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Extension to support Objectify types like Ref or Key

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/*
 * Copyright 2017 Nicolas Morel
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.github.nmorel.gwtjackson.objectify.server;

import java.io.IOException;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.BeanProperty;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.deser.ContextualDeserializer;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import com.github.nmorel.gwtjackson.objectify.shared.RefConstant;
import com.googlecode.objectify.Key;
import com.googlecode.objectify.Ref;
import com.googlecode.objectify.impl.ref.DeadRef;

public class RefStdDeserializer extends StdDeserializer> implements ContextualDeserializer {

    private final JavaType valueType;

    public RefStdDeserializer() {
        this( null );
    }

    public RefStdDeserializer( JavaType valueType ) {
        super( Ref.class );
        this.valueType = valueType;
    }

    @Override
    public JsonDeserializer createContextual( DeserializationContext ctxt, BeanProperty property ) throws JsonMappingException {
        if ( ctxt.getContextualType() == null || ctxt.getContextualType().containedType( 0 ) == null ) {
            throw JsonMappingException.from( ctxt, "Cannot deserialize Ref. Cannot find the Generic Type T." );
        }
        return new RefStdDeserializer( ctxt.getContextualType().containedType( 0 ) );
    }

    @Override
    public Ref deserialize( JsonParser jp, DeserializationContext ctxt ) throws IOException {
        if ( valueType == null ) {
            throw JsonMappingException.from( ctxt, "valueType can't be null." );
        }

        JsonNode jsonNode = jp.readValueAsTree();

        JsonParser keyJsonParser = jsonNode.get( RefConstant.KEY ).traverse();
        keyJsonParser.setCodec( jp.getCodec() );
        Key key = keyJsonParser.readValueAs( Key.class );

        Object value = null;
        if ( jsonNode.has( RefConstant.VALUE ) ) {
            JsonParser valueJsonParser = jsonNode.get( RefConstant.VALUE ).traverse();
            valueJsonParser.setCodec( jp.getCodec() );
            value = valueJsonParser.readValueAs( valueType.getRawClass() );
        }

        return value != null ? new DeadRef( key, value ) : new DeadRef( key );
    }
}