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/deser/NullValueProvider.java

package com.fasterxml.jackson.databind.deser;

import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.util.AccessPattern;

/**
 * Helper interface implemented by classes that are to be used as
 * null providers during deserialization. Most importantly implemented by
 * {@link com.fasterxml.jackson.databind.JsonDeserializer} (as a mix-in
 * interface), but also by converters used to support more configurable
 * null replacement.
 *
 * @since 2.9
 */
public interface NullValueProvider
{
    /**
     * Method called to possibly convert incoming `null` token (read via
     * underlying streaming input source) into other value of type accessor
     * supports. May return `null`, or value compatible with type binding.
     *<p>
     * NOTE: if {@link #getNullAccessPattern()} returns `ALWAYS_NULL` or
     * `CONSTANT`, this method WILL NOT use provided `ctxt` and it may thus
     * be passed as `null`.
     */
    public Object getNullValue(DeserializationContext ctxt) throws JsonMappingException;

    /**
     * Accessor that may be used to determine if and when provider must be called to
     * access null replacement value.
     */
    public AccessPattern getNullAccessPattern(); 

    /**
     * Method called to determine placeholder value to be used for cases
     * where no value was obtained from input but we must pass a value
     * nonetheless: the common case is that of Creator methods requiring
     * passing a value for every parameter.
     * Usually this is same as {@link #getNullValue} (which in turn
     * is usually simply Java {@code null}), but it can be overridden
     * for specific types: most notable scalar types must use "default"
     * values.
     *<p>
     * This method needs to be called every time a determination is made.
     *<p>
     * Default implementation simply calls and returns {@link #getNullValue}.
     *
     * @since 2.13
     */
    default Object getAbsentValue(DeserializationContext ctxt) throws JsonMappingException {
        return getNullValue(ctxt);
    }
}

⏎ com/fasterxml/jackson/databind/deser/NullValueProvider.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
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⇑⇑ Jackson - Java JSON library