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/impl/JavaUtilCollectionsDeserializers.java

package com.fasterxml.jackson.databind.deser.impl;

import java.util.*;

import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.deser.std.StdDelegatingDeserializer;
import com.fasterxml.jackson.databind.type.TypeFactory;
import com.fasterxml.jackson.databind.util.Converter;

/**
 * Helper class used to contain logic for deserializing "special" containers
 * from {@code java.util.Collections} and {@code java.util.Arrays}. This is needed
 * because they do not have usable no-arguments constructor: however, are easy enough
 * to deserialize using delegating deserializer.
 *
 * @since 2.9.4
 */
public abstract class JavaUtilCollectionsDeserializers
{
    private final static int TYPE_SINGLETON_SET = 1;
    private final static int TYPE_SINGLETON_LIST = 2;
    private final static int TYPE_SINGLETON_MAP = 3;

    private final static int TYPE_UNMODIFIABLE_SET = 4;
    private final static int TYPE_UNMODIFIABLE_LIST = 5;
    private final static int TYPE_UNMODIFIABLE_MAP = 6;

    // 2.12.1
    private final static int TYPE_SYNC_SET = 7;
    private final static int TYPE_SYNC_COLLECTION = 8;
    private final static int TYPE_SYNC_LIST = 9;
    private final static int TYPE_SYNC_MAP = 10;

    public final static int TYPE_AS_LIST = 11;

    private final static String PREFIX_JAVA_UTIL_COLLECTIONS = "java.util.Collections$";
    private final static String PREFIX_JAVA_UTIL_ARRAYS = "java.util.Arrays$";
    // for Java 11+ List.of(), Map.of() stuff
    private final static String PREFIX_JAVA_UTIL_IMMUTABLE_COLL = "java.util.ImmutableCollections$";

    public static JsonDeserializer<?> findForCollection(DeserializationContext ctxt,
            JavaType type)
        throws JsonMappingException
    {
        final String clsName = type.getRawClass().getName();
        if (!clsName.startsWith("java.util.")) {
            return null;
        }
        // 10-Jan-2017, tatu: Some types from `java.util.Collections`/`java.util.Arrays`
        //    need a bit of help...
        String localName = _findUtilCollectionsTypeName(clsName);
        if (localName != null) {
            JavaUtilCollectionsConverter conv = null;
            String name;
    
            if ((name = _findUnmodifiableTypeName(localName)) != null) {
                if (name.endsWith("Set")) {
                    conv = converter(TYPE_UNMODIFIABLE_SET, type, Set.class);
                } else if (name.endsWith("List")) {
                    conv = converter(TYPE_UNMODIFIABLE_LIST, type, List.class);
                }
            } else if ((name = _findSingletonTypeName(localName)) != null) {
                if (name.endsWith("Set")) {
                    conv = converter(TYPE_SINGLETON_SET, type, Set.class);
                } else if (name.endsWith("List")) {
                    conv = converter(TYPE_SINGLETON_LIST, type, List.class);
                }
            } else if ((name = _findSyncTypeName(localName)) != null) {
                // [databind#3009]: synchronized, too
                if (name.endsWith("Set")) {
                    conv = converter(TYPE_SYNC_SET, type, Set.class);
                } else if (name.endsWith("List")) {
                    conv = converter(TYPE_SYNC_LIST, type, List.class);
                } else if (name.endsWith("Collection")) {
                    conv = converter(TYPE_SYNC_COLLECTION, type, Collection.class);
                }
            }
    
            return (conv == null) ? null : new StdDelegatingDeserializer<Object>(conv);
        }
        if ((localName = _findUtilArrayTypeName(clsName)) != null) {
            // Typically ends with "List" but let's just look for it
            if (localName.contains("List")) {
                // 21-Aug-2022, tatu: [databind#3565] Let's try avoid making "Arrays.asList()"
                //    unmodifiable tho. Could either match "ArrayList" or just, well,
                //    default to "any" modifiable List:
                return new StdDelegatingDeserializer<Object>(
                        converter(TYPE_AS_LIST, type, List.class));
            }
            return null;
        }

        if ((localName = _findUtilCollectionsImmutableTypeName(clsName)) != null) {
            // NOTE: names are "List12" and "ListN" but... let's just look for "List"
            if (localName.contains("List")) {
                return new StdDelegatingDeserializer<Object>(
                        converter(TYPE_AS_LIST, type, List.class));
            }
            if (localName.contains("Set")) {
                return new StdDelegatingDeserializer<Object>(
                        converter(TYPE_UNMODIFIABLE_SET, type, Set.class));
            }
            return null;
        }

        return null;
    }

    public static JsonDeserializer<?> findForMap(DeserializationContext ctxt,
            JavaType type)
        throws JsonMappingException
    {
        final String clsName = type.getRawClass().getName();
        String localName;
        JavaUtilCollectionsConverter conv = null;

        if ((localName = _findUtilCollectionsTypeName(clsName)) != null) {
            String name;
        
            if ((name = _findUnmodifiableTypeName(localName)) != null) {
                if (name.contains("Map")) {
                    conv = converter(TYPE_UNMODIFIABLE_MAP, type, Map.class);
                }
            } else if ((name = _findSingletonTypeName(localName)) != null) {
                if (name.contains("Map")) {
                    conv = converter(TYPE_SINGLETON_MAP, type, Map.class);
                }
            } else if ((name = _findSyncTypeName(localName)) != null) {
                // [databind#3009]: synchronized, too
                if (name.contains("Map")) {
                    conv = converter(TYPE_SYNC_MAP, type, Map.class);
                }
            }
        } else if ((localName = _findUtilCollectionsImmutableTypeName(clsName)) != null) {
            if (localName.contains("Map")) {
                conv = converter(TYPE_UNMODIFIABLE_MAP, type, Map.class);
            }
        }
        return (conv == null) ? null : new StdDelegatingDeserializer<Object>(conv);
    }

    static JavaUtilCollectionsConverter converter(int kind,
            JavaType concreteType, Class<?> rawSuper)
    {
        return new JavaUtilCollectionsConverter(kind, concreteType.findSuperType(rawSuper));
    }

    private static String _findUtilArrayTypeName(String clsName) {
        if (clsName.startsWith(PREFIX_JAVA_UTIL_ARRAYS)) {
            return clsName.substring(PREFIX_JAVA_UTIL_ARRAYS.length());
        }
        return null;
    }

    private static String _findUtilCollectionsTypeName(String clsName) {
        if (clsName.startsWith(PREFIX_JAVA_UTIL_COLLECTIONS)) {
            return clsName.substring(PREFIX_JAVA_UTIL_COLLECTIONS.length());
        }
        return null;
    }

    private static String _findUtilCollectionsImmutableTypeName(String clsName) {
        if (clsName.startsWith(PREFIX_JAVA_UTIL_IMMUTABLE_COLL)) {
            return clsName.substring(PREFIX_JAVA_UTIL_IMMUTABLE_COLL.length());
        }
        return null;
    }

    private static String _findSingletonTypeName(String localName) {
        return localName.startsWith("Singleton") ? localName.substring(9): null;
    }

    private static String _findSyncTypeName(String localName) {
        return localName.startsWith("Synchronized") ? localName.substring(12): null;
    }

    private static String _findUnmodifiableTypeName(String localName) {
        return localName.startsWith("Unmodifiable") ? localName.substring(12): null;
    }
    
    /**
     * Implementation used for converting from various generic container
     * types ({@link java.util.Set}, {@link java.util.List}, {@link java.util.Map})
     * into more specific implementations accessible via {@code java.util.Collections}.
     */
    private static class JavaUtilCollectionsConverter implements Converter<Object,Object>
    {
        private final JavaType _inputType;

        private final int _kind;

        JavaUtilCollectionsConverter(int kind, JavaType inputType) {
            _inputType = inputType;
            _kind = kind;
        }
        
        @Override
        public Object convert(Object value) {
            if (value == null) { // is this legal to get?
                return null;
            }
            
            switch (_kind) {
            case TYPE_SINGLETON_SET:
                {
                    Set<?> set = (Set<?>) value;
                    _checkSingleton(set.size());
                    return Collections.singleton(set.iterator().next());
                }
            case TYPE_SINGLETON_LIST:
                {
                    List<?> list = (List<?>) value;
                    _checkSingleton(list.size());
                    return Collections.singletonList(list.get(0));
                }
            case TYPE_SINGLETON_MAP:
                {
                    Map<?,?> map = (Map<?,?>) value;
                    _checkSingleton(map.size());
                    Map.Entry<?,?> entry = map.entrySet().iterator().next();
                    return Collections.singletonMap(entry.getKey(), entry.getValue());
                }

            case TYPE_UNMODIFIABLE_SET:
                return Collections.unmodifiableSet((Set<?>) value);
            case TYPE_UNMODIFIABLE_LIST:
                return Collections.unmodifiableList((List<?>) value);
            case TYPE_UNMODIFIABLE_MAP:
                return Collections.unmodifiableMap((Map<?,?>) value);

            case TYPE_SYNC_SET:
                return Collections.synchronizedSet((Set<?>) value);
            case TYPE_SYNC_LIST:
                return Collections.synchronizedList((List<?>) value);
            case TYPE_SYNC_COLLECTION:
                return Collections.synchronizedCollection((Collection<?>) value);
            case TYPE_SYNC_MAP:
                return Collections.synchronizedMap((Map<?,?>) value);
                
            case TYPE_AS_LIST:
            default:
                // Here we do not actually care about impl type, just return List as-is:
                return value;
            }
        }

        @Override
        public JavaType getInputType(TypeFactory typeFactory) {
            return _inputType;
        }

        @Override
        public JavaType getOutputType(TypeFactory typeFactory) {
            // we don't actually care, so:
            return _inputType;
        }

        private void _checkSingleton(int size) {
            if (size != 1) {
                // not the best error ever but... has to do
                throw new IllegalArgumentException("Can not deserialize Singleton container from "+size+" entries");
            }
        }
    }
}

⏎ com/fasterxml/jackson/databind/deser/impl/JavaUtilCollectionsDeserializers.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 

⇒ Jackson Annotations Source Code

⇐ Download and Install Jackson Binary Package

⇑ Downloading and Reviewing jackson-*.jar

⇑⇑ Jackson - Java JSON library