什么时候JsonNode实例为null？(When is a JsonNode instance null?)

 考虑下面的Java类定义。 
 
 CrudOperation.java： 
 
package com.cyberfront.test.json.nll.demonstration;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.databind.JsonNode;

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY)
@JsonInclude(JsonInclude.Include.NON_NULL)
public class CrudOperation{
    public enum Type {CREATE,UPDATE,DELETE,READ}
    private final Type type;
    private final JsonNode op;

    public CrudOperation(Type type) { this(type, null); }
    public CrudOperation(JsonNode op) { this(Type.UPDATE, op); }

    @JsonCreator
    public CrudOperation(@JsonProperty("type") Type type, @JsonProperty("op") JsonNode op) {
        this.type = type;
        this.op = op;

        boolean isUpdate = Type.UPDATE.equals(this.getType());
        boolean isNotNull = null == this.getOp();
        boolean isValid = isUpdate ^ isNotNull;

        if (!isValid) {
            System.out.println(" isUpdate: " + String.valueOf(isUpdate));
            System.out.println("isNotNull: " + String.valueOf(isNotNull));
            System.out.println("  isValid: " + String.valueOf(isValid));

            throw new IllegalArgumentException("Operation Failed Validation: " + this.toString());
        }
    }

    @JsonProperty("type")
    public Type getType() { return this.type; };

    @JsonProperty("op")
    public JsonNode getOp() { return this.op; }

    public static <T> String nullCheck(T val) { return null == val ? "null" : val.toString(); }

    public static String toString(Type type, JsonNode op) {
        return "{\"type\":\"" + nullCheck(type) + "\",\"op\":" + nullCheck(op) + "}";
    }

    @Override
    public String toString() { return toString(this.getType(), this.getOp()); }
}
 
 在这个类中，带有@JsonCreator注解的构造函数执行验证，以确保当type参数的值为Type.UPDATE时确实存在非空JsonNode参数。 也就是说，当type值为Type.UPDATE ， op应该是非空的。 如果type有任何其他值，则op应该为空。 
 
 接下来考虑下面的类。 
 
 Main.java： 
 
package com.cyberfront.test.json.nll.demonstration;

import com.cyberfront.test.json.nll.demonstration.CrudOperation.Type;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Main {

    private static ObjectMapper mapper = new ObjectMapper();

    public static void main(String[] args) {
        CrudOperation createObject = new CrudOperation(Type.CREATE);
        System.out.println("Initial value: " + createObject.toString());

        JsonNode createDocument = mapper.valueToTree(createObject);
        System.out.println("Document value: " + createDocument.toString());

        CrudOperation reconstructedObject = null;
        try {
            reconstructedObject = mapper.treeToValue(createDocument, CrudOperation.class);
        } catch (JsonProcessingException e) {
            System.out.println(e);
        }

        System.out.println("Reconstructed: " + CrudOperation.nullCheck(reconstructedObject));
    }
}
 
 这用于测试操作类型值为Type.CREATE的CrudOperation实例的创建，这意味着op应该为null。 但是，这会产生以下输出： 
 
Initial value: {"type":"CREATE","op":null}
Document value: {"@type":"CrudOperation","type":"CREATE"}
 isUpdate: false
isNotNull: false
  isValid: false
com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of com.cyberfront.test.json.nll.demonstration.CrudOperation, problem: Operation Failed Validation: {"type":"CREATE","op":null}
 at [Source: N/A; line: -1, column: -1]
Reconstructed: null
 
 最初的CrudOperation实例化已经过验证，并且该实例转换为JsonNode实例也可以正常工作。 尝试从原始派生的JsonNode重构CrudOperation实例时，会发生此问题。 出于某种原因， op的值显示为null ，但与null进行相等性测试失败，这反过来又导致CrudOperation验证失败。 
 
 为什么会发生，怎么解决？ 

Consider the following Java class definition.
 
CrudOperation.java:
 
package com.cyberfront.test.json.nll.demonstration;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.databind.JsonNode;

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY)
@JsonInclude(JsonInclude.Include.NON_NULL)
public class CrudOperation{
    public enum Type {CREATE,UPDATE,DELETE,READ}
    private final Type type;
    private final JsonNode op;

    public CrudOperation(Type type) { this(type, null); }
    public CrudOperation(JsonNode op) { this(Type.UPDATE, op); }

    @JsonCreator
    public CrudOperation(@JsonProperty("type") Type type, @JsonProperty("op") JsonNode op) {
        this.type = type;
        this.op = op;

        boolean isUpdate = Type.UPDATE.equals(this.getType());
        boolean isNotNull = null == this.getOp();
        boolean isValid = isUpdate ^ isNotNull;

        if (!isValid) {
            System.out.println(" isUpdate: " + String.valueOf(isUpdate));
            System.out.println("isNotNull: " + String.valueOf(isNotNull));
            System.out.println("  isValid: " + String.valueOf(isValid));

            throw new IllegalArgumentException("Operation Failed Validation: " + this.toString());
        }
    }

    @JsonProperty("type")
    public Type getType() { return this.type; };

    @JsonProperty("op")
    public JsonNode getOp() { return this.op; }

    public static <T> String nullCheck(T val) { return null == val ? "null" : val.toString(); }

    public static String toString(Type type, JsonNode op) {
        return "{\"type\":\"" + nullCheck(type) + "\",\"op\":" + nullCheck(op) + "}";
    }

    @Override
    public String toString() { return toString(this.getType(), this.getOp()); }
}
 
In this class, the constructor with the @JsonCreator annotation performs validation to ensure there is a non-null JsonNode argument op exactly when the type argument has the value Type.UPDATE. That is, when type has value Type.UPDATE, op should be non-null. If type has any other value, op should be null.
 
Next consider the class below.
 
Main.java:
 
package com.cyberfront.test.json.nll.demonstration;

import com.cyberfront.test.json.nll.demonstration.CrudOperation.Type;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Main {

    private static ObjectMapper mapper = new ObjectMapper();

    public static void main(String[] args) {
        CrudOperation createObject = new CrudOperation(Type.CREATE);
        System.out.println("Initial value: " + createObject.toString());

        JsonNode createDocument = mapper.valueToTree(createObject);
        System.out.println("Document value: " + createDocument.toString());

        CrudOperation reconstructedObject = null;
        try {
            reconstructedObject = mapper.treeToValue(createDocument, CrudOperation.class);
        } catch (JsonProcessingException e) {
            System.out.println(e);
        }

        System.out.println("Reconstructed: " + CrudOperation.nullCheck(reconstructedObject));
    }
}
 
This is used to test the creation of a CrudOperation instance with operation type value of Type.CREATE, which means op should be null. Yet, this produces the following output:
 
Initial value: {"type":"CREATE","op":null}
Document value: {"@type":"CrudOperation","type":"CREATE"}
 isUpdate: false
isNotNull: false
  isValid: false
com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of com.cyberfront.test.json.nll.demonstration.CrudOperation, problem: Operation Failed Validation: {"type":"CREATE","op":null}
 at [Source: N/A; line: -1, column: -1]
Reconstructed: null
 
The initial CrudOperation instantiation is validated, and the conversion of that instance to a JsonNode instance also works properly. The problem occurs when attempting to reconstitute the CrudOperation instance from the JsonNode derived from the original. For some reason, the value of op displays as null, but the test for equality with null fails, which in turn causes the CrudOperation validation to fail.
 
Why is happening and how can it be resolved?

原文：https://stackoverflow.com/questions/47483118