/*
 * Copyright 2002-2019 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
 *
 *      https://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.codec;

import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;
import java.nio.ByteBuffer;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.Collections;
import java.util.List;
import java.util.Map;

import org.reactivestreams.Publisher;
import reactor.core.publisher.Mono;

import org.springframework.core.ResolvableType;
import org.springframework.core.codec.Hints;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.core.log.LogFormatUtils;
import org.springframework.http.MediaType;
import org.springframework.http.ReactiveHttpOutputMessage;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.MultiValueMap;

HttpMessageWriter for writing a MultiValueMap<String, String> as HTML form data, i.e. "application/x-www-form-urlencoded", to the body of a request. 
Note that unless the media type is explicitly set to MediaType.APPLICATION_FORM_URLENCODED, the canWrite method will need generic type information to confirm the target map has String values. This is because a MultiValueMap with non-String values can be used to write multipart requests. 
To support both form data and multipart requests, consider using MultipartHttpMessageWriter configured with this writer as the fallback for writing plain form data. 
Author:
Sebastien Deleuze, Rossen Stoyanchev
See Also:
MultipartHttpMessageWriter
Since:
5.0/**
 * {@link HttpMessageWriter} for writing a {@code MultiValueMap<String, String>}
 * as HTML form data, i.e. {@code "application/x-www-form-urlencoded"}, to the
 * body of a request.
 *
 * <p>Note that unless the media type is explicitly set to
 * {@link MediaType#APPLICATION_FORM_URLENCODED}, the {@link #canWrite} method
 * will need generic type information to confirm the target map has String values.
 * This is because a MultiValueMap with non-String values can be used to write
 * multipart requests.
 *
 * <p>To support both form data and multipart requests, consider using
 * {@link org.springframework.http.codec.multipart.MultipartHttpMessageWriter}
 * configured with this writer as the fallback for writing plain form data.
 *
 * @author Sebastien Deleuze
 * @author Rossen Stoyanchev
 * @since 5.0
 * @see org.springframework.http.codec.multipart.MultipartHttpMessageWriter
 */
public class FormHttpMessageWriter extends LoggingCodecSupport
		implements HttpMessageWriter<MultiValueMap<String, String>> {

	
The default charset used by the writer.
/**
	 * The default charset used by the writer.
	 */
	public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

	private static final MediaType DEFAULT_FORM_DATA_MEDIA_TYPE =
			new MediaType(MediaType.APPLICATION_FORM_URLENCODED, DEFAULT_CHARSET);

	private static final List<MediaType> MEDIA_TYPES =
			Collections.singletonList(MediaType.APPLICATION_FORM_URLENCODED);

	private static final ResolvableType MULTIVALUE_TYPE =
			ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class);


	private Charset defaultCharset = DEFAULT_CHARSET;


	
Set the default character set to use for writing form data when the response
Content-Type header does not explicitly specify it.
By default this is set to "UTF-8".
/**
	 * Set the default character set to use for writing form data when the response
	 * Content-Type header does not explicitly specify it.
	 * <p>By default this is set to "UTF-8".
	 */
	public void setDefaultCharset(Charset charset) {
		Assert.notNull(charset, "Charset must not be null");
		this.defaultCharset = charset;
	}

	
Return the configured default charset.
/**
	 * Return the configured default charset.
	 */
	public Charset getDefaultCharset() {
		return this.defaultCharset;
	}


	@Override
	public List<MediaType> getWritableMediaTypes() {
		return MEDIA_TYPES;
	}


	@Override
	public boolean canWrite(ResolvableType elementType, @Nullable MediaType mediaType) {
		if (!MultiValueMap.class.isAssignableFrom(elementType.toClass())) {
			return false;
		}
		if (MediaType.APPLICATION_FORM_URLENCODED.isCompatibleWith(mediaType)) {
			// Optimistically, any MultiValueMap with or without generics
			return true;
		}
		if (mediaType == null) {
			// Only String-based MultiValueMap
			return MULTIVALUE_TYPE.isAssignableFrom(elementType);
		}
		return false;
	}

	@Override
	public Mono<Void> write(Publisher<? extends MultiValueMap<String, String>> inputStream,
			ResolvableType elementType, @Nullable MediaType mediaType, ReactiveHttpOutputMessage message,
			Map<String, Object> hints) {

		mediaType = getMediaType(mediaType);
		message.getHeaders().setContentType(mediaType);

		Charset charset = mediaType.getCharset() != null ? mediaType.getCharset() : getDefaultCharset();

		return Mono.from(inputStream).flatMap(form -> {
			logFormData(form, hints);
			String value = serializeForm(form, charset);
			ByteBuffer byteBuffer = charset.encode(value);
			DataBuffer buffer = message.bufferFactory().wrap(byteBuffer); // wrapping only, no allocation
			message.getHeaders().setContentLength(byteBuffer.remaining());
			return message.writeWith(Mono.just(buffer));
		});
	}

	protected MediaType getMediaType(@Nullable MediaType mediaType) {
		if (mediaType == null) {
			return DEFAULT_FORM_DATA_MEDIA_TYPE;
		}
		else if (mediaType.getCharset() == null) {
			return new MediaType(mediaType, getDefaultCharset());
		}
		else {
			return mediaType;
		}
	}

	private void logFormData(MultiValueMap<String, String> form, Map<String, Object> hints) {
		LogFormatUtils.traceDebug(logger, traceOn -> Hints.getLogPrefix(hints) + "Writing " +
				(isEnableLoggingRequestDetails() ?
						LogFormatUtils.formatValue(form, !traceOn) :
						"form fields " + form.keySet() + " (content masked)"));
	}

	protected String serializeForm(MultiValueMap<String, String> formData, Charset charset) {
		StringBuilder builder = new StringBuilder();
		formData.forEach((name, values) ->
				values.forEach(value -> {
					try {
						if (builder.length() != 0) {
							builder.append('&');
						}
						builder.append(URLEncoder.encode(name, charset.name()));
						if (value != null) {
							builder.append('=');
							builder.append(URLEncoder.encode(value, charset.name()));
						}
					}
					catch (UnsupportedEncodingException ex) {
						throw new IllegalStateException(ex);
					}
				}));
		return builder.toString();
	}

}