/*
 * 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.web.servlet.view.json;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.fasterxml.jackson.core.JsonEncoding;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;

import org.springframework.util.Assert;
import org.springframework.util.CollectionUtils;
import org.springframework.validation.BindingResult;
import org.springframework.web.servlet.View;
import org.springframework.web.servlet.view.AbstractView;

/**
 * Spring MVC {@link View} that renders JSON content by serializing the model for the current request
 * using <a href="http://jackson.codehaus.org/">Jackson 2's</a> {@link ObjectMapper}.
 *
 * <p>By default, the entire contents of the model map (with the exception of framework-specific classes)
 * will be encoded as JSON. If the model contains only one key, you can have it extracted encoded as JSON
 * alone via  {@link #setExtractValueFromSingleKeyModel}.
 *
 * @author Jeremy Grelle
 * @author Arjen Poutsma
 * @author Rossen Stoyanchev
 * @author Juergen Hoeller
 * @since 3.1.2
 * @see org.springframework.http.converter.json.MappingJackson2HttpMessageConverter
 */
public class MappingJackson2JsonView extends AbstractView {

	/**
	 * Default content type: "application/json".
	 * Overridable through {@link #setContentType}.
	 */
	public static final String DEFAULT_CONTENT_TYPE = "application/json";


	private ObjectMapper objectMapper = new ObjectMapper();

	private JsonEncoding encoding = JsonEncoding.UTF8;

	private String jsonPrefix;

	private Boolean prettyPrint;

	private Set<String> modelKeys;

	private boolean extractValueFromSingleKeyModel = false;

	private boolean disableCaching = true;

	private boolean updateContentLength = false;


	/**
	 * Construct a new {@code MappingJackson2JsonView}, setting the content type to {@code application/json}.
	 */
	public MappingJackson2JsonView() {
		setContentType(DEFAULT_CONTENT_TYPE);
		setExposePathVariables(false);
	}


	/**
	 * Set the {@code ObjectMapper} for this view.
	 * If not set, a default {@link ObjectMapper#ObjectMapper() ObjectMapper} will be used.
	 * <p>Setting a custom-configured {@code ObjectMapper} is one way to take further control of
	 * the JSON serialization process. The other option is to use Jackson's provided annotations
	 * on the types to be serialized, in which case a custom-configured ObjectMapper is unnecessary.
	 */
	public void setObjectMapper(ObjectMapper objectMapper) {
		Assert.notNull(objectMapper, "'objectMapper' must not be null");
		this.objectMapper = objectMapper;
		configurePrettyPrint();
	}

	/**
	 * Return the {@code ObjectMapper} for this view.
	 */
	public final ObjectMapper getObjectMapper() {
		return this.objectMapper;
	}

	/**
	 * Set the {@code JsonEncoding} for this view.
	 * By default, {@linkplain JsonEncoding#UTF8 UTF-8} is used.
	 */
	public void setEncoding(JsonEncoding encoding) {
		Assert.notNull(encoding, "'encoding' must not be null");
		this.encoding = encoding;
	}

	/**
	 * Return the {@code JsonEncoding} for this view.
	 */
	public final JsonEncoding getEncoding() {
		return this.encoding;
	}

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

	/**
	 * Indicates whether the JSON output by this view should be prefixed with <tt>"{} && "</tt>.
	 * 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);
	}

	/**
	 * Whether to use the default pretty printer when writing JSON.
	 * This is a shortcut for setting up an {@code ObjectMapper} as follows:
	 * <pre class="code">
	 * ObjectMapper mapper = new ObjectMapper();
	 * mapper.configure(SerializationFeature.INDENT_OUTPUT, true);
	 * </pre>
	 * <p>The default value is {@code false}.
	 */
	public void setPrettyPrint(boolean prettyPrint) {
		this.prettyPrint = prettyPrint;
		configurePrettyPrint();
	}

	private void configurePrettyPrint() {
		if (this.prettyPrint != null) {
			this.objectMapper.configure(SerializationFeature.INDENT_OUTPUT, this.prettyPrint);
		}
	}

	/**
	 * Set the attribute in the model that should be rendered by this view.
	 * When set, all other model attributes will be ignored.
	 */
	public void setModelKey(String modelKey) {
		this.modelKeys = Collections.singleton(modelKey);
	}

	/**
	 * Set the attributes in the model that should be rendered by this view.
	 * When set, all other model attributes will be ignored.
	 */
	public void setModelKeys(Set<String> modelKeys) {
		this.modelKeys = modelKeys;
	}

	/**
	 * Return the attributes in the model that should be rendered by this view.
	 */
	public final Set<String> getModelKeys() {
		return this.modelKeys;
	}

	/**
	 * Set the attributes in the model that should be rendered by this view.
	 * When set, all other model attributes will be ignored.
	 * @deprecated use {@link #setModelKeys(Set)} instead
	 */
	@Deprecated
	public void setRenderedAttributes(Set<String> renderedAttributes) {
		this.modelKeys = renderedAttributes;
	}

	/**
	 * Return the attributes in the model that should be rendered by this view.
	 * @deprecated use {@link #getModelKeys()} instead
	 */
	@Deprecated
	public final Set<String> getRenderedAttributes() {
		return this.modelKeys;
	}

	/**
	 * Set whether to serialize models containing a single attribute as a map or whether to
	 * extract the single value from the model and serialize it directly.
	 * <p>The effect of setting this flag is similar to using {@code MappingJackson2HttpMessageConverter}
	 * with an {@code @ResponseBody} request-handling method.
	 * <p>Default is {@code false}.
	 */
	public void setExtractValueFromSingleKeyModel(boolean extractValueFromSingleKeyModel) {
		this.extractValueFromSingleKeyModel = extractValueFromSingleKeyModel;
	}

	/**
	 * Disables caching of the generated JSON.
	 * <p>Default is {@code true}, which will prevent the client from caching the generated JSON.
	 */
	public void setDisableCaching(boolean disableCaching) {
		this.disableCaching = disableCaching;
	}

	/**
	 * Whether to update the 'Content-Length' header of the response. When set to
	 * {@code true}, the response is buffered in order to determine the content
	 * length and set the 'Content-Length' header of the response.
	 * <p>The default setting is {@code false}.
	 */
	public void setUpdateContentLength(boolean updateContentLength) {
		this.updateContentLength = updateContentLength;
	}


	@Override
	protected void prepareResponse(HttpServletRequest request, HttpServletResponse response) {
		setResponseContentType(request, response);
		response.setCharacterEncoding(this.encoding.getJavaName());
		if (this.disableCaching) {
			response.addHeader("Pragma", "no-cache");
			response.addHeader("Cache-Control", "no-cache, no-store, max-age=0");
			response.addDateHeader("Expires", 1L);
		}
	}

	@Override
	protected void renderMergedOutputModel(Map<String, Object> model, HttpServletRequest request,
			HttpServletResponse response) throws Exception {

		OutputStream stream = (this.updateContentLength ? createTemporaryOutputStream() : response.getOutputStream());
		Object value = filterModel(model);
		writeContent(stream, value, this.jsonPrefix);
		if (this.updateContentLength) {
			writeToResponse(response, (ByteArrayOutputStream) stream);
		}
	}

	/**
	 * Filter out undesired attributes from the given model.
	 * The return value can be either another {@link Map} or a single value object.
	 * <p>The default implementation removes {@link BindingResult} instances and entries
	 * not included in the {@link #setRenderedAttributes renderedAttributes} property.
	 * @param model the model, as passed on to {@link #renderMergedOutputModel}
	 * @return the value to be rendered
	 */
	protected Object filterModel(Map<String, Object> model) {
		Map<String, Object> result = new HashMap<String, Object>(model.size());
		Set<String> renderedAttributes = (!CollectionUtils.isEmpty(this.modelKeys) ? this.modelKeys : model.keySet());
		for (Map.Entry<String, Object> entry : model.entrySet()) {
			if (!(entry.getValue() instanceof BindingResult) && renderedAttributes.contains(entry.getKey())) {
				result.put(entry.getKey(), entry.getValue());
			}
		}
		return (this.extractValueFromSingleKeyModel && result.size() == 1 ? result.values().iterator().next() : result);
	}

	/**
	 * Write the actual JSON content to the stream.
	 * @param stream the output stream to use
	 * @param value the value to be rendered, as returned from {@link #filterModel}
	 * @param jsonPrefix the prefix for this view's JSON output
	 * (as indicated through {@link #setJsonPrefix}/{@link #setPrefixJson})
	 * @throws IOException if writing failed
	 */
	protected void writeContent(OutputStream stream, Object value, String jsonPrefix) throws IOException {
		JsonGenerator generator = this.objectMapper.getJsonFactory().createJsonGenerator(stream, this.encoding);

		// A workaround for JsonGenerators not applying serialization features
		// https://github.com/FasterXML/jackson-databind/issues/12
		if (this.objectMapper.isEnabled(SerializationFeature.INDENT_OUTPUT)) {
			generator.useDefaultPrettyPrinter();
		}

		if (jsonPrefix != null) {
			generator.writeRaw(jsonPrefix);
		}
		this.objectMapper.writeValue(generator, value);
	}

}
