Jackson is "the Java JSON library" or "the best JSON parser for Java". Or simply as "JSON for Java".

Jackson Databind Source Code files are provided in the source packge (jackson-databind-2.14.0-sources.jar). You can download it at Jackson Maven Website.

You can also browse Jackson Databind Source Code below:

⏎ com/fasterxml/jackson/databind/node/NullNode.java

package com.fasterxml.jackson.databind.node;

import java.io.IOException;

import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.SerializerProvider;


/**
 * This singleton value class is used to contain explicit JSON null
 * value.
 */
public class NullNode
    extends ValueNode
{
    // // Just need a fly-weight singleton

    private static final long serialVersionUID = 1L;

    public final static NullNode instance = new NullNode();

    /**
     *<p>
     * NOTE: visibility raised to `protected` in 2.9.3 to allow custom subtypes.
     */
    protected NullNode() { }

    // To support JDK serialization, recovery of Singleton instance
    protected Object readResolve() {
        return instance;
    }
    
    public static NullNode getInstance() { return instance; }

    @Override
    public JsonNodeType getNodeType() {
        return JsonNodeType.NULL;
    }

    @Override public JsonToken asToken() { return JsonToken.VALUE_NULL; }

    @Override public String asText(String defaultValue) { return defaultValue; }
    @Override public String asText() { return "null"; }

    @SuppressWarnings("unchecked")
    @Override
    public JsonNode requireNonNull() {
        return _reportRequiredViolation("requireNonNull() called on `NullNode`");
    }

    // as with MissingNode, not considered number node; hence defaults are returned if provided
    
    /*
    public int asInt(int defaultValue);
    public long asLong(long defaultValue);
    public double asDouble(double defaultValue);
    public boolean asBoolean(boolean defaultValue);
    */
    
    @Override
    public final void serialize(JsonGenerator g, SerializerProvider provider)
        throws IOException
    {
        provider.defaultSerializeNull(g);
    }

    @Override
    public boolean equals(Object o) {
        // 29-Aug-2019, tatu: [databind#2433] Since custom sub-classes are allowed (bad idea probably),
        //     need to do better comparison
        return (o == this) || (o instanceof NullNode);
    }

    @Override
    public int hashCode() {
        return JsonNodeType.NULL.ordinal();
    }
}

⏎ com/fasterxml/jackson/databind/node/NullNode.java

Or download all of them as a single archive file:

File name: jackson-databind-2.14.0-sources.jar
File size: 1187952 bytes
Release date: 2022-11-05
Download 

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⇑⇑ Jackson - Java JSON library