setCustomer(customer);
        order1.setCustomer(customer);

        //3insert 2update
//        entityManager.persist(order);
//        entityManager.persist(order);
//
//        entityManager.persist(customer);

        //如果使用了mappedBy = "customer" 则只用3条insert语句
        entityManager.persist(customer);

        entityManager.persist(order);
        entityManager.persist(order);


    }

    //可以使用 @OneToMany 的 fetch 属性来修改默认的加载策略,可以使用 @OneToMany 的 fetch 属性来修改默认的加载策略
    @Test
    public void testTowWayOneToManyFind(){
        Customer customer = entityManager.find(Customer.class, 1);
        System.out.println(customer.getLastName());

        System.out.println(customer.getOrders().size());
    }

    //默认情况下, 若删除 1 的一端, 则会先把关联的 n 的一端的外键置空, 然后进行删除.
    //可以通过 @OneToMany 的 cascade 属性来修改默认的删除策略. CascadeType.REMOVE 级联删除
    @Test
    public void tesTowWayOneToManyRemove(){
        Customer customer = entityManager.find(Customer.class, 3);
        entityManager.remove(customer);
    }

    @Test
    public void testTowWayUpdate(){
        Customer customer = entityManager.find(Customer.class, 1);
        customer.getOrders().iterator().next().setOrderName("O-XXX-1");

    }

}

映射双向一对一的关联关系

• 基于外键的 1-1 关联关系：在双向的一对一关联中，需要在关系被维护端(inverse side)中的 @OneToOne 注释中指定 mappedBy，以指定是这一关联中的被维护端。同时需要在关系维护端(owner side)建立外键列指向关系被维护端的主键列。

• 如果延迟加载要起作用, 就必须设置一个代理对象.

• Manager 其实可以不关联一个 Department

• 如果有 Department 关联就设置为代理对象而延迟加载, 如果不存在关联的 Department 就设置 null, 因为外键字段是定义在 Department 表中的,Hibernate 在不读取 Department 表的情况是无法判断是否有关联有 Deparmtment, 因此无法判断设置 null 还是代理对象, 而统一设置为代理对象,也无法满足不关联的情况, 所以无法使用延迟加载,只 有显式读取 Department.

• Department 类

package com.ifox.hgx.jpa.entity;

import javax.persistence.*;

@Table(name="JPA_DEPARTMENTS")
@Entity
public class Department {

    private Integer id;
    private String deptName;

    private Manager mgr;

    @GeneratedValue
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name="DEPT_NAME")
    public String getDeptName() {
        return deptName;
    }

    public void setDeptName(String deptName) {
        this.deptName = deptName;
    }

    //使用 @OneToOne 来映射 1-1 关联关系。
    //若需要在当前数据表中添加主键则需要使用 @JoinColumn 来进行映射. 注意, 1-1 关联关系, 所以需要添加 unique=true
    @JoinColumn(name="MGR_ID", unique=true)
    @OneToOne(fetch=FetchType.LAZY)
    public Manager getMgr() {
        return mgr;
    }

    public void setMgr(Manager mgr) {
        this.mgr = mgr;
    }
}

• Manager 类

package com.ifox.hgx.jpa.entity;

import javax.persistence.*;

@Table(name = "JPA_MANAGERS")
@Entity
public class Manager {

    private Integer id;
    private String mgrName;

    private Department dept;

    @GeneratedValue
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name = "MGR_NAME")
    public String getMgrName() {
        return mgrName;
    }

    public void setMgrName(String mgrName) {
        this.mgrName = mgrName;
    }

    //对于不维护关联关系, 没有外键的一方, 使用 @OneToOne 来进行映射, 建议设置 mappedBy=true
    @OneToOne(mappedBy = "mgr")
    public Department getDept() {
        return dept;
    }

    public void setDept(Department dept) {
        this.dept = dept;
    }
}

测试示例:

package com.ifox.hgx.jpa.test;

import com.ifox.hgx.jpa.entity.Department;
import com.ifox.hgx.jpa.entity.Manager;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

public class TestMapping_TwoWayOneToOne {

    private EntityManagerFactory entityManagerFactory;
    private EntityManager entityManager;
    private EntityTransaction transaction;

    @Before
    public void init() {
        entityManagerFactory = Persistence.createEntityManagerFactory("jpa_m1");
        entityManager = entityManagerFactory.createEntityManager();
        transaction = entityManager.getTransaction();
        transaction.begin();
    }

    @After
    public void destroy() {
        transaction.commit();
        entityManager.close();
        entityManagerFactory.close();
    }

    //双向 1-1 的关联关系, 建议先保存不维护关联关系的一方, 即没有外键的一方, 这样不会多出 UPDATE 语句.
    @Test
    public void testTwoWayOneToOnePersist(){
        Manager mgr = new Manager();
        mgr.setMgrName("M-BB");

        Department dept = new Department();
        dept.setDeptName("D-BB");

        //设置关联关系
        mgr.setDept(dept);
        dept.setMgr(mgr);

        //执行保存操作
        entityManager.persist(mgr);
        entityManager.persist(dept);
    }

    //1.默认情况下, 若获取维护关联关系的一方, 则会通过左外连接获取其关联的对象.
    //但可以通过 @OntToOne 的 fetch 属性来修改加载策略. lazy 下: com.ifox.hgx.jpa.entity.Manager$HibernateProxy$vI5wL1QS 代理对象
    @Test
    public void testTwoWayOneToOneFind(){
        Department dept = entityManager.find(Department.class, 2);
        System.out.println(dept.getDeptName());
        System.out.println(dept.getMgr().getClass().getName());
    }

    //1. 默认情况下, 若获取不维护关联关系的一方, 则也会通过左外连接获取其关联的对象.
    //可以通过 @OneToOne 的 fetch 属性来修改加载策略. 但依然会再发送 SQL 语句来初始化其关联的对象
    //这说明在不维护关联关系的一方, 不建议修改 fetch 属性.
    @Test
    public void testTwoWayOneToOneFind2(){
        Manager mgr = entityManager.find(Manager.class, 1);
        System.out.println(mgr.getMgrName());
        System.out.println(mgr.getDept().getClass().getName());
    }
}

persistence.xml中加入:

 <!--添加持久化类-->
<class>com.ifox.hgx.jpa.entity.Manager</class>
<class>com.ifox.hgx.jpa.entity.Department</class>

映射双向多对多的关联关系

• 在双向多对多关系中，我们必须指定一个关系维护端(owner side),可以通过 @ManyToMany 注释中指定 mappedBy 属性来标识其为关系维护端。

• Item 实体类

package com.ifox.hgx.jpa.entity;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Table(name="JPA_ITEMS")
@Entity
public class Item {

    private Integer id;
    private String itemName;

    private Set<Category> categories = new HashSet<>();

    @GeneratedValue
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name="ITEM_NAME")
    public String getItemName() {
        return itemName;
    }

    public void setItemName(String itemName) {
        this.itemName = itemName;
    }

    //使用 @ManyToMany 注解来映射多对多关联关系
    //使用 @JoinTable 来映射中间表
    //1. name 指向中间表的名字
    //2. joinColumns 映射当前类所在的表在中间表中的外键
    //2.1 name 指定外键列的列名
    //2.2 referencedColumnName 指定外键列关联当前表的哪一列
    //3. inverseJoinColumns 映射关联的类所在中间表的外键
    @JoinTable(name="ITEM_CATEGORY",
            joinColumns={@JoinColumn(name="ITEM_ID", referencedColumnName="ID")},
            inverseJoinColumns={@JoinColumn(name="CATEGORY_ID", referencedColumnName="ID")})
    @ManyToMany
    public Set<Category> getCategories() {
        return categories;
    }

    public void setCategories(Set<Category> categories) {
        this.categories = categories;
    }
}

• Category 实体类

package com.ifox.hgx.jpa.entity;


import java.util.HashSet;
import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Table(name="JPA_CATEGORIES")
@Entity
public class Category {

    private Integer id;
    private String categoryName;

    private Set<Item> items = new HashSet<>();

    @GeneratedValue
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name="CATEGORY_NAME")
    public String getCategoryName() {
        return categoryName;
    }

    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }

    @ManyToMany(mappedBy="categories")
    public Set<Item> getItems() {
        return items;
    }

    public void setItems(Set<Item> items) {
        this.items = items;
    }
}

• persistence.xml中加入:

<class>com.ifox.hgx.jpa.entity.Category</class>
<class>com.ifox.hgx.jpa.entity.Item</class>

• 测试示例:

package com.ifox.hgx.jpa.test;

import com.ifox.hgx.jpa.entity.Category;
import com.ifox.hgx.jpa.entity.Item;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

public class Test_TwoWayManyToMany {


    private EntityManagerFactory entityManagerFactory;
    private EntityManager entityManager;
    private EntityTransaction transaction;

    @Before
    public void init() {
        entityManagerFactory = Persistence.createEntityManagerFactory("jpa_m1");
        entityManager = entityManagerFactory.createEntityManager();
        transaction = entityManager.getTransaction();
        transaction.begin();
    }

    @After
    public void destroy() {
        transaction.commit();
        entityManager.close();
        entityManagerFactory.close();
    }

    //多对多的保存
    @Test
    public void testManyToManyPersist(){
        Item i1 = new Item();
        i1.setItemName("i-1");

        Item i2 = new Item();
        i2.setItemName("i-2");

        Category c1 = new Category();
        c1.setCategoryName("C-1");

        Category c2 = new Category();
        c2.setCategoryName("C-2");

        //设置关联关系
        i1.getCategories().add(c1);
        i1.getCategories().add(c2);

        i2.getCategories().add(c1);
        i2.getCategories().add(c2);

        c1.getItems().add(i1);
        c1.getItems().add(i2);

        c2.getItems().add(i1);
        c2.getItems().add(i2);

        //执行保存
        entityManager.persist(i1);
        entityManager.persist(i2);
        entityManager.persist(c1);
        entityManager.persist(c2);
    }


    //对于关联的集合对象, 默认使用懒加载的策略.
    //使用维护关联关系的一方获取, 还是使用不维护关联关系的一方获取, SQL 语句相同.
    @Test
    public void testManyToManyFind(){
//        Item item = entityManager.find(Item.class, 5);
//        System.out.println(item.getItemName());
//
//        System.out.println(item.getCategories().size());

        Category category = entityManager.find(Category.class, 7);
        System.out.println(category.getCategoryName());
        System.out.println(category.getItems().size());
    }
}

报错的话

尝试:

pom.xml

<!--&lt;!&ndash; https://mvnrepository.com/artifact/org.hibernate/hibernate-entitymanager &ndash;&gt;-->
        <!--<dependency>-->
            <!--<groupId>org.hibernate</groupId>-->
            <!--<artifactId>hibernate-entitymanager</artifactId>-->
            <!--<version>5.3.1.Final</version>-->
        <!--</dependency>-->

        <!-- https://mvnrepository.com/artifact/hibernate/hibernate-entitymanager -->
        <dependency>
            <groupId>hibernate</groupId>
            <artifactId>hibernate-entitymanager</artifactId>
            <version>3.4.0.GA</version>
            <type>pom</type>
        </dependency>