com.ibm.watson.app.common.util.rest.JSONResponseHandler.java Source code

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Here is the source code for com.ibm.watson.app.common.util.rest.JSONResponseHandler.java

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/* Copyright IBM Corp. 2015
 *
 * 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.ibm.watson.app.common.util.rest;

import java.io.IOException;
import java.io.InvalidObjectException;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;

import org.apache.http.HttpEntity;
import org.apache.http.util.EntityUtils;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

import com.google.gson.Gson;
import com.google.gson.JsonSyntaxException;
import com.ibm.watson.app.common.util.rest.MessageKey;

/**
 * This response handler deserializes the HTTP entity as JSON.
 * 
 * @param <T> The type representing the JSON data
 */
public abstract class JSONResponseHandler<T> extends HttpStatusAwareResponseHandler<T> {
    private static final Logger logger = LogManager.getLogger();

    private final Gson gson;
    private static final Charset CHARSET = StandardCharsets.UTF_8; // JSON should always be UTF-8

    protected Class<T> classOfT;

    public JSONResponseHandler(Class<T> classOfT) {
        this(new Gson(), classOfT);
    }

    public JSONResponseHandler(Gson gson, Class<T> classOfT) {
        this.gson = gson;
        this.classOfT = classOfT;
    }

    @Override
    protected T handleEntity(HttpEntity entity) throws IOException {
        final String jsonString = EntityUtils.toString(entity, CHARSET);
        if (logger.isTraceEnabled()) {
            logger.trace("Response JSON: " + jsonString);
        }

        if (jsonString == null || jsonString.isEmpty()) {
            if (logger.isDebugEnabled()) {
                logger.debug("Received null or empty JSON string '" + jsonString + "'");
            }
            return getDefaultReturnValue();
        }

        try {
            T retval = parseJSON(jsonString);
            validate(retval);
            return retval;
        } catch (JsonSyntaxException e) {
            logger.error(MessageKey.AQWEGA04001E_unable_parse_json_1.getMessage(e.getMessage()));
            logger.catching(e);
            return getDefaultReturnValue();
        }
    }

    /**
     * Parse the supplied string as JSON. 
     * This method will never receive a {@code null} or empty string.
     * @param jsonString The JSON string
     * @return T An object representing the deserialized JSON 
     * @throws JsonSyntaxException
     */
    protected T parseJSON(String jsonString) throws JsonSyntaxException {
        return gson.fromJson(jsonString, classOfT);
    }

    /**
     * Check if the object returned from deserialization is valid.
     * This method will never receive {@code null}.
     * @param T An object representing the deserialized JSON 
     * @throws InvalidObjectException if the supplied object is not valid
     */
    protected abstract void validate(T object) throws InvalidObjectException;
}