/*
 * Copyright 2002-2014 the original author or authors.
 *
 * 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 org.springframework.http.converter.json;

import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.Reader;
import java.lang.reflect.Type;
import java.nio.charset.Charset;

import com.google.gson.Gson;
import com.google.gson.JsonIOException;
import com.google.gson.JsonParseException;
import com.google.gson.reflect.TypeToken;

import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.http.converter.AbstractHttpMessageConverter;
import org.springframework.http.converter.GenericHttpMessageConverter;
import org.springframework.http.converter.HttpMessageNotReadableException;
import org.springframework.http.converter.HttpMessageNotWritableException;
import org.springframework.util.Assert;

/**
 * Implementation of {@link org.springframework.http.converter.HttpMessageConverter}
 * that can read and write JSON using the
 * <a href="https://code.google.com/p/google-gson/">Google Gson</a> library's
 * {@link Gson} class.
 *
 * <p>This converter can be used to bind to typed beans or untyped {@code HashMap}s.
 * By default, it supports {@code application/json} and {@code application/*+json}.
 *
 * <p>Tested against Gson 2.3; compatible with Gson 2.0 and higher.
 *
 * @author Roy Clarkson
 * @since 4.1
 * @see #setGson
 * @see #setSupportedMediaTypes
 */
public class GsonHttpMessageConverter extends AbstractHttpMessageConverter<Object>
		implements GenericHttpMessageConverter<Object> {

	public static final Charset DEFAULT_CHARSET = Charset.forName("UTF-8");


	private Gson gson = new Gson();

	private String jsonPrefix;


	/**
	 * Construct a new {@code GsonHttpMessageConverter}.
	 */
	public GsonHttpMessageConverter() {
		super(new MediaType("application", "json", DEFAULT_CHARSET),
				new MediaType("application", "*+json", DEFAULT_CHARSET));
	}


	/**
	 * Set the {@code Gson} instance to use.
	 * If not set, a default {@link Gson#Gson() Gson} instance is used.
	 * <p>Setting a custom-configured {@code Gson} is one way to take further
	 * control of the JSON serialization process.
	 */
	public void setGson(Gson gson) {
		Assert.notNull(gson, "'gson' is required");
		this.gson = gson;
	}

	/**
	 * Return the configured {@code Gson} instance for this converter.
	 */
	public Gson getGson() {
		return this.gson;
	}

	/**
	 * Specify a custom prefix to use for JSON output. Default is none.
	 * @see #setPrefixJson
	 */
	public void setJsonPrefix(String jsonPrefix) {
		this.jsonPrefix = jsonPrefix;
	}

	/**
	 * Indicate whether the JSON output by this view should be prefixed with "{} &&".
	 * Default is {@code false}.
	 * <p>Prefixing the JSON string in this manner is used to help prevent JSON
	 * Hijacking. The prefix renders the string syntactically invalid as a script
	 * so that it cannot be hijacked. This prefix does not affect the evaluation
	 * of JSON, but if JSON validation is performed on the string, the prefix
	 * would need to be ignored.
	 * @see #setJsonPrefix
	 */
	public void setPrefixJson(boolean prefixJson) {
		this.jsonPrefix = (prefixJson ? "{} && " : null);
	}


	@Override
	public boolean canRead(Class<?> clazz, MediaType mediaType) {
		return canRead(mediaType);
	}

	@Override
	public boolean canRead(Type type, Class<?> contextClass, MediaType mediaType) {
		return canRead(mediaType);
	}

	@Override
	public boolean canWrite(Class<?> clazz, MediaType mediaType) {
		return canWrite(mediaType);
	}

	@Override
	protected boolean supports(Class<?> clazz) {
		// should not be called, since we override canRead/Write instead
		throw new UnsupportedOperationException();
	}

	@Override
	protected Object readInternal(Class<?> clazz, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException {

		TypeToken<?> token = getTypeToken(clazz);
		return readTypeToken(token, inputMessage);
	}

	@Override
	public Object read(Type type, Class<?> contextClass, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException {

		TypeToken<?> token = getTypeToken(type);
		return readTypeToken(token, inputMessage);
	}

	/**
	 * Return the Gson {@link TypeToken} for the specified type.
	 * <p>The default implementation returns {@code TypeToken.get(type)}, but
	 * this can be overridden in subclasses to allow for custom generic
	 * collection handling. For instance:
	 * <pre class="code">
	 * protected TypeToken<?> getTypeToken(Type type) {
	 *   if (type instanceof Class && List.class.isAssignableFrom((Class<?>) type)) {
	 *     return new TypeToken<ArrayList<MyBean>>() {};
	 *   }
	 *   else {
	 *     return super.getTypeToken(type);
	 *   }
	 * }
	 * </pre>
	 * @param type the type for which to return the TypeToken
	 * @return the type token
	 */
	protected TypeToken<?> getTypeToken(Type type) {
		return TypeToken.get(type);
	}

	private Object readTypeToken(TypeToken<?> token, HttpInputMessage inputMessage) throws IOException {
		Reader json = new InputStreamReader(inputMessage.getBody(), getCharset(inputMessage.getHeaders()));
		try {
			return this.gson.fromJson(json, token.getType());
		}
		catch (JsonParseException ex) {
			throw new HttpMessageNotReadableException("Could not read JSON: " + ex.getMessage(), ex);
		}
	}

	private Charset getCharset(HttpHeaders headers) {
		if (headers == null || headers.getContentType() == null || headers.getContentType().getCharSet() == null) {
			return DEFAULT_CHARSET;
		}
		return headers.getContentType().getCharSet();
	}

	@Override
	protected void writeInternal(Object o, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException {

		Charset charset = getCharset(outputMessage.getHeaders());
		OutputStreamWriter writer = new OutputStreamWriter(outputMessage.getBody(), charset);
		try {
			if (this.jsonPrefix != null) {
				writer.append(this.jsonPrefix);
			}
			this.gson.toJson(o, writer);
			writer.close();
		}
		catch (JsonIOException ex) {
			throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getMessage(), ex);
		}
	}

}
