Java 如何通过在Hibernate中引入二级缓存来解决N+1问题？

在Hibernate文档中指出：

对于N+1选择的问题，一种完全不同的方法是使用 二级缓存

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我不明白它如何解决这个问题。什么是真实世界的例子和解释？

很简单。假设您拥有以下域模型：

@Entity(name = "Post")
public class Post {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String name;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "post")
    private List<Comment> comments = new ArrayList<>();

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<Comment> getComments() {
        return comments;
    }

    public void addComment(Comment comment) {
        comments.add(comment);
        comment.setPost(this);
    }
}

@Entity(name = "Comment")
public class Comment {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @ManyToOne
    private Post post;

    public Comment() {
    }

    public Comment(String review) {
        this.review = review;
    }

    private String review;

    public Long getId() {
        return id;
    }

    public Post getPost() {
        return post;
    }

    public void setPost(Post post) {
        this.post = post;
    }

    public void setReview(String review) {
        this.review = review;
    }
}

然后Hibernate首先进入第二级缓存以加载实体，并且只有在没有找到缓存项时才会访问数据库

更简单的解决方案是：

这样您就不会遇到N+1查询问题，也不需要二级缓存，当数据库在HibernateAPI之外更新时，二级缓存容易出现不一致

List<Comment> comments = session.createQuery(
    "select c from Comment c ").list();
for(Comment comment : comments) {
    Post post = comment.getPost();
}

@Entity(name = "Post")
@Cacheable
@org.hibernate.annotations.Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class Post {
    ...
}

List<Comment> comments = session.createQuery(
    "select c from Comment c fetch c.post ").list();