使用Jackson

Question

我是Spring Boot，Jackson和JPA的新手。我正在尝试构建一个RESTful应用程序。根据我对Spring Boot文档的理解，反序列化数据和返回JSON的大部分工作都是＃34;神奇地＆＃34;由Spring框架处理（当包含适当的启动器/依赖项时）。

我的控制器端点很简单。

import java.util.List;
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.midamcorp.roominspections.models.*;
import com.midamcorp.roominspections.service.ItemService;

 @RestController
@RequestMapping("/item")
public class ItemController {
    @Autowired
    private ItemService itemService;

@RequestMapping(method= RequestMethod.GET)
public List<Item> getItems() {
    return itemService.findAll();
}

@RequestMapping(method=RequestMethod.GET, value="/{id}")
public Item getItem(@PathVariable int id) {
    return itemService.findOne(id);
}

@RequestMapping(method=RequestMethod.GET,value="/page/{pageNumber}")
public List<Item> getItemsByPage(@PathVariable int pageNumber) {
    return itemService.findByPage(pageNumber);
}
}

但是，我的类（JPA实体）表现出一些关系。例如：

import java.util.Set;
import javax.persistence.*;

       @Entity
@Table(name="Items")
public class Item {

    public Item() {}
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int ItemID;

@ManyToOne(optional=false)
@JoinColumn(name="CommentCategoryID", insertable = false, updatable = false)
private CommentCategory commentCategory;

@ManyToOne(optional=false)
@JoinColumn(name="ItemCategoryID", insertable = false, updatable = false)
private ItemCategory itemCategory; 

@OneToMany(mappedBy="item",targetEntity=ReportDetail.class)
private Set<ReportDetail> detail;

private String ItemName;
private String ItemDescription;
private String ItemType;
@Column(name="PageNumber")
private int pageNumber;
private int PossPoints;
private int CommentCategoryID;
private int ItemCategoryID;

// getters and setters follow
}

每当我尝试访问项目端点时，都会收到以下错误：

 Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: could not deserialize; nested exception is com.fasterxml.jackson.databind.JsonMappingException: could not deserialize (through reference chain: java.util.ArrayList[0]->com.midamcorp.roominspections.models.Item["commentCategory"]->com.midamcorp.roominspections.models.CommentCategory["comments"]->org.hibernate.collection.internal.PersistentSet[0]->com.midamcorp.roominspections.models.Comment["reportDetails"])

现在我明白问题在于反序列化数据（通过杰克逊），问题＆＃34;结束＆＃34;使用Comment实体的reportDetails属性。但是，我已经检查了Comment和ReportDetails实体，并且没有发现任何错误。

注释

import java.util.Set;
import javax.persistence.*;

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

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int CommentID;

public Comment() {}

@OneToMany(mappedBy="comment", targetEntity=ReportDetail.class)
private Set<ReportDetail> reportDetails;

@ManyToOne(optional=false)
@JoinColumn(name="CommentCategoryID", insertable = false, updatable = false)
private CommentCategory commentCategory;

private int CommentCategoryID;
private String CommentBody;

// getters and setters
}

ReportDetail

import javax.persistence.*;

    @Entity
@Table(name="ReportDetailRows")

public class ReportDetail {

    public ReportDetail() {}

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int ReportRowID;
private int ReportID;
private int ItemID;
private int CommentID;
private int PointsDeducted;
private String ItemIMG;

@ManyToOne(optional=false)
@JoinColumn(name="CommentID", insertable = false, updatable = false)
private Comment comment;

@ManyToOne(optional=false)
@JoinColumn(name="ReportID", insertable = false, updatable = false)
private ReportSummary summary;

@ManyToOne(optional=false)
@JoinColumn(name="ItemID", insertable = false, updatable = false)
private Item item;

// getters and setters
}

使用存储库接口（扩展CrudRepository）和服务层。

以下是一个示例服务：

import java.util.List;
import javax.transaction.Transactional;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import com.midamcorp.roominspections.models.Item;
import com.midamcorp.roominspections.models.ItemRepo;

   @Service
    @Transactional
    public class ItemServiceImpl implements ItemService {
    @Autowired
    private ItemRepo itemRepo;

    @Override
    public List<Item> findAll() {
    return (List<Item>) itemRepo.findAll();
    }

    @Override
    public List<Item> findByPage(int pageNumber) {
    return itemRepo.findByPageNumber(pageNumber);
    }

    @Override
    public Item findOne(int id) {
    return itemRepo.findOne(id);
    }

}

根据我使用.NET的经验，我必须创建DTO（使用诸如AutoMapper之类的库）来展平实体以进行传输。但是，根据我对Jackson文档的回顾，似乎图书馆自动处理这样的扁平化（注意，我发现传输如此大量的数据并不是一个好习惯;因为我是技术新手，我只是想要让它先工作）。如果需要DTO，它们将位于何处？例如，在Controller，Service，Repository等中？

我真的很感激任何建议。我一直在搜索，但一直无法为我的特定问题找到解决方案。