Add actuator specific ObjectMapper

Prior to this commit, Actuator endpoints would use the application
ObjectMapper instance for serializing payloads as JSON. This was
problematic in several cases:

* application-specific configuration would change the actuator endpoint
output.
* choosing a different JSON mapper implementation in the application
would break completely some endpoints.

Spring Boot Actuator already has a hard dependency on Jackson, and this
commit uses that fact to configure a shared `ObjectMapper` instance that
will be used by the Actuator infrastructure consistently, without
polluting the application context.

This `ObjectMapper` is used in Actuator for:

* JMX endpoints
* Spring MVC endpoints with an HTTP message converter
* Spring WebFlux endpoints with an `Encoder`
* Jersey endpoints with a `ContextResolver<ObjectMapper>`

For all web endpoints, this configuration is limited to the
actuator-specific media types such as
`"application/vnd.spring-boot.actuator.v3+json"`.

Fixes gh-12951

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

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2012-2020 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.boot.actuate.endpoint.json;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.SerializationFeature;
+
+import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;
+
+/**
+ * Factory for an {@code ObjectMapper} instance to be shared within Actuator
+ * infrastructure.
+ *
+ * <p>
+ * The goal is to have a Jackson configuration separate from the rest of the application
+ * to keep a consistent serialization behavior between applications.
+ *
+ * @author Brian Clozel
+ * @since 2.3.0
+ */
+public class ActuatorJsonMapperProvider {
+
+	private ObjectMapper objectMapper;
+
+	public ObjectMapper getInstance() {
+		if (this.objectMapper == null) {
+			Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder();
+			builder.featuresToDisable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
+			builder.featuresToDisable(SerializationFeature.WRITE_DURATIONS_AS_TIMESTAMPS);
+			this.objectMapper = builder.build();
+		}
+		return this.objectMapper;
+	}
+
+}

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/json/package-info.java

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2012-2020 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.
+ */
+
+/**
+ * JSON support for actuator endpoints.
+ */
+package org.springframework.boot.actuate.endpoint.json;