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/ExternalTypeHandler.java

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

import java.io.IOException;
import java.util.*;

import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.deser.SettableBeanProperty;
import com.fasterxml.jackson.databind.jsontype.TypeDeserializer;
import com.fasterxml.jackson.databind.util.TokenBuffer;

/**
 * Helper class that is used to flatten JSON structure when using
 * "external type id" (see {@link com.fasterxml.jackson.annotation.JsonTypeInfo.As#EXTERNAL_PROPERTY}).
 * This is needed to store temporary state and buffer tokens, as the structure is
 * rearranged a bit so that actual type deserializer can resolve type and 
 * finalize deserialization.
 */
public class ExternalTypeHandler
{
    private final JavaType _beanType;

    private final ExtTypedProperty[] _properties;

    /**
     * Mapping from external property ids to one or more indexes;
     * in most cases single index as <code>Integer</code>, but
     * occasionally same name maps to multiple ones: if so,
     * <code>List&lt;Integer&gt;</code>.
     */
    private final Map<String, Object> _nameToPropertyIndex;

    private final String[] _typeIds;
    private final TokenBuffer[] _tokens;

    protected ExternalTypeHandler(JavaType beanType,
            ExtTypedProperty[] properties,
            Map<String, Object> nameToPropertyIndex,
            String[] typeIds, TokenBuffer[] tokens)
    {
        _beanType = beanType;
        _properties = properties;
        _nameToPropertyIndex = nameToPropertyIndex;
        _typeIds = typeIds;
        _tokens = tokens;
    }

    protected ExternalTypeHandler(ExternalTypeHandler h)
    {
        _beanType = h._beanType;
        _properties = h._properties;
        _nameToPropertyIndex = h._nameToPropertyIndex;
        int len = _properties.length;
        _typeIds = new String[len];
        _tokens = new TokenBuffer[len];
    }

    /**
     * @since 2.9
     */
    public static Builder builder(JavaType beanType) {
        return new Builder(beanType);
    }

    /**
     * Method called to start collection process by creating non-blueprint
     * instances.
     */
    public ExternalTypeHandler start() {
        return new ExternalTypeHandler(this);
    }

    /**
     * Method called to see if given property/value pair is an external type
     * id; and if so handle it. This is <b>only</b> to be called in case
     * containing POJO has similarly named property as the external type id AND
     * value is of scalar type:
     * otherwise {@link #handlePropertyValue} should be called instead.
     */
    @SuppressWarnings("unchecked")
    public boolean handleTypePropertyValue(JsonParser p, DeserializationContext ctxt,
            String propName, Object bean)
        throws IOException
    {
        Object ob = _nameToPropertyIndex.get(propName);
        if (ob == null) {
            return false;
        }
        final String typeId = p.getText();
        // 28-Nov-2016, tatu: For [databind#291], need separate handling
        if (ob instanceof List<?>) {
            boolean result = false;
            for (Integer index : (List<Integer>) ob) {
                if (_handleTypePropertyValue(p, ctxt, propName, bean,
                        typeId, index.intValue())) {
                    result = true;
                }
            }
            return result;
        }
        return _handleTypePropertyValue(p, ctxt, propName, bean,
                typeId, ((Integer) ob).intValue());
    }

    private final boolean _handleTypePropertyValue(JsonParser p, DeserializationContext ctxt,
            String propName, Object bean, String typeId, int index)
        throws IOException
    {
        ExtTypedProperty prop = _properties[index];
        if (!prop.hasTypePropertyName(propName)) { // when could/should this ever happen?
            return false;
        }
        // note: can NOT skip child values (should always be String anyway)
        boolean canDeserialize = (bean != null) && (_tokens[index] != null);
        // Minor optimization: deserialize properties as soon as we have all we need:
        if (canDeserialize) {
            _deserializeAndSet(p, ctxt, bean, index, typeId);
            // clear stored data, to avoid deserializing+setting twice:
            _tokens[index] = null;
        } else {
            _typeIds[index] = typeId;
        }
        return true;
    }

    /**
     * Method called to ask handler to handle value of given property,
     * at point where parser points to the first token of the value.
     * Handling can mean either resolving type id it contains (if it matches type
     * property name), or by buffering the value for further use.
     * 
     * @return True, if the given property was properly handled
     */
    @SuppressWarnings("unchecked")
    public boolean handlePropertyValue(JsonParser p, DeserializationContext ctxt,
            String propName, Object bean) throws IOException
    {
        Object ob = _nameToPropertyIndex.get(propName);
        if (ob == null) {
            return false;
        }
        // 28-Nov-2016, tatu: For [databind#291], need separate handling
        if (ob instanceof List<?>) {
            Iterator<Integer> it = ((List<Integer>) ob).iterator();
            Integer index = it.next();

            ExtTypedProperty prop = _properties[index];
            // For now, let's assume it's same type (either type id OR value)
            // for all mappings, so we'll only check first one
            if (prop.hasTypePropertyName(propName)) {
                String typeId = p.getText();
                p.skipChildren();
                _typeIds[index] = typeId;
                while (it.hasNext()) {
                    _typeIds[it.next()] = typeId;
                }
            } else {
                TokenBuffer tokens = ctxt.bufferAsCopyOfValue(p);
                _tokens[index] = tokens;
                while (it.hasNext()) {
                    _tokens[it.next()] = tokens;
                }
            }
            return true;
        }

        // Otherwise only maps to a single value, in which case we can
        // handle things in bit more optimal way...
        int index = ((Integer) ob).intValue();
        ExtTypedProperty prop = _properties[index];
        boolean canDeserialize;
        if (prop.hasTypePropertyName(propName)) {
            // 19-Feb-2021, tatu: as per [databind#3008], don't use "getText()"
            //    since that'll coerce null value into String "null"...
            _typeIds[index] = p.getValueAsString();
            p.skipChildren();
            canDeserialize = (bean != null) && (_tokens[index] != null);
        } else {
            @SuppressWarnings("resource")
            TokenBuffer tokens = ctxt.bufferAsCopyOfValue(p);
            _tokens[index] = tokens;
            canDeserialize = (bean != null) && (_typeIds[index] != null);
        }
        // Minor optimization: let's deserialize properties as soon as
        // we have all pertinent information:
        if (canDeserialize) {
            String typeId = _typeIds[index];
            // clear stored data, to avoid deserializing+setting twice:
            _typeIds[index] = null;
            _deserializeAndSet(p, ctxt, bean, index, typeId);
            _tokens[index] = null;
        }
        return true;
    }

    /**
     * Method called after JSON Object closes, and has to ensure that all external
     * type ids have been handled.
     */
    @SuppressWarnings("resource")
    public Object complete(JsonParser p, DeserializationContext ctxt, Object bean)
        throws IOException
    {
        for (int i = 0, len = _properties.length; i < len; ++i) {
            String typeId = _typeIds[i];
            final ExtTypedProperty extProp = _properties[i];
            if (typeId == null) {
                TokenBuffer tokens = _tokens[i];
                // let's allow missing both type and property (may already have been set, too)
                // but not just one
                if (tokens == null) {
                    continue;
                }
                // [databind#118]: Need to mind natural types, for which no type id
                // will be included.
                JsonToken t = tokens.firstToken();
                if (t.isScalarValue()) { // can't be null as we never store empty buffers
                    JsonParser buffered = tokens.asParser(p);
                    buffered.nextToken();
                    SettableBeanProperty prop = extProp.getProperty();
                    Object result = TypeDeserializer.deserializeIfNatural(buffered, ctxt, prop.getType());
                    if (result != null) {
                        prop.set(bean, result);
                        continue;
                    }
                }
                // 26-Oct-2012, tatu: As per [databind#94], must allow use of 'defaultImpl'
                if (!extProp.hasDefaultType()) {
                    ctxt.reportPropertyInputMismatch(_beanType, extProp.getProperty().getName(),
                            "Missing external type id property '%s' (and no 'defaultImpl' specified)",
                            extProp.getTypePropertyName());
                } else  {
                    typeId = extProp.getDefaultTypeId();
                    if (typeId == null) {
                        ctxt.reportPropertyInputMismatch(_beanType, extProp.getProperty().getName(),
"Invalid default type id for property '%s': `null` returned by TypeIdResolver",
                                extProp.getTypePropertyName());
                    }
                }
            } else if (_tokens[i] == null) {
                SettableBeanProperty prop = extProp.getProperty();

                if (prop.isRequired() ||
                        ctxt.isEnabled(DeserializationFeature.FAIL_ON_MISSING_EXTERNAL_TYPE_ID_PROPERTY)) {
                    ctxt.reportPropertyInputMismatch(bean.getClass(), prop.getName(),
                            "Missing property '%s' for external type id '%s'",
                            prop.getName(), extProp.getTypePropertyName());
                }
                return bean;
            }
            _deserializeAndSet(p, ctxt, bean, i, typeId);
        }
        return bean;
    }

    /**
     * Variant called when creation of the POJO involves buffering of creator properties
     * as well as property-based creator.
     */
    public Object complete(JsonParser p, DeserializationContext ctxt,
            PropertyValueBuffer buffer, PropertyBasedCreator creator)
        throws IOException
    {
        // first things first: deserialize all data buffered:
        final int len = _properties.length;
        Object[] values = new Object[len];
        for (int i = 0; i < len; ++i) {
            String typeId = _typeIds[i];
            final ExtTypedProperty extProp = _properties[i];
            if (typeId == null) {
                // let's allow missing both type and property (may already have been set, too)
                TokenBuffer tb = _tokens[i];
                if ((tb == null)
                        // 19-Feb-2021, tatu: Both missing value and explicit `null`
                        //    should be accepted...
                        || (tb.firstToken() == JsonToken.VALUE_NULL)
                    ) {
                    continue;
                }
                // but not just one
                // 26-Oct-2012, tatu: As per [databind#94], must allow use of 'defaultImpl'
                if (!extProp.hasDefaultType()) {
                    ctxt.reportPropertyInputMismatch(_beanType, extProp.getProperty().getName(),
                            "Missing external type id property '%s'",
                            extProp.getTypePropertyName());
                } else {
                    typeId = extProp.getDefaultTypeId();
                }
            }

            if (_tokens[i] != null) {
                values[i] = _deserialize(p, ctxt, i, typeId);
            } else {
                if (ctxt.isEnabled(DeserializationFeature.FAIL_ON_MISSING_EXTERNAL_TYPE_ID_PROPERTY)) {
                    SettableBeanProperty prop = extProp.getProperty();
                    ctxt.reportPropertyInputMismatch(_beanType, prop.getName(),
                            "Missing property '%s' for external type id '%s'",
                            prop.getName(), _properties[i].getTypePropertyName());
                }
                // 03-Aug-2022, tatu: [databind#3533] to handle absent value matching
                //    present type id
                values[i] = _deserializeMissingToken(p, ctxt, i, typeId);
            }

            final SettableBeanProperty prop = extProp.getProperty();
            // also: if it's creator prop, fill in
            if (prop.getCreatorIndex() >= 0) {
                buffer.assignParameter(prop, values[i]);

                // [databind#999] And maybe there's creator property for type id too?
                SettableBeanProperty typeProp = extProp.getTypeProperty();
                // for now, should only be needed for creator properties, too
                if ((typeProp != null) && (typeProp.getCreatorIndex() >= 0)) {
                    // 31-May-2018, tatu: [databind#1328] if id is NOT plain `String`, need to
                    //    apply deserializer... fun fun.
                    final Object v;
                    if (typeProp.getType().hasRawClass(String.class)) {
                        v = typeId;
                    } else {
                        TokenBuffer tb = ctxt.bufferForInputBuffering(p);
                        tb.writeString(typeId);
                        v = typeProp.getValueDeserializer().deserialize(tb.asParserOnFirstToken(), ctxt);
                        tb.close();
                    }
                    buffer.assignParameter(typeProp, v);
                }
            }
        }
        Object bean = creator.build(ctxt, buffer);
        // third: assign non-creator properties
        for (int i = 0; i < len; ++i) {
            SettableBeanProperty prop = _properties[i].getProperty();
            if (prop.getCreatorIndex() < 0) {
                prop.set(bean, values[i]);
            }
        }
        return bean;
    }

    @SuppressWarnings("resource")
    protected final Object _deserialize(JsonParser p, DeserializationContext ctxt,
            int index, String typeId) throws IOException
    {
        JsonParser p2 = _tokens[index].asParser(p);
        JsonToken t = p2.nextToken();
        // 29-Sep-2015, tatu: As per [databind#942], nulls need special support
        if (t == JsonToken.VALUE_NULL) {
            return null;
        }
        TokenBuffer merged = ctxt.bufferForInputBuffering(p);
        merged.writeStartArray();
        merged.writeString(typeId);
        merged.copyCurrentStructure(p2);
        merged.writeEndArray();

        // needs to point to START_OBJECT (or whatever first token is)
        JsonParser mp = merged.asParser(p);
        mp.nextToken();
        return _properties[index].getProperty().deserialize(mp, ctxt);
    }

    // 03-Aug-2022, tatu: [databind#3533] to handle absent value matching:
    @SuppressWarnings("resource")
    protected final Object _deserializeMissingToken(JsonParser p, DeserializationContext ctxt,
            int index, String typeId) throws IOException
    {
        TokenBuffer merged = ctxt.bufferForInputBuffering(p);
        merged.writeStartArray();
        merged.writeString(typeId);
        merged.writeEndArray();

        // needs to point to START_OBJECT (or whatever first token is)
        JsonParser mp = merged.asParser(p);
        mp.nextToken();
        return _properties[index].getProperty().deserialize(mp, ctxt);
    }

    @SuppressWarnings("resource")
    protected final void _deserializeAndSet(JsonParser p, DeserializationContext ctxt,
            Object bean, int index, String typeId) throws IOException
    {
        // 11-Nov-2020, tatu: Should never get `null` passed this far,
        if (typeId == null) {
            ctxt.reportInputMismatch(_beanType, "Internal error in external Type Id handling: `null` type id passed");
        }

        // Ok: time to mix type id, value; and we will actually use "wrapper-array"
        // style to ensure we can handle all kinds of JSON constructs.
        JsonParser p2 = _tokens[index].asParser(p);
        JsonToken t = p2.nextToken();
        // 29-Sep-2015, tatu: As per [databind#942], nulls need special support
        if (t == JsonToken.VALUE_NULL) {
            _properties[index].getProperty().set(bean, null);
            return;
        }
        TokenBuffer merged = ctxt.bufferForInputBuffering(p);
        merged.writeStartArray();
        merged.writeString(typeId);

        merged.copyCurrentStructure(p2);
        merged.writeEndArray();
        // needs to point to START_OBJECT (or whatever first token is)
        JsonParser mp = merged.asParser(p);
        mp.nextToken();
        _properties[index].getProperty().deserializeAndSet(mp, ctxt, bean);
    }

    /*
    /**********************************************************
    /* Helper classes
    /**********************************************************
     */
    
    public static class Builder
    {
        private final JavaType _beanType;

        private final List<ExtTypedProperty> _properties = new ArrayList<>();
        private final Map<String, Object> _nameToPropertyIndex = new HashMap<>();

        protected Builder(JavaType t) {
            _beanType = t;
        }

        public void addExternal(SettableBeanProperty property, TypeDeserializer typeDeser)
        {
            Integer index = _properties.size();
            _properties.add(new ExtTypedProperty(property, typeDeser));
            _addPropertyIndex(property.getName(), index);
            _addPropertyIndex(typeDeser.getPropertyName(), index);
        }

        private void _addPropertyIndex(String name, Integer index) {
            Object ob = _nameToPropertyIndex.get(name);
            if (ob == null) {
                _nameToPropertyIndex.put(name, index);
            } else if (ob instanceof List<?>) {
                @SuppressWarnings("unchecked")
                List<Object> list = (List<Object>) ob;
                list.add(index);
            } else {
                List<Object> list = new LinkedList<>();
                list.add(ob);
                list.add(index);
                _nameToPropertyIndex.put(name, list);
            }
        }
        
        /**
         * Method called after all external properties have been assigned, to further
         * link property with polymorphic value with possible property for type id
         * itself. This is needed to support type ids as Creator properties.
         *
         * @since 2.8
         */
        public ExternalTypeHandler build(BeanPropertyMap otherProps) {
            // 21-Jun-2016, tatu: as per [databind#999], may need to link type id property also
            final int len = _properties.size();
            ExtTypedProperty[] extProps = new ExtTypedProperty[len];
            for (int i = 0; i < len; ++i) {
                ExtTypedProperty extProp = _properties.get(i);
                String typePropId = extProp.getTypePropertyName();
                SettableBeanProperty typeProp = otherProps.find(typePropId);
                if (typeProp != null) {
                    extProp.linkTypeProperty(typeProp);
                }
                extProps[i] = extProp;
            }
            return new ExternalTypeHandler(_beanType, extProps, _nameToPropertyIndex,
                    null, null);
        }
    }

    private final static class ExtTypedProperty
    {
        private final SettableBeanProperty _property;
        private final TypeDeserializer _typeDeserializer;
        private final String _typePropertyName;

        /**
         * @since 2.8
         */
        private SettableBeanProperty _typeProperty;

        public ExtTypedProperty(SettableBeanProperty property, TypeDeserializer typeDeser)
        {
            _property = property;
            _typeDeserializer = typeDeser;
            _typePropertyName = typeDeser.getPropertyName();
        }

        /**
         * @since 2.8
         */
        public void linkTypeProperty(SettableBeanProperty p) {
            _typeProperty = p;
        }

        public boolean hasTypePropertyName(String n) {
            return n.equals(_typePropertyName);
        }

        public boolean hasDefaultType() {
            return _typeDeserializer.hasDefaultImpl();
        }

        /**
         * Specialized called when we need to expose type id of `defaultImpl` when
         * serializing: we may need to expose it for assignment to a property, or
         * it may be requested as visible for some other reason.
         */
        public String getDefaultTypeId() {
            Class<?> defaultType = _typeDeserializer.getDefaultImpl();
            if (defaultType == null) {
                return null;
            }
            return _typeDeserializer.getTypeIdResolver().idFromValueAndType(null, defaultType);
        }

        public String getTypePropertyName() { return _typePropertyName; }

        public SettableBeanProperty getProperty() {
            return _property;
        }

        /**
         * @since 2.8
         */
        public SettableBeanProperty getTypeProperty() {
            return _typeProperty;
        }
    }
}

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