java - Spring Boot REST Controller 问题

Question

我在使用 Rest Controller 时遇到了一个非常奇怪的问题。我有一个非常基本的休息 Controller 。

package com.therealdanvega.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

import com.therealdanvega.domain.Post;
import com.therealdanvega.service.PostService;

@RestController
public class PostController {

    private PostService postService;

    @Autowired
    public PostController(PostService postService){
        this.postService = postService;
    }

    @RequestMapping("posts/test")
    public String test(){
        return "test...";
    }

    @RequestMapping( name="/posts/", method=RequestMethod.GET )
    public Iterable<Post> list(){
        return postService.list();
    }


}

调用服务

package com.therealdanvega.service;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import com.therealdanvega.domain.Post;
import com.therealdanvega.repository.PostRepository;

@Service
public class PostService {

    private PostRepository postRepository;

    @Autowired
    public PostService(PostRepository postRepository){
        this.postRepository = postRepository;
    }

    public Iterable<Post> list(){
        return postRepository.findAll();
    }

}

调用存储库来获取数据。

package com.therealdanvega.repository;

import java.util.List;

import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;

import com.therealdanvega.domain.Post;

@Repository
public interface PostRepository extends CrudRepository<Post, Long> {

    Post findFirstByOrderByPostedOnDesc();

    List<Post> findAllByOrderByPostedOnDesc();

    Post findBySlug(String slug);

}

我正在内存数据库中使用 H2，并且其中只有一条 Post 记录，可以通过转到 H2 控制台并再次运行选择 Post 表来确认这一点。

如果我访问/test URL，我会得到我所期望的结果，即打印到浏览器的字符串“test...”。如果我尝试列出所有帖子(同样只有 1 个帖子)，浏览器就会开始一遍又一遍地循环，并继续多次打印代表第 1 篇帖子的 JSON，以至于应用程序崩溃，我在控制台中看到了这一点

2015-11-07 17:58:42.959 ERROR 5546 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet dispatcherServlet threw exception java.lang.IllegalStateException: getOutputStream() has already been called for this response

这就是我访问/posts 时浏览器的样子，它应该只列出 1

Post Domain Class

package com.therealdanvega.domain;

import java.util.Date;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

import org.springframework.data.annotation.CreatedDate;


@Entity
public class Post {

    @Id @GeneratedValue
    private Long id;
    private String title;

    @Column(columnDefinition = "TEXT")
    private String body;

    @Column(columnDefinition = "TEXT")
    private String teaser;

    private String slug;

    @CreatedDate 
    @Temporal(TemporalType.TIMESTAMP)
    private Date postedOn;

    @ManyToOne
    private Author author;

    @SuppressWarnings("unused")
    private Post(){
    }

    public Post(String title){
        this.setTitle(title);
    }

    // getters & setters
}

有谁知道我在这里做错或错过了什么？为什么不只显示 JSON 格式的 1 条记录？

Answer 1

您的 Post 对象似乎有循环引用。 Post 对象中的 Author 对象有一个 Posts 对象列表等。尝试将 @JsonIgnore 注释放在帖子对象的作者属性上。

您还可以使用@JsonBackReference和@JsonManagedReference来解决问题。

来自Jackson documentation :

Object references, identity

@JsonManagedReference, @JsonBackReference: pair of annotations used toindicate and handle parent/child relationships expressed with pair ofmatching properties @JsonIdentityInfo: class/property annotation usedto indicate that Object Identity is to be used whenserializing/deserializing values, such that multiple references to asingle Java Object can be properly deserialized. This can be used toproperly deal with cyclic object graphs and directed-acyclic graphs.