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

Jackson Annotations Source Code files are provided in the source packge (jackson-annotations-2.12.4-sources.jar). You can download it at Jackson Maven Website.

You can also browse Jackson Annotations Source Code below:

⏎ com/fasterxml/jackson/databind/deser/std/StringCollectionDeserializer.java

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

import java.io.IOException;
import java.util.Collection;
import java.util.Objects;

import com.fasterxml.jackson.annotation.JsonFormat;

import com.fasterxml.jackson.core.*;

import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.annotation.JacksonStdImpl;
import com.fasterxml.jackson.databind.deser.ContextualDeserializer;
import com.fasterxml.jackson.databind.deser.NullValueProvider;
import com.fasterxml.jackson.databind.deser.ValueInstantiator;
import com.fasterxml.jackson.databind.introspect.AnnotatedWithParams;
import com.fasterxml.jackson.databind.jsontype.TypeDeserializer;
import com.fasterxml.jackson.databind.type.LogicalType;

/**
 * Specifically optimized version for {@link java.util.Collection}s
 * that contain String values; reason is that this is a very common
 * type and we can make use of the fact that Strings are final.
 */
@JacksonStdImpl
public final class StringCollectionDeserializer
    extends ContainerDeserializerBase<Collection<String>>
    implements ContextualDeserializer
{
    private static final long serialVersionUID = 1L;

    // // Configuration

    /**
     * Value deserializer to use, if NOT the standard one
     * (if it is, will be null).
     */
    protected final JsonDeserializer<String> _valueDeserializer;

    // // Instance construction settings:
    
    /**
     * Instantiator used in case custom handling is needed for creation.
     */
    protected final ValueInstantiator _valueInstantiator;

    /**
     * Deserializer that is used iff delegate-based creator is
     * to be used for deserializing from JSON Object.
     */
    protected final JsonDeserializer<Object> _delegateDeserializer;

    // NOTE: no PropertyBasedCreator, as JSON Arrays have no properties

    /*
    /**********************************************************
    /* Life-cycle
    /**********************************************************
     */
    
    public StringCollectionDeserializer(JavaType collectionType,
            JsonDeserializer<?> valueDeser, ValueInstantiator valueInstantiator)
    {
        this(collectionType, valueInstantiator, null, valueDeser, valueDeser, null);
    }

    @SuppressWarnings("unchecked")
    protected StringCollectionDeserializer(JavaType collectionType,
            ValueInstantiator valueInstantiator, JsonDeserializer<?> delegateDeser,
            JsonDeserializer<?> valueDeser,
            NullValueProvider nuller, Boolean unwrapSingle)
    {
        super(collectionType, nuller, unwrapSingle);
        _valueDeserializer = (JsonDeserializer<String>) valueDeser;
        _valueInstantiator = valueInstantiator;
        _delegateDeserializer = (JsonDeserializer<Object>) delegateDeser;
    }

    protected StringCollectionDeserializer withResolved(JsonDeserializer<?> delegateDeser,
            JsonDeserializer<?> valueDeser,
            NullValueProvider nuller, Boolean unwrapSingle)
    {
        if ((Objects.equals(_unwrapSingle, unwrapSingle)) && (_nullProvider == nuller)
                && (_valueDeserializer == valueDeser) && (_delegateDeserializer == delegateDeser)) {
            return this;
        }
        return new StringCollectionDeserializer(_containerType, _valueInstantiator,
                delegateDeser, valueDeser, nuller, unwrapSingle);
    }

    @Override // since 2.5
    public boolean isCachable() {
        // 26-Mar-2015, tatu: Important: prevent caching if custom deserializers via annotations
        //    are involved
        return (_valueDeserializer == null) && (_delegateDeserializer == null);
    }

    @Override // since 2.12
    public LogicalType logicalType() {
        return LogicalType.Collection;
    }

    /*
    /**********************************************************
    /* Validation, post-processing
    /**********************************************************
     */
    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext ctxt,
            BeanProperty property) throws JsonMappingException
    {
        // May need to resolve types for delegate-based creators:
        JsonDeserializer<Object> delegate = null;
        if (_valueInstantiator != null) {
            // [databind#2324]: check both array-delegating and delegating
            AnnotatedWithParams delegateCreator = _valueInstantiator.getArrayDelegateCreator();
            if (delegateCreator != null) {
                JavaType delegateType = _valueInstantiator.getArrayDelegateType(ctxt.getConfig());
                delegate = findDeserializer(ctxt, delegateType, property);
            } else if ((delegateCreator = _valueInstantiator.getDelegateCreator()) != null) {
                JavaType delegateType = _valueInstantiator.getDelegateType(ctxt.getConfig());
                delegate = findDeserializer(ctxt, delegateType, property);
            }
        }
        JsonDeserializer<?> valueDeser = _valueDeserializer;
        final JavaType valueType = _containerType.getContentType();
        if (valueDeser == null) {
            // [databind#125]: May have a content converter
            valueDeser = findConvertingContentDeserializer(ctxt, property, valueDeser);
            if (valueDeser == null) {
            // And we may also need to get deserializer for String
                valueDeser = ctxt.findContextualValueDeserializer(valueType, property);
            }
        } else { // if directly assigned, probably not yet contextual, so:
            valueDeser = ctxt.handleSecondaryContextualization(valueDeser, property, valueType);
        }
        // 11-Dec-2015, tatu: Should we pass basic `Collection.class`, or more refined? Mostly
        //   comes down to "List vs Collection" I suppose... for now, pass Collection
        Boolean unwrapSingle = findFormatFeature(ctxt, property, Collection.class,
                JsonFormat.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY);
        NullValueProvider nuller = findContentNullProvider(ctxt, property, valueDeser);
        if (isDefaultDeserializer(valueDeser)) {
            valueDeser = null;
        }
        return withResolved(delegate, valueDeser, nuller, unwrapSingle);
    }
    
    /*
    /**********************************************************
    /* ContainerDeserializerBase API
    /**********************************************************
     */

    @SuppressWarnings("unchecked")
    @Override
    public JsonDeserializer<Object> getContentDeserializer() {
        JsonDeserializer<?> deser = _valueDeserializer;
        return (JsonDeserializer<Object>) deser;
    }

    @Override
    public ValueInstantiator getValueInstantiator() {
        return _valueInstantiator;
    }

    /*
    /**********************************************************
    /* JsonDeserializer API
    /**********************************************************
     */
    
    @SuppressWarnings("unchecked")
    @Override
    public Collection<String> deserialize(JsonParser p, DeserializationContext ctxt)
        throws IOException
    {
        if (_delegateDeserializer != null) {
            return (Collection<String>) _valueInstantiator.createUsingDelegate(ctxt,
                    _delegateDeserializer.deserialize(p, ctxt));
        }
        final Collection<String> result = (Collection<String>) _valueInstantiator.createUsingDefault(ctxt);
        return deserialize(p, ctxt, result);
    }

    @Override
    public Collection<String> deserialize(JsonParser p, DeserializationContext ctxt,
            Collection<String> result)
        throws IOException
    {
        // Ok: must point to START_ARRAY
        if (!p.isExpectedStartArrayToken()) {
            return handleNonArray(p, ctxt, result);
        }

        if (_valueDeserializer != null) {
            return deserializeUsingCustom(p, ctxt, result, _valueDeserializer);
        }
        try {
            while (true) {
                // First the common case:
                String value = p.nextTextValue();
                if (value != null) {
                    result.add(value);
                    continue;
                }
                JsonToken t = p.currentToken();
                if (t == JsonToken.END_ARRAY) {
                    break;
                }
                if (t == JsonToken.VALUE_NULL) {
                    if (_skipNullValues) {
                        continue;
                    }
                    value = (String) _nullProvider.getNullValue(ctxt);
                } else {
                    value = _parseString(p, ctxt);
                }
                result.add(value);
            }
        } catch (Exception e) {
            throw JsonMappingException.wrapWithPath(e, result, result.size());
        }
        return result;
    }
    
    private Collection<String> deserializeUsingCustom(JsonParser p, DeserializationContext ctxt,
            Collection<String> result, final JsonDeserializer<String> deser) throws IOException
    {
        try {
            while (true) {
                /* 30-Dec-2014, tatu: This may look odd, but let's actually call method
                 *   that suggest we are expecting a String; this helps with some formats,
                 *   notably XML. Note, however, that while we can get String, we can't
                 *   assume that's what we use due to custom deserializer
                 */
                String value;
                if (p.nextTextValue() == null) {
                    JsonToken t = p.currentToken();
                    if (t == JsonToken.END_ARRAY) {
                        break;
                    }
                    // Ok: no need to convert Strings, but must recognize nulls
                    if (t == JsonToken.VALUE_NULL) {
                        if (_skipNullValues) {
                            continue;
                        }
                        value = (String) _nullProvider.getNullValue(ctxt);
                    } else {
                        value = deser.deserialize(p, ctxt);
                    }
                } else {
                    value = deser.deserialize(p, ctxt);
                }
                result.add(value);
            }
        } catch (Exception e) {
            throw JsonMappingException.wrapWithPath(e, result, result.size());
        }
        return result;
    }

    @Override
    public Object deserializeWithType(JsonParser p, DeserializationContext ctxt,
            TypeDeserializer typeDeserializer) throws IOException {
        // In future could check current token... for now this should be enough:
        return typeDeserializer.deserializeTypedFromArray(p, ctxt);
    }

    /**
     * Helper method called when current token is not START_ARRAY. Will either
     * throw an exception, or try to handle value as if member of implicit
     * array, depending on configuration.
     */
    @SuppressWarnings("unchecked")
    private final Collection<String> handleNonArray(JsonParser p, DeserializationContext ctxt,
            Collection<String> result) throws IOException
    {
        // implicit arrays from single values?
        boolean canWrap = (_unwrapSingle == Boolean.TRUE) ||
                ((_unwrapSingle == null) &&
                        ctxt.isEnabled(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY));
        if (!canWrap) {
            if (p.hasToken(JsonToken.VALUE_STRING)) {
                return _deserializeFromString(p, ctxt);
            }
            return (Collection<String>) ctxt.handleUnexpectedToken(_containerType, p);
        }
        // Strings are one of "native" (intrinsic) types, so there's never type deserializer involved
        JsonDeserializer<String> valueDes = _valueDeserializer;
        JsonToken t = p.currentToken();

        String value;
        
        if (t == JsonToken.VALUE_NULL) {
            // 03-Feb-2017, tatu: Does this work?
            if (_skipNullValues) {
                return result;
            }
            value = (String) _nullProvider.getNullValue(ctxt);
        } else {
            try {
                value = (valueDes == null) ? _parseString(p, ctxt) : valueDes.deserialize(p, ctxt);
            } catch (Exception e) {
                throw JsonMappingException.wrapWithPath(e, result, result.size());
            }
        }
        result.add(value);
        return result;
    }
}

⏎ com/fasterxml/jackson/databind/deser/std/StringCollectionDeserializer.java

 

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