SpringBoot结合SpringData Jpa--注解部分（三）

SpringBoot结合SpringData Jpa–注解部分（三）

承接上篇博客：https://blog.youkuaiyun.com/qq_33608923/article/details/102792383
上篇主要讲了@ManyToMany和@OneToOne两个注解，遗留了一个问题，就是在实体关系为一对多或者多对多的时候，hibernate会先执行查询主表信息的一条sql，再根据主表外键查询副表的信息，这样就会执行1 + n条sql。
本篇来解决这个问题，Spring Data JPA引入了@NameEntityGraph注解：

import javax.persistence.*;
import java.util.List;

/**
*Permission
*author BenJerry
*version 1.0
*/
@NamedEntityGraph(name = "Permission.roles", attributeNodes = {
        @NamedAttributeNode("roles")
})
@Table(name = "t_permission")
@Entity
public class Permission {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String permission;

    @ManyToMany
    @JoinTable(name = "t_role_permission", joinColumns = @JoinColumn(name = "pid", referencedColumnName = "id"),
        inverseJoinColumns = @JoinColumn(name = "rid", referencedColumnName = "id"))
    private List<Role> roles;

    public Long getId() {
        return id;
    }

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

    public String getPermission() {
        return permission;
    }

    public void setPermission(String permission) {
        this.permission = permission;
    }

    public List<Role> getRoles() {
        return roles;
    }

    public void setRoles(List<Role> roles) {
        this.roles = roles;
    }
}

在PermissionRepository中重写findAll方法：

import com.github.jpa.demo.entity.Permission;
import org.springframework.data.jpa.repository.EntityGraph;
import org.springframework.data.jpa.repository.JpaRepository;

import java.util.List;

/**
 * PermissionRepository
 * author BenJerry
 * version 1.0
 */
public interface PermissionRepository extends JpaRepository<Permission, Long> {

    @Override
    @EntityGraph(value = "Permission.roles")
    List<Permission> findAll();

}

实际执行的sql：

Hibernate: select permission0_.id as id1_1_0_, role2_.id as id1_2_1_, permission0_.permission as permissi2_1_0_, role2_.pid as pid2_2_1_, role2_.role as role3_2_1_, roles1_.pid as pid1_3_0__, roles1_.rid as rid2_3_0__ from t_permission permission0_ left outer join t_role_permission roles1_ on permission0_.id=roles1_.pid left outer join t_role role2_ on roles1_.rid=role2_.id

可以看到jpa一条sql就搞定了，使用的是left outer join，其中 @NamedAttributeNode这个注解可以写多个用以加入更多的表。