Add kotlinx.serialization JSON support to Spring MVC

Closes gh-21188

Co-authored-by: Sebastien Deleuze <sdeleuze@vmware.com>

spring-web/src/main/java/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverter.java

@@ -0,0 +1,154 @@
+/*
+ * Copyright 2002-2020 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.converter.json;
+
+import java.io.IOException;
+import java.lang.reflect.Type;
+import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
+import java.util.Map;
+
+import kotlinx.serialization.KSerializer;
+import kotlinx.serialization.SerializationException;
+import kotlinx.serialization.SerializersKt;
+import kotlinx.serialization.json.Json;
+
+import org.springframework.http.HttpInputMessage;
+import org.springframework.http.HttpOutputMessage;
+import org.springframework.http.MediaType;
+import org.springframework.http.converter.AbstractGenericHttpMessageConverter;
+import org.springframework.http.converter.HttpMessageNotReadableException;
+import org.springframework.http.converter.HttpMessageNotWritableException;
+import org.springframework.lang.Nullable;
+import org.springframework.util.ConcurrentReferenceHashMap;
+import org.springframework.util.StreamUtils;
+
+/**
+ * Implementation of {@link org.springframework.http.converter.HttpMessageConverter} that can read and write JSON using
+ * <a href="https://github.com/Kotlin/kotlinx.serialization">kotlinx.serialization</a>.
+ *
+ * <p>This converter can be used to bind {@code @Serializable} Kotlin classes. It supports {@code application/json} and
+ * {@code application/*+json} with various character sets, {@code UTF-8} being the default.
+ *
+ * @author Andreas Ahlenstorf
+ * @author Sebastien Deleuze
+ * @since 5.3
+ */
+public class KotlinSerializationJsonHttpMessageConverter extends AbstractGenericHttpMessageConverter<Object> {
+
+	private static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;
+
+	private static final Map<Type, KSerializer<Object>> serializerCache = new ConcurrentReferenceHashMap<>();
+
+	private final Json json;
+
+	/**
+	 * Construct a new {@code KotlinSerializationJsonHttpMessageConverter} with the default configuration.
+	 */
+	public KotlinSerializationJsonHttpMessageConverter() {
+		this(Json.Default);
+	}
+
+	/**
+	 * Construct a new {@code KotlinSerializationJsonHttpMessageConverter} with a custom configuration.
+	 */
+	public KotlinSerializationJsonHttpMessageConverter(Json json) {
+		super(MediaType.APPLICATION_JSON, new MediaType("application", "*+json"));
+		this.json = json;
+	}
+
+	@Override
+	protected boolean supports(Class<?> clazz) {
+		try {
+			resolve(clazz);
+			return true;
+		}
+		catch (Exception ex) {
+			return false;
+		}
+	}
+
+	@Override
+	protected Object readInternal(Class<?> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
+		return this.read(clazz, null, inputMessage);
+	}
+
+	@Override
+	public Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
+		MediaType contentType = inputMessage.getHeaders().getContentType();
+		String jsonText = StreamUtils.copyToString(inputMessage.getBody(), getCharsetToUse(contentType));
+		try {
+			// TODO Use stream based API when available
+			return this.json.decodeFromString(resolve(type), jsonText);
+		}
+		catch (SerializationException ex) {
+			throw new HttpMessageNotReadableException("Could not read JSON: " + ex.getMessage(), ex, inputMessage);
+		}
+	}
+
+	@Override
+	protected void writeInternal(Object o, HttpOutputMessage outputMessage) throws HttpMessageNotWritableException {
+		try {
+			this.writeInternal(o, o.getClass(), outputMessage);
+		}
+		catch (IOException ex) {
+			throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getMessage(), ex);
+		}
+	}
+
+	@Override
+	protected void writeInternal(Object o, @Nullable Type type, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
+		try {
+			String json = this.json.encodeToString(resolve(type), o);
+			MediaType contentType = outputMessage.getHeaders().getContentType();
+			outputMessage.getBody().write(json.getBytes(getCharsetToUse(contentType)));
+			outputMessage.getBody().flush();
+		}
+		catch (IOException ex) {
+			throw ex;
+		}
+		catch (Exception ex) {
+			throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getMessage(), ex);
+		}
+	}
+
+	private Charset getCharsetToUse(@Nullable MediaType contentType) {
+		if (contentType != null && contentType.getCharset() != null) {
+			return contentType.getCharset();
+		}
+		return DEFAULT_CHARSET;
+	}
+
+	/**
+	 * Tries to find a serializer that can marshall or unmarshall instances of the given type using
+	 * kotlinx.serialization. If no serializer can be found, an exception is thrown.
+	 * <p>
+	 * Resolved serializers are cached and cached results are returned on successive calls.
+	 *
+	 * @param type to find a serializer for.
+	 * @return resolved serializer for the given type.
+	 * @throws RuntimeException if no serializer supporting the given type can be found.
+	 */
+	private KSerializer<Object> resolve(Type type) {
+		KSerializer<Object> serializer = serializerCache.get(type);
+		if (serializer == null) {
+			serializer = SerializersKt.serializer(type);
+			serializerCache.put(type, serializer);
+		}
+		return serializer;
+	}
+}

spring-web/src/main/java/org/springframework/http/converter/support/AllEncompassingFormHttpMessageConverter.java

@@ -20,6 +20,7 @@ import org.springframework.core.SpringProperties;
 import org.springframework.http.converter.FormHttpMessageConverter;
 import org.springframework.http.converter.json.GsonHttpMessageConverter;
 import org.springframework.http.converter.json.JsonbHttpMessageConverter;
+import org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverter;
 import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
 import org.springframework.http.converter.smile.MappingJackson2SmileHttpMessageConverter;
 import org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter;
@@ -57,6 +58,8 @@ public class AllEncompassingFormHttpMessageConverter extends FormHttpMessageConv
 
 	private static final boolean jsonbPresent;
 
+	private static final boolean kotlinSerializationJsonPresent;
+
 	static {
 		ClassLoader classLoader = AllEncompassingFormHttpMessageConverter.class.getClassLoader();
 		jaxb2Present = ClassUtils.isPresent("javax.xml.bind.Binder", classLoader);
@@ -66,6 +69,7 @@ public class AllEncompassingFormHttpMessageConverter extends FormHttpMessageConv
 		jackson2SmilePresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.smile.SmileFactory", classLoader);
 		gsonPresent = ClassUtils.isPresent("com.google.gson.Gson", classLoader);
 		jsonbPresent = ClassUtils.isPresent("javax.json.bind.Jsonb", classLoader);
+		kotlinSerializationJsonPresent = ClassUtils.isPresent("kotlinx.serialization.json.Json", classLoader);
 	}
 
 
@@ -92,6 +96,9 @@ public class AllEncompassingFormHttpMessageConverter extends FormHttpMessageConv
 		else if (jsonbPresent) {
 			addPartConverter(new JsonbHttpMessageConverter());
 		}
+		else if (kotlinSerializationJsonPresent) {
+			addPartConverter(new KotlinSerializationJsonHttpMessageConverter());
+		}
 
 		if (jackson2XmlPresent && !shouldIgnoreXml) {
 			addPartConverter(new MappingJackson2XmlHttpMessageConverter());

spring-web/src/main/java/org/springframework/web/client/RestTemplate.java

@@ -50,6 +50,7 @@ import org.springframework.http.converter.feed.AtomFeedHttpMessageConverter;
 import org.springframework.http.converter.feed.RssChannelHttpMessageConverter;
 import org.springframework.http.converter.json.GsonHttpMessageConverter;
 import org.springframework.http.converter.json.JsonbHttpMessageConverter;
+import org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverter;
 import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
 import org.springframework.http.converter.smile.MappingJackson2SmileHttpMessageConverter;
 import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;
@@ -114,6 +115,8 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 
 	private static final boolean jsonbPresent;
 
+	private static final boolean kotlinSerializationJsonPresent;
+
 	static {
 		ClassLoader classLoader = RestTemplate.class.getClassLoader();
 		romePresent = ClassUtils.isPresent("com.rometools.rome.feed.WireFeed", classLoader);
@@ -126,6 +129,7 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 		jackson2CborPresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.cbor.CBORFactory", classLoader);
 		gsonPresent = ClassUtils.isPresent("com.google.gson.Gson", classLoader);
 		jsonbPresent = ClassUtils.isPresent("javax.json.bind.Jsonb", classLoader);
+		kotlinSerializationJsonPresent = ClassUtils.isPresent("kotlinx.serialization.json.Json", classLoader);
 	}
 
 
@@ -179,6 +183,9 @@ public class RestTemplate extends InterceptingHttpAccessor implements RestOperat
 		else if (jsonbPresent) {
 			this.messageConverters.add(new JsonbHttpMessageConverter());
 		}
+		else if (kotlinSerializationJsonPresent) {
+			this.messageConverters.add(new KotlinSerializationJsonHttpMessageConverter());
+		}
 
 		if (jackson2SmilePresent) {
 			this.messageConverters.add(new MappingJackson2SmileHttpMessageConverter());