跳至主要内容

JPA 2.1: Entity Graph


JPA 2.1: Entity Graph

Entity Graph is a means to specify the structure of a graph of entities, it defines the return path and boundaries when the entity is loaded.

Define Entity Graph

You can define an Entity graph via @NamedEntityGraph annotation.
@NamedEntityGraph(
        name = "post",
        attributeNodes = {
            @NamedAttributeNode("title"),
            @NamedAttributeNode(value = "comments", subgraph = "comments")
        },
        subgraphs = {
            @NamedSubgraph(
                    name = "comments",
                    attributeNodes = {
                        @NamedAttributeNode("content")}
            )
        }
)
or create a EntityGraph dynamically via createEntitGraph method of EntityManager.
EntityGraph postEntityGraph=em.createEntityGraph(Post.class);
postEntityGraph.addAttributeNodes("title");
postEntityGraph.addSubgraph("comments").addAttributeNodes("content");

Apply the Entity Graph

Get the EntityGraph.
EntityGraph postGraph=em.getEntityGraph("post");
Set the value of javax.persistence.fetchgraph
em.createQuery("select p from Post p where p.id=:id", Post.class)
  .setHint("javax.persistence.fetchgraph", postGraph)
  .setParameter("id", this.id)
  .getResultList()
  .get(0);
There are two hints available in JPA 2.1 for configuring the Entity Graph loading strategy.
javax.persistence.fetchgraph will load all attributes defined in the EntityGraph, and all unlisted attributes will use LAZY to load.
javax.persistence.loadgraph will load all attributes defined in the EntityGraph, and all unlisted attributes will apply it's fetch settings.

What problem it resolved

If you are using Hibernate, you have to encounter the famous LazyInitializedExcpetion when you try to fetch values from association attributes of an entity outside of an active Session.
There are some existed solutions to resolve this problem. In a web application, most of case, this exception could be thrown in the view layer, one solution is introduce Open Session in View pattern, in Spring application, an OpenInView AOP Interceptor(for none web application) and an OpenInView web Filter(for web application) are provided.
Now, the Entity Graph could be another robust solution for resolving this issue. For those cases which require to hint the lazy association of an Entity, create a Entity Graph to load them when the query is executed.
Some JPA providers, such as OpenJPA, provide a Fetch Plan feature, which is similar with the Entity Graph.

Summary

The Entity Graph overrides the default loading strategy, and provides flexibility of loading the association attributes of an Entity.
The sample codes are hosted on my github.com account, check out and play it yourself.
标签：

评论

发表评论

此博客中的热门博文

AngularJS CakePHP Sample codes

Introduction This sample is a Blog application which has the same features with the official CakePHP Blog tutorial, the difference is AngularJS was used as frontend solution, and CakePHP was only use for building backend RESR API. Technologies AngularJS   is a popular JS framework in these days, brought by Google. In this example application, AngularJS and Bootstrap are used to implement the frontend pages. CakePHP   is one of the most popular PHP frameworks in the world. CakePHP is used as the backend REST API producer. MySQL   is used as the database in this sample application. A PHP runtime environment is also required, I was using   WAMP   under Windows system. Post links I assume you have some experience of PHP and CakePHP before, and know well about Apache server. Else you could read the official PHP introduction( php.net ) and browse the official CakePHP Blog tutorial to have basic knowledge about CakePHP. In these posts, I tried to follow the steps describ
2 条评论
阅读全文

JPA 2.1: Attribute Converter

JPA 2.1: Attribute Converter If you are using Hibernate, and want a customized type is supported in your Entity class, you could have to write a custom Hibernate Type. JPA 2.1 brings a new feature named attribute converter, which can help you convert your custom class type to JPA supported type. Create an Entity Reuse the   Post   entity class as example. @Entity @Table(name="POSTS") public class Post implements Serializable { private static final long serialVersionUID = 1L; @Id @GeneratedValue(strategy = GenerationType.AUTO) @Column(name="ID") private Long id; @Column(name="TITLE") private String title; @Column(name="BODY") private String body; @Temporal(javax.persistence.TemporalType.DATE) @Column(name="CREATED") private Date created; @Column(name="TAGS") private List<String> tags=new ArrayList<>(); } Create an attribute convert
发表评论
阅读全文

Auditing with Hibernate Envers

Auditing with Hibernate Envers The approaches provided in JPA lifecyle hook and Spring Data auditing only track the creation and last modification info of an Entity, but all the modification history are not tracked. Hibernate Envers fills the blank table. Since Hibernate 3.5, Envers is part of Hibernate core project. Configuration Configure Hibernate Envers in your project is very simple, just need to add   hibernate-envers   as project dependency. <dependency> <groupId>org.hibernate</groupId> <artifactId>hibernate-envers</artifactId> </dependency> Done. No need extra Event listeners configuration as the early version. Basic Usage Hibernate Envers provides a simple   @Audited   annotation, you can place it on an Entity class or property of an Entity. @Audited private String description; If   @Audited   annotation is placed on a property, this property can be tracked. @Entity @Audited public class Signup implements Serializa
发表评论
阅读全文