JSONApiTypeAdapterFactory.java

package com.twobox.evaluategson_jsonapi.deserialize;


import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;

import org.json.JSONObject;

import java.io.IOException;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

/**
 * This is a implementation to unflatten JSON which is structured according to http://jsonapi.org/ specification (and hence is flat)
 * and make it mappable to regular POJOs.
 *
 * The idea is based on http://stackoverflow.com/questions/11271375/gson-custom-seralizer-for-one-variable-of-many-in-an-object-using-typeadapter
 *
 * Created by wischweh1 on 02.07.15.
 */
public class JSONApiTypeAdapterFactory<C>
        implements TypeAdapterFactory {
    private final Class<C> customizedClass;

    public static final String JSONAPI_KEYWORD_ID="id";
    public static final String JSONAPI_KEYWORD_TYPE="type";
    public static final String JSONAPI_KEYWORD_ATTRIBUTES="attributes";


    public JSONApiTypeAdapterFactory(Class<C> customizedClass) {
        this.customizedClass = customizedClass;
    }

    @SuppressWarnings("unchecked") // we use a runtime check to guarantee that 'C' and 'T' are equal
    public final <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        return type.getRawType() == customizedClass
                ? (TypeAdapter<T>) customizeMyClassAdapter(gson, (TypeToken<C>) type)
                : null;
    }

    private TypeAdapter<C> customizeMyClassAdapter(Gson gson, TypeToken<C> type) {
        final TypeAdapter<C> delegate = gson.getDelegateAdapter(this, type);
        final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
        return new TypeAdapter<C>() {
            @Override public void write(JsonWriter out, C value) throws IOException {
                JsonElement tree = delegate.toJsonTree(value);
                beforeWrite(value, tree);
                elementAdapter.write(out, tree);
            }
            @Override public C read(JsonReader in) throws IOException {
                JsonElement tree = elementAdapter.read(in);
                JsonElement unflattenedTree=unflatten(tree);
                return delegate.fromJsonTree(unflattenedTree);
            }
        };
    }

    /**
     * Override this to muck with {@code toSerialize} before it is written to
     * the outgoing JSON stream.
     */
    protected void beforeWrite(C source, JsonElement toSerialize) {
        // TODO: Implement this
    }

    /**
     * Override this to muck with {@code deserialized} before it parsed into
     * the application type.
     */
    protected JsonElement unflatten(JsonElement deserialized) {
        JsonObject result=new JsonObject();
        JsonObject dataNode=deserialized.getAsJsonObject();
        result.add(JSONAPI_KEYWORD_ID, dataNode.get(JSONAPI_KEYWORD_ID));
        JsonObject attributes=dataNode.getAsJsonObject(JSONAPI_KEYWORD_ATTRIBUTES);
        for (Map.Entry<String, JsonElement> entry : attributes.entrySet()) {
            result.add(entry.getKey(),entry.getValue());
        }
        return result;
    }
}