假设我们的应用程序使用以下实体:Post
、PostComment
、PostDetails
和Tag
,它们形成了一对多、一对一和多对多表关系:
![JPA Criteria Metamodel](https://istack.dev59.com/JGxfQ.webp)
如何生成JPA Criteria元模型
Hibernate ORM提供的hibernate-jpamodelgen
工具可用于扫描项目实体并生成JPA Criteria元模型。只需在Maven pom.xml
配置文件中的maven-compiler-plugin
中添加以下annotationProcessorPath
即可:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>${maven-compiler-plugin.version}</version>
<configuration>
<annotationProcessorPaths>
<annotationProcessorPath>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-jpamodelgen</artifactId>
<version>${hibernate.version}</version>
</annotationProcessorPath>
</annotationProcessorPaths>
</configuration>
</plugin>
现在,当项目被编译时,您可以看到在target
文件夹中生成了以下Java类:
> tree target/generated-sources/
target/generated-sources/
└── annotations
└── com
└── vladmihalcea
└── book
└── hpjp
└── hibernate
├── forum
│ ├── PostComment_.java
│ ├── PostDetails_.java
│ ├── Post_.java
│ └── Tag_.java
标签实体元模型
如果要将Tag
实体映射为以下内容:
@Entity
@Table(name = "tag")
public class Tag {
@Id
private Long id;
private String name;
}
Tag_
元模型类是这样生成的:
@Generated(value = "org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor")
@StaticMetamodel(Tag.class)
public abstract class Tag_ {
public static volatile SingularAttribute<Tag, String> name;
public static volatile SingularAttribute<Tag, Long> id;
public static final String NAME = "name";
public static final String ID = "id";
}
SingularAttribute
用于基本的id
和name
Tag
JPA实体属性。
文章实体元模型
Post
实体的映射如下:
@Entity
@Table(name = "post")
public class Post {
@Id
private Long id;
private String title;
@OneToMany(
mappedBy = "post",
cascade = CascadeType.ALL,
orphanRemoval = true
)
private List<PostComment> comments = new ArrayList<>();
@OneToOne(
mappedBy = "post",
cascade = CascadeType.ALL,
fetch = FetchType.LAZY
)
@LazyToOne(LazyToOneOption.NO_PROXY)
private PostDetails details;
@ManyToMany
@JoinTable(
name = "post_tag",
joinColumns = @JoinColumn(name = "post_id"),
inverseJoinColumns = @JoinColumn(name = "tag_id")
)
private List<Tag> tags = new ArrayList<>();
}
Post
实体具有两个基本属性id
和title
,一个一对多的comments
集合,一个一对一的details
关联,以及一个多对多的tags
集合。
Post_
元模型类生成如下:
@Generated(value = "org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor")
@StaticMetamodel(Post.class)
public abstract class Post_ {
public static volatile ListAttribute<Post, PostComment> comments;
public static volatile SingularAttribute<Post, PostDetails> details;
public static volatile SingularAttribute<Post, Long> id;
public static volatile SingularAttribute<Post, String> title;
public static volatile ListAttribute<Post, Tag> tags;
public static final String COMMENTS = "comments";
public static final String DETAILS = "details";
public static final String ID = "id";
public static final String TITLE = "title";
public static final String TAGS = "tags";
}
SingularAttribute
表示基本的
id
和
title
属性以及一对一的
details
关联,而JPA
ListAttribute
表示
comments
和
tags
集合。
PostDetails
实体模型如下:
@Entity
@Table(name = "post_details")
public class PostDetails {
@Id
@GeneratedValue
private Long id;
@Column(name = "created_on")
private Date createdOn;
@Column(name = "created_by")
private String createdBy;
@OneToOne(fetch = FetchType.LAZY)
@MapsId
@JoinColumn(name = "id")
private Post post;
}
所有实体属性都将由关联的 PostDetails_
元模型类中的 JPA SingularAttribute
表示:
@Generated(value = "org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor")
@StaticMetamodel(PostDetails.class)
public abstract class PostDetails_ {
public static volatile SingularAttribute<PostDetails, Post> post;
public static volatile SingularAttribute<PostDetails, String> createdBy;
public static volatile SingularAttribute<PostDetails, Long> id;
public static volatile SingularAttribute<PostDetails, Date> createdOn;
public static final String POST = "post";
public static final String CREATED_BY = "createdBy";
public static final String ID = "id";
public static final String CREATED_ON = "createdOn";
}
PostComment实体元模型
PostComment
的映射如下所示:
@Entity
@Table(name = "post_comment")
public class PostComment {
@Id
private Long id;
@ManyToOne(fetch = FetchType.LAZY)
private Post post;
private String review;
}
此外,所有实体属性都由关联的PostComments_
元模型类中的JPA SingularAttribute
表示:
@Generated(value = "org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor")
@StaticMetamodel(PostComment.class)
public abstract class PostComment_ {
public static volatile SingularAttribute<PostComment, Post> post;
public static volatile SingularAttribute<PostComment, String> review;
public static volatile SingularAttribute<PostComment, Long> id;
public static final String POST = "post";
public static final String REVIEW = "review";
public static final String ID = "id";
}
使用JPA Criteria元模型
如果没有JPA元模型,需要通过标准API查询PostComment
实体并根据关联的Post
标题进行过滤,代码如下:
CriteriaBuilder builder = entityManager.getCriteriaBuilder()
CriteriaQuery<PostComment> query = builder.createQuery(PostComment.class)
Root<PostComment> postComment = query.from(PostComment.class)
Join<PostComment, Post> post = postComment.join("post")
query.where(
builder.equal(
post.get("title"),
"High-Performance Java Persistence"
)
)
List<PostComment> comments = entityManager
.createQuery(query)
.getResultList()
注意我们创建Join
实例时使用了post
字符串,引用Post
title
时使用了title
字符串。
JPA元模型允许我们避免硬编码实体属性,如下面的示例所示:
CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<PostComment> query = builder.createQuery(PostComment.class);
Root<PostComment> postComment = query.from(PostComment.class);
Join<PostComment, Post> post = postComment.join(PostComment_.post);
query.where(
builder.equal(
post.get(Post_.title),
"High-Performance Java Persistence"
)
);
List<PostComment> comments = entityManager
.createQuery(query)
.getResultList();
或者,假设我们想在过滤Post
的title
和PostDetails
的createdOn
属性时获取DTO投影。
我们可以在创建联接属性时使用元模型,以及在构建DTO投影列别名或引用需要筛选的实体属性时使用元模型:
CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<Object[]> query = builder.createQuery(Object[].class);
Root<PostComment> postComment = query.from(PostComment.class);
Join<PostComment, Post> post = postComment.join(PostComment_.post);
query.multiselect(
postComment.get(PostComment_.id).alias(PostComment_.ID),
postComment.get(PostComment_.review).alias(PostComment_.REVIEW),
post.get(Post_.title).alias(Post_.TITLE)
);
query.where(
builder.and(
builder.like(
post.get(Post_.title),
"%Java Persistence%"
),
builder.equal(
post.get(Post_.details).get(PostDetails_.CREATED_BY),
"Vlad Mihalcea"
)
)
);
List<PostCommentSummary> comments = entityManager
.createQuery(query)
.unwrap(Query.class)
.setResultTransformer(Transformers.aliasToBean(PostCommentSummary.class))
.getResultList();
很酷,对吧?