JPA笔记3 OneToOne

package one_to_one;

import javax.persistence.Entity;

import javax.persistence.FetchType;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.JoinColumn;

import javax.persistence.OneToOne;

import javax.persistence.Table;

import com.sun.org.apache.xml.internal.resolver.readers.TR9401CatalogReader;

@Table(name = "JPA_DEPARTMENT")

@Entity

public class Department {

    private Integer id;

    private String name;

    private Manager manager;

    @GeneratedValue(strategy = GenerationType.AUTO)

    @Id

    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;

    }

    //注意：1-2关联关系，需要添加unique = true

    @JoinColumn(name = "mgr_id", unique = true)

    @OneToOne(fetch=FetchType.LAZY)

    public Manager getManager() {

        return manager;

    }

    public void setManager(Manager manager) {

        this.manager = manager;

    }

}

package one_to_one;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.OneToOne;

import javax.persistence.Table;

@Table(name = "JPA_MANAGER")

@Entity

public class Manager {

    private Integer id;

    private String mgrName;

    private Department department;

    @GeneratedValue(strategy = GenerationType.AUTO)

    @Id

    public Integer getId() {

        return id;

    }

    public void setId(Integer id) {

        this.id = id;

    }

    public String getMgrName() {

        return mgrName;

    }

    public void setMgrName(String mgrName) {

        this.mgrName = mgrName;

    }

    @OneToOne(mappedBy = "manager")

    public Department getDepartment() {

        return department;

    }

    public void setDepartment(Department department) {

        this.department = department;

    }

}

package one_to_one;

import javax.persistence.EntityManager;

import javax.persistence.EntityManagerFactory;

import javax.persistence.EntityTransaction;

import javax.persistence.Persistence;

public class Main3 {

    public static void main(String[] args) {

        // TODO Auto-generated method stub

        String persistenceUnitName = "jpa-1";

        EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory(persistenceUnitName);

        EntityManager entityManager = entityManagerFactory.createEntityManager();

        EntityTransaction transaction = entityManager.getTransaction();

        transaction.begin();

        // oneToOnePersist(entityManager);

        oneToOneFind(entityManager);

        transaction.commit();

        entityManager.close();

        entityManagerFactory.close();

    }

    // 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

    private static void oneToOnePersist(EntityManager entityManager) {

        Manager manager = new Manager();

        manager.setMgrName("M-AA");

        Department department = new Department();

        department.setName("D-AA");

        manager.setDepartment(department);

        department.setManager(manager);

        entityManager.persist(manager);

        entityManager.persist(department);

    }

    // 默认通过左连接加载对象

    // 如果通过设置@OneToOne的fetch属性为lazy，还是会用左连接，且会多查询

    private static void oneToOneFind(EntityManager entityManager) {

        Department dept = entityManager.find(Department.class, 1);

        System.out.println(dept);

        System.out.println(dept.getManager().getMgrName());

    }

}