Allow operations to produce different output

Update the actuator @Enpoint` infrastructure code so that operations
may inject enums that indicate the type of output to produce. A new
`Producible` interface can be implemented by any enum that indicates
the mime-type that an enum value produces.

The new `OperationArgumentResolver` provides a general strategy for
resolving operation arguments with `ProducibleOperationArgumentResolver`
providing support for `Producible` enums. Existing injection support has
been refactored to use the new resolver.

See gh-25738

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/InvocationContext.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -16,7 +16,12 @@
 
 package org.springframework.boot.actuate.endpoint;
 
+import java.security.Principal;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
 import java.util.Map;
+import java.util.function.Supplier;
 
 import org.springframework.boot.actuate.endpoint.http.ApiVersion;
 import org.springframework.boot.actuate.endpoint.invoke.OperationInvoker;
@@ -31,11 +36,9 @@ import org.springframework.util.Assert;
  */
 public class InvocationContext {
 
-	private final SecurityContext securityContext;
-
 	private final Map<String, Object> arguments;
 
-	private final ApiVersion apiVersion;
+	private final List<OperationArgumentResolver> argumentResolvers;
 
 	/**
 	 * Creates a new context for an operation being invoked by the given
@@ -54,13 +57,34 @@ public class InvocationContext {
 	 * @param securityContext the current security context. Never {@code null}
 	 * @param arguments the arguments available to the operation. Never {@code null}
 	 * @since 2.2.0
+	 * @deprecated since 2.5.0 in favor of
+	 * {@link #InvocationContext(SecurityContext, Map, List)}
 	 */
+	@Deprecated
 	public InvocationContext(ApiVersion apiVersion, SecurityContext securityContext, Map<String, Object> arguments) {
+		this(securityContext, arguments, Arrays.asList(new FixedValueArgumentResolver<>(ApiVersion.class, apiVersion)));
+	}
+
+	/**
+	 * Creates a new context for an operation being invoked by the given
+	 * {@code securityContext} with the given available {@code arguments}.
+	 * @param securityContext the current security context. Never {@code null}
+	 * @param arguments the arguments available to the operation. Never {@code null}
+	 * @param argumentResolvers resolvers for additional arguments should be available to
+	 * the operation.
+	 */
+	public InvocationContext(SecurityContext securityContext, Map<String, Object> arguments,
+			List<OperationArgumentResolver> argumentResolvers) {
 		Assert.notNull(securityContext, "SecurityContext must not be null");
 		Assert.notNull(arguments, "Arguments must not be null");
-		this.apiVersion = (apiVersion != null) ? apiVersion : ApiVersion.LATEST;
-		this.securityContext = securityContext;
 		this.arguments = arguments;
+		this.argumentResolvers = new ArrayList<>();
+		if (argumentResolvers != null) {
+			this.argumentResolvers.addAll(argumentResolvers);
+		}
+		this.argumentResolvers.add(new FixedValueArgumentResolver<>(SecurityContext.class, securityContext));
+		this.argumentResolvers.add(new SuppliedValueArgumentResolver<>(Principal.class, securityContext::getPrincipal));
+		this.argumentResolvers.add(new FixedValueArgumentResolver<>(ApiVersion.class, ApiVersion.LATEST));
 	}
 
 	/**
@@ -69,15 +93,17 @@ public class InvocationContext {
 	 * @since 2.2.0
 	 */
 	public ApiVersion getApiVersion() {
-		return this.apiVersion;
+		return resolveArgument(ApiVersion.class);
 	}
 
 	/**
 	 * Return the security context to use for the invocation.
 	 * @return the security context
+	 * @deprecated since 2.5.0 in favor of {@link #resolveArgument(Class)}
 	 */
+	@Deprecated
 	public SecurityContext getSecurityContext() {
-		return this.securityContext;
+		return resolveArgument(SecurityContext.class);
 	}
 
 	/**
@@ -88,4 +114,92 @@ public class InvocationContext {
 		return this.arguments;
 	}
 
+	/**
+	 * Resolves an argument with the given {@code argumentType}.
+	 * @param <T> type of the argument
+	 * @param argumentType type of the argument
+	 * @return resolved argument of the required type or {@code null}
+	 * @since 2.5.0
+	 * @see #canResolve(Class)
+	 */
+	public <T> T resolveArgument(Class<T> argumentType) {
+		for (OperationArgumentResolver argumentResolver : this.argumentResolvers) {
+			if (argumentResolver.canResolve(argumentType)) {
+				T result = argumentResolver.resolve(argumentType);
+				if (result != null) {
+					return result;
+				}
+			}
+		}
+		return null;
+	}
+
+	/**
+	 * Returns whether or not the context is capable of resolving an argument of the given
+	 * {@code type}. Note that, even when {@code true} is returned,
+	 * {@link #resolveArgument argument resolution} will return {@code null} if no
+	 * argument of the required type is available.
+	 * @param type argument type
+	 * @return {@code true} if resolution of arguments of the given type is possible,
+	 * otherwise {@code false}.
+	 * @since 2.5.0
+	 * @see #resolveArgument(Class)
+	 */
+	public boolean canResolve(Class<?> type) {
+		for (OperationArgumentResolver argumentResolver : this.argumentResolvers) {
+			if (argumentResolver.canResolve(type)) {
+				return true;
+			}
+		}
+		return false;
+	}
+
+	private static final class FixedValueArgumentResolver<T> implements OperationArgumentResolver {
+
+		private final Class<T> argumentType;
+
+		private final T value;
+
+		private FixedValueArgumentResolver(Class<T> argumentType, T value) {
+			this.argumentType = argumentType;
+			this.value = value;
+		}
+
+		@SuppressWarnings("unchecked")
+		@Override
+		public <U> U resolve(Class<U> type) {
+			return (U) this.value;
+		}
+
+		@Override
+		public boolean canResolve(Class<?> type) {
+			return this.argumentType.equals(type);
+		}
+
+	}
+
+	private static final class SuppliedValueArgumentResolver<T> implements OperationArgumentResolver {
+
+		private final Class<T> argumentType;
+
+		private final Supplier<T> value;
+
+		private SuppliedValueArgumentResolver(Class<T> argumentType, Supplier<T> value) {
+			this.argumentType = argumentType;
+			this.value = value;
+		}
+
+		@SuppressWarnings("unchecked")
+		@Override
+		public <U> U resolve(Class<U> type) {
+			return (U) this.value.get();
+		}
+
+		@Override
+		public boolean canResolve(Class<?> type) {
+			return this.argumentType.equals(type);
+		}
+
+	}
+
 }

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/OperationArgumentResolver.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2012-2021 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.boot.actuate.endpoint;
+
+/**
+ * Resolver for an argument of an {@link Operation}.
+ *
+ * @author Andy Wilkinson
+ * @since 2.5.0
+ */
+public interface OperationArgumentResolver {
+
+	/**
+	 * Resolves an argument of the given {@code type}.
+	 * @param <T> required type of the argument
+	 * @param type argument type
+	 * @return an argument of the required type, or {@code null}
+	 */
+	<T> T resolve(Class<T> type);
+
+	/**
+	 * Return whether an argument of the given {@code type} can be resolved.
+	 * @param type argument type
+	 * @return {@code true} if an argument of the required type can be resolved, otherwise
+	 * {@code false}
+	 */
+	boolean canResolve(Class<?> type);
+
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/http/ApiVersion.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -20,6 +20,7 @@ import java.util.List;
 import java.util.Map;
 
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.MimeType;
 import org.springframework.util.MimeTypeUtils;
 
 /**
@@ -29,17 +30,17 @@ import org.springframework.util.MimeTypeUtils;
  * @author Phillip Webb
  * @since 2.2.0
  */
-public enum ApiVersion {
+public enum ApiVersion implements Producible<ApiVersion> {
 
 	/**
 	 * Version 2 (supported by Spring Boot 2.0+).
 	 */
-	V2,
+	V2(ActuatorMediaType.V2_JSON),
 
 	/**
 	 * Version 3 (supported by Spring Boot 2.2+).
 	 */
-	V3;
+	V3(ActuatorMediaType.V3_JSON);
 
 	private static final String MEDIA_TYPE_PREFIX = "application/vnd.spring-boot.actuator.";
 
@@ -87,4 +88,15 @@ public enum ApiVersion {
 		return (candidateOrdinal > existingOrdinal) ? candidate : existing;
 	}
 
+	private final MimeType mimeType;
+
+	ApiVersion(String mimeType) {
+		this.mimeType = MimeTypeUtils.parseMimeType(mimeType);
+	}
+
+	@Override
+	public MimeType getMimeType() {
+		return this.mimeType;
+	}
+
 }

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/http/Producible.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2012-2021 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.boot.actuate.endpoint.http;
+
+import org.springframework.util.MimeType;
+
+/**
+ * Interface to be implemented by an {@link Enum} that can be injected into an operation
+ * on a web endpoint. The value of the {@code Producible} enum is resolved using the
+ * {@code Accept} header of the request. When multiple values are equally acceptable, the
+ * value with the highest {@link Enum#ordinal() ordinal} is used.
+ *
+ * @param <E> enum type that implements this interface
+ * @author Andy Wilkinson
+ * @since 2.5.0
+ */
+public interface Producible<E extends Enum<E> & Producible<E>> {
+
+	/**
+	 * Mime type that can be produced.
+	 * @return the producible mime type
+	 */
+	MimeType getMimeType();
+
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/http/ProducibleOperationArgumentResolver.java

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2012-2021 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.boot.actuate.endpoint.http;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
+import org.springframework.boot.actuate.endpoint.OperationArgumentResolver;
+import org.springframework.util.CollectionUtils;
+import org.springframework.util.MimeType;
+import org.springframework.util.MimeTypeUtils;
+
+/**
+ * An {@link OperationArgumentResolver} for {@link Producible producible enums}.
+ *
+ * @author Andy Wilkinson
+ * @since 2.5.0
+ */
+public class ProducibleOperationArgumentResolver implements OperationArgumentResolver {
+
+	private final Map<String, List<String>> httpHeaders;
+
+	public ProducibleOperationArgumentResolver(Map<String, List<String>> httpHeaders) {
+		this.httpHeaders = httpHeaders;
+	}
+
+	@Override
+	public boolean canResolve(Class<?> type) {
+		return Producible.class.isAssignableFrom(type) && Enum.class.isAssignableFrom(type);
+	}
+
+	@SuppressWarnings("unchecked")
+	@Override
+	public <T> T resolve(Class<T> type) {
+		return (T) resolveProducible((Class<Enum<? extends Producible<?>>>) type);
+	}
+
+	private Enum<? extends Producible<?>> resolveProducible(Class<Enum<? extends Producible<?>>> type) {
+		List<String> accepts = this.httpHeaders.get("Accept");
+		List<Enum<? extends Producible<?>>> values = Arrays.asList(type.getEnumConstants());
+		Collections.reverse(values);
+		if (CollectionUtils.isEmpty(accepts)) {
+			return values.get(0);
+		}
+		Enum<? extends Producible<?>> result = null;
+		for (String accept : accepts) {
+			for (String mimeType : MimeTypeUtils.tokenize(accept)) {
+				result = mostRecent(result, forType(values, MimeTypeUtils.parseMimeType(mimeType)));
+			}
+		}
+		return result;
+	}
+
+	private static Enum<? extends Producible<?>> mostRecent(Enum<? extends Producible<?>> existing,
+			Enum<? extends Producible<?>> candidate) {
+		int existingOrdinal = (existing != null) ? ((Enum<?>) existing).ordinal() : -1;
+		int candidateOrdinal = (candidate != null) ? ((Enum<?>) candidate).ordinal() : -1;
+		return (candidateOrdinal > existingOrdinal) ? candidate : existing;
+	}
+
+	private static Enum<? extends Producible<?>> forType(List<Enum<? extends Producible<?>>> candidates,
+			MimeType mimeType) {
+		for (Enum<? extends Producible<?>> candidate : candidates) {
+			if (mimeType.isCompatibleWith(((Producible<?>) candidate).getMimeType())) {
+				return candidate;
+			}
+		}
+		return null;
+	}
+
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/invoke/reflect/ReflectiveOperationInvoker.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -17,13 +17,10 @@
 package org.springframework.boot.actuate.endpoint.invoke.reflect;
 
 import java.lang.reflect.Method;
-import java.security.Principal;
 import java.util.Set;
 import java.util.stream.Collectors;
 
 import org.springframework.boot.actuate.endpoint.InvocationContext;
-import org.springframework.boot.actuate.endpoint.SecurityContext;
-import org.springframework.boot.actuate.endpoint.http.ApiVersion;
 import org.springframework.boot.actuate.endpoint.invoke.MissingParametersException;
 import org.springframework.boot.actuate.endpoint.invoke.OperationInvoker;
 import org.springframework.boot.actuate.endpoint.invoke.OperationParameter;
@@ -89,13 +86,7 @@ public class ReflectiveOperationInvoker implements OperationInvoker {
 		if (!parameter.isMandatory()) {
 			return false;
 		}
-		if (ApiVersion.class.equals(parameter.getType())) {
-			return false;
-		}
-		if (Principal.class.equals(parameter.getType())) {
-			return context.getSecurityContext().getPrincipal() == null;
-		}
-		if (SecurityContext.class.equals(parameter.getType())) {
+		if (context.canResolve(parameter.getType())) {
 			return false;
 		}
 		return context.getArguments().get(parameter.getName()) == null;
@@ -107,14 +98,9 @@ public class ReflectiveOperationInvoker implements OperationInvoker {
 	}
 
 	private Object resolveArgument(OperationParameter parameter, InvocationContext context) {
-		if (ApiVersion.class.equals(parameter.getType())) {
-			return context.getApiVersion();
-		}
-		if (Principal.class.equals(parameter.getType())) {
-			return context.getSecurityContext().getPrincipal();
-		}
-		if (SecurityContext.class.equals(parameter.getType())) {
-			return context.getSecurityContext();
+		Object resolvedByType = context.resolveArgument(parameter.getType());
+		if (resolvedByType != null) {
+			return resolvedByType;
 		}
 		Object value = context.getArguments().get(parameter.getName());
 		return this.parameterValueMapper.mapParameterValue(parameter, value);

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/invoker/cache/CachingOperationInvoker.java

@@ -79,7 +79,8 @@ public class CachingOperationInvoker implements OperationInvoker {
 		}
 		long accessTime = System.currentTimeMillis();
 		ApiVersion contextApiVersion = context.getApiVersion();
-		CacheKey cacheKey = new CacheKey(contextApiVersion, context.getSecurityContext().getPrincipal());
+		Principal principal = context.resolveArgument(Principal.class);
+		CacheKey cacheKey = new CacheKey(contextApiVersion, principal);
 		CachedResponse cached = this.cachedResponses.get(cacheKey);
 		if (cached == null || cached.isStale(accessTime, this.timeToLive)) {
 			Object response = this.invoker.invoke(context);

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/web/WebEndpointResponse.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -17,6 +17,7 @@
 package org.springframework.boot.actuate.endpoint.web;
 
 import org.springframework.boot.actuate.endpoint.web.annotation.EndpointWebExtension;
+import org.springframework.util.MimeType;
 
 /**
  * A {@code WebEndpointResponse} can be returned by an operation on a
@@ -70,6 +71,8 @@ public final class WebEndpointResponse<T> {
 
 	private final int status;
 
+	private final MimeType contentType;
+
 	/**
 	 * Creates a new {@code WebEndpointResponse} with no body and a 200 (OK) status.
 	 */
@@ -87,7 +90,7 @@ public final class WebEndpointResponse<T> {
 	}
 
 	/**
-	 * Creates a new {@code WebEndpointResponse} with then given body and a 200 (OK)
+	 * Creates a new {@code WebEndpointResponse} with the given body and a 200 (OK)
 	 * status.
 	 * @param body the body
 	 */
@@ -96,13 +99,44 @@ public final class WebEndpointResponse<T> {
 	}
 
 	/**
-	 * Creates a new {@code WebEndpointResponse} with then given body and status.
+	 * Creates a new {@code WebEndpointResponse} with the given body and content type and
+	 * a 200 (OK) status.
+	 * @param body the body
+	 * @param contentType the content type of the response
+	 * @since 2.5.0
+	 */
+	public WebEndpointResponse(T body, MimeType contentType) {
+		this(body, STATUS_OK, contentType);
+	}
+
+	/**
+	 * Creates a new {@code WebEndpointResponse} with the given body and status.
 	 * @param body the body
 	 * @param status the HTTP status
 	 */
 	public WebEndpointResponse(T body, int status) {
+		this(body, status, null);
+	}
+
+	/**
+	 * Creates a new {@code WebEndpointResponse} with the given body and status.
+	 * @param body the body
+	 * @param status the HTTP status
+	 * @param contentType the content type of the response
+	 * @since 2.5.0
+	 */
+	public WebEndpointResponse(T body, int status, MimeType contentType) {
 		this.body = body;
 		this.status = status;
+		this.contentType = contentType;
+	}
+
+	/**
+	 * Returns the content type of the response.
+	 * @return the content type;
+	 */
+	public MimeType getContentType() {
+		return this.contentType;
 	}
 
 	/**

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/web/jersey/JerseyEndpointResourceFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -21,6 +21,7 @@ import java.io.InputStream;
 import java.nio.charset.StandardCharsets;
 import java.security.Principal;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
@@ -43,7 +44,7 @@ import reactor.core.publisher.Mono;
 import org.springframework.boot.actuate.endpoint.InvalidEndpointRequestException;
 import org.springframework.boot.actuate.endpoint.InvocationContext;
 import org.springframework.boot.actuate.endpoint.SecurityContext;
-import org.springframework.boot.actuate.endpoint.http.ApiVersion;
+import org.springframework.boot.actuate.endpoint.http.ProducibleOperationArgumentResolver;
 import org.springframework.boot.actuate.endpoint.web.EndpointLinksResolver;
 import org.springframework.boot.actuate.endpoint.web.EndpointMapping;
 import org.springframework.boot.actuate.endpoint.web.EndpointMediaTypes;
@@ -151,9 +152,9 @@ public class JerseyEndpointResourceFactory {
 			arguments.putAll(extractPathParameters(data));
 			arguments.putAll(extractQueryParameters(data));
 			try {
-				ApiVersion apiVersion = ApiVersion.fromHttpHeaders(data.getHeaders());
 				JerseySecurityContext securityContext = new JerseySecurityContext(data.getSecurityContext());
-				InvocationContext invocationContext = new InvocationContext(apiVersion, securityContext, arguments);
+				InvocationContext invocationContext = new InvocationContext(securityContext, arguments,
+						Arrays.asList(new ProducibleOperationArgumentResolver(data.getHeaders())));
 				Object response = this.operation.invoke(invocationContext);
 				return convertToJaxRsResponse(response, data.getRequest().getMethod());
 			}
@@ -215,6 +216,7 @@ public class JerseyEndpointResourceFactory {
 				}
 				WebEndpointResponse<?> webEndpointResponse = (WebEndpointResponse<?>) response;
 				return Response.status(webEndpointResponse.getStatus())
+						.header("Content-Type", webEndpointResponse.getContentType())
 						.entity(convertIfNecessary(webEndpointResponse.getBody())).build();
 			}
 			catch (IOException ex) {

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/web/reactive/AbstractWebFluxEndpointHandlerMapping.java

@@ -19,6 +19,7 @@ package org.springframework.boot.actuate.endpoint.web.reactive;
 import java.lang.reflect.Method;
 import java.nio.charset.StandardCharsets;
 import java.security.Principal;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedHashMap;
@@ -33,7 +34,7 @@ import org.springframework.boot.actuate.endpoint.InvalidEndpointRequestException
 import org.springframework.boot.actuate.endpoint.InvocationContext;
 import org.springframework.boot.actuate.endpoint.OperationType;
 import org.springframework.boot.actuate.endpoint.SecurityContext;
-import org.springframework.boot.actuate.endpoint.http.ApiVersion;
+import org.springframework.boot.actuate.endpoint.http.ProducibleOperationArgumentResolver;
 import org.springframework.boot.actuate.endpoint.invoke.OperationInvoker;
 import org.springframework.boot.actuate.endpoint.web.EndpointMapping;
 import org.springframework.boot.actuate.endpoint.web.EndpointMediaTypes;
@@ -43,6 +44,7 @@ import org.springframework.boot.actuate.endpoint.web.WebOperation;
 import org.springframework.boot.actuate.endpoint.web.WebOperationRequestPredicate;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.security.access.AccessDecisionVoter;
 import org.springframework.security.access.SecurityConfig;
@@ -297,7 +299,6 @@ public abstract class AbstractWebFluxEndpointHandlerMapping extends RequestMappi
 
 		@Override
 		public Mono<ResponseEntity<Object>> handle(ServerWebExchange exchange, Map<String, String> body) {
-			ApiVersion apiVersion = ApiVersion.fromHttpHeaders(exchange.getRequest().getHeaders());
 			Map<String, Object> arguments = getArguments(exchange, body);
 			String matchAllRemainingPathSegmentsVariable = this.operation.getRequestPredicate()
 					.getMatchAllRemainingPathSegmentsVariable();
@@ -306,7 +307,8 @@ public abstract class AbstractWebFluxEndpointHandlerMapping extends RequestMappi
 						tokenizePathSegments((String) arguments.get(matchAllRemainingPathSegmentsVariable)));
 			}
 			return this.securityContextSupplier.get()
-					.map((securityContext) -> new InvocationContext(apiVersion, securityContext, arguments))
+					.map((securityContext) -> new InvocationContext(securityContext, arguments,
+							Arrays.asList(new ProducibleOperationArgumentResolver(exchange.getRequest().getHeaders()))))
 					.flatMap((invocationContext) -> handleResult((Publisher<?>) this.invoker.invoke(invocationContext),
 							exchange.getRequest().getMethod()));
 		}
@@ -348,7 +350,10 @@ public abstract class AbstractWebFluxEndpointHandlerMapping extends RequestMappi
 				return new ResponseEntity<>(response, HttpStatus.OK);
 			}
 			WebEndpointResponse<?> webEndpointResponse = (WebEndpointResponse<?>) response;
-			return ResponseEntity.status(webEndpointResponse.getStatus()).body(webEndpointResponse.getBody());
+			MediaType contentType = (webEndpointResponse.getContentType() != null)
+					? new MediaType(webEndpointResponse.getContentType()) : null;
+			return ResponseEntity.status(webEndpointResponse.getStatus()).contentType(contentType)
+					.body(webEndpointResponse.getBody());
 		}
 
 		@Override

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/web/servlet/AbstractWebMvcEndpointHandlerMapping.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -33,7 +33,7 @@ import org.springframework.beans.factory.InitializingBean;
 import org.springframework.boot.actuate.endpoint.InvalidEndpointRequestException;
 import org.springframework.boot.actuate.endpoint.InvocationContext;
 import org.springframework.boot.actuate.endpoint.SecurityContext;
-import org.springframework.boot.actuate.endpoint.http.ApiVersion;
+import org.springframework.boot.actuate.endpoint.http.ProducibleOperationArgumentResolver;
 import org.springframework.boot.actuate.endpoint.invoke.OperationInvoker;
 import org.springframework.boot.actuate.endpoint.web.EndpointMapping;
 import org.springframework.boot.actuate.endpoint.web.EndpointMediaTypes;
@@ -44,6 +44,7 @@ import org.springframework.boot.actuate.endpoint.web.WebOperationRequestPredicat
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.server.ServletServerHttpRequest;
 import org.springframework.util.AntPathMatcher;
@@ -284,9 +285,9 @@ public abstract class AbstractWebMvcEndpointHandlerMapping extends RequestMappin
 			HttpHeaders headers = new ServletServerHttpRequest(request).getHeaders();
 			Map<String, Object> arguments = getArguments(request, body);
 			try {
-				ApiVersion apiVersion = ApiVersion.fromHttpHeaders(headers);
 				ServletSecurityContext securityContext = new ServletSecurityContext(request);
-				InvocationContext invocationContext = new InvocationContext(apiVersion, securityContext, arguments);
+				InvocationContext invocationContext = new InvocationContext(securityContext, arguments,
+						Arrays.asList(new ProducibleOperationArgumentResolver(headers)));
 				return handleResult(this.operation.invoke(invocationContext), HttpMethod.resolve(request.getMethod()));
 			}
 			catch (InvalidEndpointRequestException ex) {
@@ -352,7 +353,9 @@ public abstract class AbstractWebMvcEndpointHandlerMapping extends RequestMappin
 				return result;
 			}
 			WebEndpointResponse<?> response = (WebEndpointResponse<?>) result;
-			return ResponseEntity.status(response.getStatus()).body(response.getBody());
+			MediaType contentType = (response.getContentType() != null) ? new MediaType(response.getContentType())
+					: null;
+			return ResponseEntity.status(response.getStatus()).contentType(contentType).body(response.getBody());
 		}
 
 	}

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/InvocationContextTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -39,11 +39,24 @@ class InvocationContextTests {
 	private final Map<String, Object> arguments = Collections.singletonMap("test", "value");
 
 	@Test
+	@SuppressWarnings("deprecation")
 	void createWhenApiVersionIsNullUsesLatestVersion() {
 		InvocationContext context = new InvocationContext(null, this.securityContext, this.arguments);
 		assertThat(context.getApiVersion()).isEqualTo(ApiVersion.LATEST);
 	}
 
+	@Test
+	void whenCreatedWithoutApiVersionThenGetApiVersionReturnsLatestVersion() {
+		InvocationContext context = new InvocationContext(this.securityContext, this.arguments);
+		assertThat(context.getApiVersion()).isEqualTo(ApiVersion.LATEST);
+	}
+
+	@Test
+	void whenCreatedWithoutApiVersionThenResolveApiVersionReturnsLatestVersion() {
+		InvocationContext context = new InvocationContext(this.securityContext, this.arguments);
+		assertThat(context.resolveArgument(ApiVersion.class)).isEqualTo(ApiVersion.LATEST);
+	}
+
 	@Test
 	void createWhenSecurityContextIsNullThrowsException() {
 		assertThatIllegalArgumentException().isThrownBy(() -> new InvocationContext(null, this.arguments))
@@ -57,17 +70,25 @@ class InvocationContextTests {
 	}
 
 	@Test
+	@SuppressWarnings("deprecation")
 	void getApiVersionReturnsApiVersion() {
 		InvocationContext context = new InvocationContext(ApiVersion.V2, this.securityContext, this.arguments);
 		assertThat(context.getApiVersion()).isEqualTo(ApiVersion.V2);
 	}
 
 	@Test
+	@SuppressWarnings("deprecation")
 	void getSecurityContextReturnsSecurityContext() {
 		InvocationContext context = new InvocationContext(this.securityContext, this.arguments);
 		assertThat(context.getSecurityContext()).isEqualTo(this.securityContext);
 	}
 
+	@Test
+	void resolveSecurityContextReturnsSecurityContext() {
+		InvocationContext context = new InvocationContext(this.securityContext, this.arguments);
+		assertThat(context.resolveArgument(SecurityContext.class)).isEqualTo(this.securityContext);
+	}
+
 	@Test
 	void getArgumentsReturnsArguments() {
 		InvocationContext context = new InvocationContext(this.securityContext, this.arguments);

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/http/ApiVersionTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -82,6 +82,12 @@ class ApiVersionTests {
 		assertThat(version).isEqualTo(ApiVersion.V3);
 	}
 
+	@Test
+	void fromHttpHeadersWhenAcceptsEverythingReturnsLatest() {
+		ApiVersion version = ApiVersion.fromHttpHeaders(acceptHeader("*/*"));
+		assertThat(version).isEqualTo(ApiVersion.V3);
+	}
+
 	private Map<String, List<String>> acceptHeader(String... types) {
 		List<String> value = Arrays.asList(types);
 		return value.isEmpty() ? Collections.emptyMap() : Collections.singletonMap("Accept", value);

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/http/ProducibleOperationArgumentResolverTests.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2012-2021 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.boot.actuate.endpoint.http;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Test for {@link ProducibleOperationArgumentResolver}.
+ *
+ * @author Andy Wilkinson
+ */
+class ProducibleOperationArgumentResolverTests {
+
+	@Test
+	void whenAcceptHeaderIsEmptyThenHighestOrdinalIsReturned() {
+		assertThat(resolve(acceptHeader())).isEqualTo(ApiVersion.V3);
+	}
+
+	@Test
+	void whenEverythingIsAcceptableThenHighestOrdinalIsReturned() {
+		assertThat(resolve(acceptHeader("*/*"))).isEqualTo(ApiVersion.V3);
+	}
+
+	@Test
+	void whenNothingIsAcceptableThenNullIsReturned() {
+		assertThat(resolve(acceptHeader("image/png"))).isEqualTo(null);
+	}
+
+	@Test
+	void whenSingleValueIsAcceptableThenMatchingEnumValueIsReturned() {
+		assertThat(new ProducibleOperationArgumentResolver(acceptHeader(ActuatorMediaType.V2_JSON))
+				.resolve(ApiVersion.class)).isEqualTo(ApiVersion.V2);
+		assertThat(new ProducibleOperationArgumentResolver(acceptHeader(ActuatorMediaType.V3_JSON))
+				.resolve(ApiVersion.class)).isEqualTo(ApiVersion.V3);
+	}
+
+	@Test
+	void whenMultipleValuesAreAcceptableThenHighestOrdinalIsReturned() {
+		assertThat(resolve(acceptHeader(ActuatorMediaType.V2_JSON, ActuatorMediaType.V3_JSON)))
+				.isEqualTo(ApiVersion.V3);
+	}
+
+	@Test
+	void whenMultipleValuesAreAcceptableAsSingleHeaderThenHighestOrdinalIsReturned() {
+		assertThat(resolve(acceptHeader(ActuatorMediaType.V2_JSON + "," + ActuatorMediaType.V3_JSON)))
+				.isEqualTo(ApiVersion.V3);
+	}
+
+	private Map<String, List<String>> acceptHeader(String... types) {
+		List<String> value = Arrays.asList(types);
+		return value.isEmpty() ? Collections.emptyMap() : Collections.singletonMap("Accept", value);
+	}
+
+	private ApiVersion resolve(Map<String, List<String>> httpHeaders) {
+		return new ProducibleOperationArgumentResolver(httpHeaders).resolve(ApiVersion.class);
+	}
+
+}

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/invoker/cache/CachingOperationInvokerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -18,6 +18,7 @@ package org.springframework.boot.actuate.endpoint.invoker.cache;
 
 import java.security.Principal;
 import java.time.Duration;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
@@ -28,6 +29,7 @@ import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import org.springframework.boot.actuate.endpoint.InvocationContext;
+import org.springframework.boot.actuate.endpoint.OperationArgumentResolver;
 import org.springframework.boot.actuate.endpoint.SecurityContext;
 import org.springframework.boot.actuate.endpoint.http.ApiVersion;
 import org.springframework.boot.actuate.endpoint.invoke.MissingParametersException;
@@ -200,10 +202,10 @@ class CachingOperationInvokerTests {
 		OperationInvoker target = mock(OperationInvoker.class);
 		Object expectedV2 = new Object();
 		Object expectedV3 = new Object();
-		InvocationContext contextV2 = new InvocationContext(ApiVersion.V2, mock(SecurityContext.class),
-				Collections.emptyMap());
-		InvocationContext contextV3 = new InvocationContext(ApiVersion.V3, mock(SecurityContext.class),
-				Collections.emptyMap());
+		InvocationContext contextV2 = new InvocationContext(mock(SecurityContext.class), Collections.emptyMap(),
+				Arrays.asList(new ApiVersionArgumentResolver(ApiVersion.V2)));
+		InvocationContext contextV3 = new InvocationContext(mock(SecurityContext.class), Collections.emptyMap(),
+				Arrays.asList(new ApiVersionArgumentResolver(ApiVersion.V3)));
 		given(target.invoke(contextV2)).willReturn(expectedV2);
 		given(target.invoke(contextV3)).willReturn(expectedV3);
 		CachingOperationInvoker invoker = new CachingOperationInvoker(target, CACHE_TTL);
@@ -240,4 +242,25 @@ class CachingOperationInvokerTests {
 
 	}
 
+	private static final class ApiVersionArgumentResolver implements OperationArgumentResolver {
+
+		private final ApiVersion apiVersion;
+
+		private ApiVersionArgumentResolver(ApiVersion apiVersion) {
+			this.apiVersion = apiVersion;
+		}
+
+		@SuppressWarnings("unchecked")
+		@Override
+		public <T> T resolve(Class<T> type) {
+			return (T) this.apiVersion;
+		}
+
+		@Override
+		public boolean canResolve(Class<?> type) {
+			return ApiVersion.class.equals(type);
+		}
+
+	}
+
 }