JPA(Hibernate) @OneToMany 两种例子

环境：Spring Data Jpa，hibernate或者其他jpa实现也是一样的；Spring Boot

场景：User和Role，一个User要对应多个Role。

第一种方式，没有中间关系表，直接在role表中添加一个user_id字段

User：

import javax.persistence.*;
import javax.validation.constraints.NotNull;
import java.util.HashSet;
import java.util.Set;

/**
 * Created by zhangpeng on 16-6-15.
 */
@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    // The user email
    @NotNull
    private String email;
    // The user name
    @NotNull
    private String name;

    private String password;

    @OneToMany(mappedBy = "user", cascade = {CascadeType.ALL}, fetch = FetchType.EAGER)
    private Set<Role> roles = new HashSet<>();

   //add getter and setter
}

Role：

import javax.persistence.*;

/**
 * Created by zhangpeng on 16-6-17.
 */
@Entity
@Table(name = "roles")
public class Role {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    Long id;
    String roleName;
    @ManyToOne
    @JoinColumn(name = "user_id")
    User user;

   //add getter and setter

}

需要注意User类中mappedBy = "user"，这个user就是Role中所持有的User对象的名字。Role中@JoinColumn(name = "user_id") 是指定role表中user标识符的字段名。cascade = {CascadeType.ALL}是用来设定级联操作的，这里开启了所有的级联操作。fetch = FetchType.EAGER是用来设置加载类型的。默认是懒加载，如果是懒加载，那就意味着User里的roles在你查询出来这个user时不同时查询出来，而是等访问user里的roles对象时才进行加载。我这里设置的是查询user时就把roles加载过来。

第二种，个人感觉更好一点的，user和role都不持有对方的引用，而是生成中间表：

User：

import javax.persistence.*;
import javax.validation.constraints.NotNull;
import java.util.HashSet;
import java.util.Set;

/**
 * Created by zhangpeng on 16-6-15.
 */
@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    // The user email
    @NotNull
    private String email;
    // The user name
    @NotNull
    private String name;

    private String password;

//    private String role;

    @OneToMany( cascade = {CascadeType.ALL}, fetch = FetchType.EAGER)
    private Set<Role> roles = new HashSet<>();

//add getter and setter
}

Role：

import javax.persistence.*;

/**
 * Created by zhangpeng on 16-6-17.
 */
@Entity
@Table(name = "roles")
public class Role {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    Long id;
    String roleName;
//    @ManyToOne
//    @JoinColumn(name = "user_id")
//    User user;

//    public User getUser() {
//        return user;
//    }
//
//    public void setUser(User user) {
//        this.user = user;
//    }

   //add getter and setter
}

这样就可以了。

测试：

import com.guduo.fenghui.dao.RoleDao;
import com.guduo.fenghui.dao.UserDao;
import com.guduo.fenghui.entity.Role;
import com.guduo.fenghui.entity.User;
import org.junit.Assert;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.SpringApplicationConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
import org.springframework.test.context.web.WebAppConfiguration;

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = Application.class)
@WebAppConfiguration
public class Test {

    @Autowired
    UserDao userDao;

    @Autowired
    RoleDao roleDao;

    @Test
    public void test() {
        User user = new User();
        user.setEmail("csonezp@gmail.com");
        user.setName("zhangpeng");
        user.setPassword("123456");
        for (int i = 0; i <= 2; i++) {
            Role role = new Role();
            role.setRoleName(i + "j");
//如果选择的是第一种方式，那要加上这一句。因为第一种方式配置的有mappedby，控制权是在role这边
//            role.setUser(user);
            user.getRoles().add(role);

        }
        userDao.save(user);

        user = userDao.findByName("zhangpeng");

        Assert.assertEquals(user.getRoles().size(), 3);
        user.getRoles().get(0).setRoleName("asdasd");
        userDao.save(user);

        user = userDao.findByName("zhangpeng");

        Assert.assertEquals("asdasd", user.getRoles().get(0).getRoleName());

        userDao.delete(user.getId());
    }

}

这一段测试代码中，体现了级联插入，级联查询，级联更新，级联删除。