Json和Java - 循环参考 [英] Json and Java - Circular Reference

问题描述

我正在使用并发布循环引用。

I'm having and issue with the Circular reference.

我有Rest Webservices将对象返回到前端，问题是当我尝试返回对象时有几个引用，结果我得到无限响应，生成

I have Rest Webservices which returns objects to the front end, the issue is when I try to return objects that have several references so as the result I get an infinite response, which generate

java.lang.IllegalStateException: 
    Cannot call sendError() after the response has been committed

对象由Hibernate代码生成自动生成我需要在后端使用循环引用，我只需要在使用Jackson将信息发送到前端之前将其删除。

The objects are generated automatically by Hibernate Code Generation and I need to have the circular reference in the backend, I've just need to remove it before send the information to the frontend using Jackson.

控制器方法标题是：

@RequestMapping(value="/list", method=RequestMethod.POST)
public @ResponseBody eventResponse list(@RequestBody String sessionID) {

我没有做任何明确的转换为Json，我是这是一个新手，我认为那个插孔自动解决这个问题。

I'm not doing anything explicite to convert to Json, I'm a newby with this and I think that jackson resolved this automatically.

推荐答案

有两种方法可以解决这个问题。如果您必须将您的实体暴露给外界，我建议添加 @JsonIgnore 导致循环引用的属性。这将告诉杰克逊不要序列化该属性。

There are two ways you can go about this. If you must expose your entity to the outside world, I recommend adding @JsonIgnore on the property that is causing the circular reference. This will tell Jackson not to serialize that property.

另一种方法是使用Jackson提供的双向功能。您可以使用 @ JsonManagedReference 或 @JsonBackReference 。 @JsonManagedReference 是属性的前进部分，它将正常序列化。 @JsonBackReference 是引用的后退部分;它不会被序列化，但会在反向类型被反序列化时重建。

Another way is to use the bidirectional features provided by Jackson. You can either use @JsonManagedReference or @JsonBackReference. @JsonManagedReference is the "forward" part of the property and it will get serialized normally. @JsonBackReference is the "back" part of the reference; it will not be serialized, but will be reconstructed when the "forward" type is deserialized.

你可以查看示例这里。

这解决了你的评论：我想你在这种情况下可能想做什么是使用外部世界可见的DTO。我喜欢这种方法，因为我不想将我的实体暴露在外面。这意味着杰克逊注释将在DTO上，而不是在实体上。您需要某种将实体转换为DTO的映射器或转换器。现在，当您对实体进行更改时，除非您修改映射器/转换器，否则它们不会传播到DTO。我认为这没关系，因为当您对实体进行更改时，您可以决定是否要公开更改。

This addresses your comment: I think what you might want to do in this case is use a DTO that is visible to the outside world. I like this approach because I don't want to expose my entities to the outside. This means that the Jackson annotations would be on the DTO and not on the enity. You would need some sort of mapper or converter that converts the entity to the DTO. Now when you make changes to your entity, they won't get propagated to the DTO unless you modify your mapper/converter. I think this is ok, because when you make a change to your entity you can decide if you want that change to be exposed or not.

更新

有一篇好文章这里详细介绍了在Jackson中处理双向关系的各种方法。它描述了使用 @JsonIgnore ， @JsonManagedReference 和 @JsonBackReference ， @JsonIdentityInfo ， @JsonView 以及自定义序列化程序。这是你可以使用的各种技术的全面综述。

There is a good blog post here that goes into detail about the various ways you can handle bidirectional relationships in Jackson. It describes solutions that use @JsonIgnore, @JsonManagedReference and @JsonBackReference, @JsonIdentityInfo, @JsonView and a custom serializer as well. It's a pretty comprehensive writeup of the various techniques that you can use.

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