使用Jackson对JSON进行对象引用的数据结构序列化

Question

我想使用杰克逊2.13.3将数据结构序列化为JSON。我创建了一个简化的示例数据结构来显示所需的序列化是什么。

数据结构由一个主Container组成，其中包含一个Element的列表。元素之间有一些链接。

在示例中，我创建了元素的以下链接结构：

startTop <--> endTop
    ^          ^
    |          |
    |          |
    v          v
    startBottom     <-->     endBottom

目标是序列化通过链接信息的ID表示链接数据。元素的完全序列化应该只发生在容器的顶级列表中。这与jackson在序列化过程中遇到元素的顺序不一致。

我想得到以下输出:

{
    "allElements": [{
            "id": "startBottom",
            "successor": "endBottom",
            "predecessor": null,
            "upperNeighbours": ["startTop", "endTop"],
            "lowerNeighbours": null
        },
        {
            "id": "endBottom",
            "successor": null,
            "predecessor": "startBottom",
            "upperNeighbours": null,
            "lowerNeighbours": null

        },
        {
            "id": "startTop",
            "successor": "endTop",
            "predecessor": null,
            "upperNeighbours": null,
            "lowerNeighbours": ["startBottom"]
        },
        {
            "id": "endTop",
            "successor": null,
            "predecessor": "startTop",
            "upperNeighbours": null,
            "lowerNeighbours": ["startBottom"]
        }
    ]
}

import com.fasterxml.jackson.annotation.JsonIdentityInfo;
import com.fasterxml.jackson.annotation.ObjectIdGenerators;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ObjectWriter;
import java.util.List;

public class Test {
  public static void main(String[] args) throws JsonProcessingException {
    Element startBottom = new Element("startBottom");
    Element endBottom = new Element("endBottom");
    Element startTop = new Element("startTop");
    Element endTop = new Element("endTop");

    startBottom.setSuccessor(endBottom);
    startTop.setSuccessor(endTop);
    endBottom.setPredecessor(startBottom);
    endTop.setPredecessor(startTop);

    startBottom.setUpperNeighbours(List.of(startTop, endTop));
    startTop.setLowerNeighbours(List.of(startBottom));
    endTop.setLowerNeighbours(List.of(startBottom));

    Container container = new Container();
    container.setAllElements(List.of(startBottom, endBottom, startTop, endTop));

    ObjectMapper mapper =
        new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    ObjectWriter prettyPrintWriter = mapper.writerWithDefaultPrettyPrinter();
    System.out.println(prettyPrintWriter.writeValueAsString(container));
  }
}

class Container {
  public List<Element> getAllElements() {return allElements;}
  public void setAllElements(List<Element> allElements) {this.allElements = allElements;}
  private List<Element> allElements;
}

@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class,
    property = "id")
class Element {
  Element(String id) {this.id = id;}

  private String id;

  // May be null
  private Element successor;

  // May be null
  private Element predecessor;

  // May be empty, which for us is the same as being null
  private List<Element> upperNeighbours;

  // May be empty, which for us is the same as being null
  private List<Element> lowerNeighbours;

  public String getId() {return id;}

  public void setId(String id) {this.id = id;}

  public Element getSuccessor() {return successor;}

  public void setSuccessor(Element successor) {this.successor = successor;}

  public Element getPredecessor() {return predecessor;}

  public void setPredecessor(Element predecessor) {this.predecessor = predecessor;}

  public List<Element> getUpperNeighbours() {return upperNeighbours;}

  public void setUpperNeighbours(List<Element> upperNeighbours) {this.upperNeighbours = upperNeighbours;}

  public List<Element> getLowerNeighbours() {return lowerNeighbours;}

  public void setLowerNeighbours(List<Element> lowerNeighbours) {this.lowerNeighbours = lowerNeighbours;}
}

Answer 1

只需用批注< code>@JsonIgnore标记您的类< code >元素中的成员< code>successor和< code>predecessor，以防止无限循环。你的代码应该可以工作。但我甚至可以提出一些建议，让事情变得更简单。我编写了自己的实用程序，允许您将任何类序列化为Json，并从Json反序列化回类的实例。它使用引擎盖下的杰克逊图书馆。为您完成这项工作的代码很简单，如下所示:

try {
  System.out.println(JsonUtils.writeObjectToJsonString(container));
}catch(IOException ioe) {
  //Handle exception
}

其中容器是容器类的实例。下面是 Javadoc 的类 JsonUtils。这个类是我编写和维护的MgntUtils库的一部分。该库可以作为 Maven 工件或在 Github 上获得（包括源html" target="_blank">代码和 Javadoc）