使用杰克逊序列化递归对象达一定深度

Question

我有一个树对象结构，需要进行序列化，并且我希望每次能够使用jackson（或任何其他库-我打开）基于参数来控制序列化的深度。

我的课是这样的：

class Node {
    ...
    private String id;
    private Node child;
    ...
}

这是我想根据深度级别获得的2个序列化json的示例

深度级别设置为3

{
  "id": "A",
  "child": {
    "id": "B",
    "child": {
      "id": "C",
      "child": {}
    }
  }
}

深度级别设置为2

{
  "id": "A",
  "child": {
    "id": "B",
    "child": {}
  }
}

有什么方法可以控制递归对象中序列化的深度？

谢谢

Answer 1

您需要实现自定义序列化程序，在其中需要计算已经处理的Node对象的数量。对于每个序列化过程，我们需要在max depth每次Node发现类时提供值并递减值。示例实现：

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ObjectWriter;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.json.JsonMapper;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.ser.BeanSerializer;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;
import com.fasterxml.jackson.databind.ser.std.BeanSerializerBase;

import java.io.IOException;
import java.util.concurrent.atomic.AtomicInteger;

public class JsonApp {

    public static void main(String[] args) throws IOException {
        Node nodeD = new Node("D", null);
        Node nodeC = new Node("C", nodeD);
        Node nodeB = new Node("B", nodeC);
        Node nodeA = new Node("A", nodeB);

        for (int i = 0; i < 4; i++) {
            System.out.println("Depth: " + i);
            System.out.println(serialiseWithDepth(nodeA, i));
        }
    }

    private static ObjectMapper mapper = JsonMapper.builder()
            .enable(SerializationFeature.INDENT_OUTPUT)
            .addModule(createNodeModule())
            .build();

    private static String serialiseWithDepth(Node node, int maxDepth) throws JsonProcessingException {
        ObjectWriter writer = mapper.writerFor(Node.class)
                .withAttribute(NodeDepthBeanSerializer.DEPTH_KEY, new AtomicInteger(maxDepth));

        return writer.writeValueAsString(node);
    }

    private static SimpleModule createNodeModule() {
        SimpleModule nodeModule = new SimpleModule("NodeModule");
        nodeModule.setSerializerModifier(new BeanSerializerModifier() {
            @Override
            public JsonSerializer<?> modifySerializer(SerializationConfig config, BeanDescription beanDesc, JsonSerializer<?> serializer) {
                if (beanDesc.getBeanClass() == Node.class) {
                    return new NodeDepthBeanSerializer((BeanSerializerBase) serializer);
                }
                return super.modifySerializer(config, beanDesc, serializer);
            }
        });
        return nodeModule;
    }
}

class NodeDepthBeanSerializer extends BeanSerializer {

    public static final String DEPTH_KEY = "maxDepthSize";

    public NodeDepthBeanSerializer(BeanSerializerBase src) {
        super(src);
    }

    @Override
    protected void serializeFields(Object bean, JsonGenerator gen, SerializerProvider provider) throws IOException {
        AtomicInteger depth = (AtomicInteger) provider.getAttribute(DEPTH_KEY);
        if (depth.decrementAndGet() >= 0) {
            super.serializeFields(bean, gen, provider);
        }
    }
}

上面的代码打印：

Depth: 0
Node{id='A', child=Node{id='B', child=Node{id='C', child=Node{id='D', child=null}}}} => 0
{ }
Depth: 1
Node{id='A', child=Node{id='B', child=Node{id='C', child=Node{id='D', child=null}}}} => 1
Node{id='B', child=Node{id='C', child=Node{id='D', child=null}}} => 0
{
  "id" : "A",
  "child" : { }
}
Depth: 2
Node{id='A', child=Node{id='B', child=Node{id='C', child=Node{id='D', child=null}}}} => 2
Node{id='B', child=Node{id='C', child=Node{id='D', child=null}}} => 1
Node{id='C', child=Node{id='D', child=null}} => 0
{
  "id" : "A",
  "child" : {
    "id" : "B",
    "child" : { }
  }
}
Depth: 3
Node{id='A', child=Node{id='B', child=Node{id='C', child=Node{id='D', child=null}}}} => 3
Node{id='B', child=Node{id='C', child=Node{id='D', child=null}}} => 2
Node{id='C', child=Node{id='D', child=null}} => 1
Node{id='D', child=null} => 0
{
  "id" : "A",
  "child" : {
    "id" : "B",
    "child" : {
      "id" : "C",
      "child" : { }
    }
  }
}