Extract ApplicationContextHeaderFilter

Extract ApplicationContextHeaderFilter to a top-level class from
EndpointWebMvcAutoConfiguration and make the header field public.

Fixes gh-5726
Closes gh-5784

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/autoconfigure/EndpointWebMvcAutoConfiguration.java

@@ -16,13 +16,7 @@
 
 package org.springframework.boot.actuate.autoconfigure;
 
-import java.io.IOException;
-
-import javax.servlet.FilterChain;
 import javax.servlet.Servlet;
-import javax.servlet.ServletException;
-import javax.servlet.http.HttpServletRequest;
-import javax.servlet.http.HttpServletResponse;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -60,6 +54,7 @@ import org.springframework.boot.bind.RelaxedPropertyResolver;
 import org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext;
 import org.springframework.boot.context.embedded.EmbeddedServletContainerFactory;
 import org.springframework.boot.context.embedded.EmbeddedWebApplicationContext;
+import org.springframework.boot.context.web.ApplicationContextHeaderFilter;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
 import org.springframework.context.ApplicationListener;
@@ -77,7 +72,6 @@ import org.springframework.core.env.PropertySource;
 import org.springframework.core.type.AnnotatedTypeMetadata;
 import org.springframework.core.type.MethodMetadata;
 import org.springframework.web.context.WebApplicationContext;
-import org.springframework.web.filter.OncePerRequestFilter;
 import org.springframework.web.servlet.DispatcherServlet;
 
 /**
@@ -93,6 +87,7 @@ import org.springframework.web.servlet.DispatcherServlet;
  * @author Andy Wilkinson
  * @author Johannes Edmeier
  * @author Eddú Meléndez
+ * @author Venil Noronha
  */
 @Configuration
 @ConditionalOnClass({ Servlet.class, DispatcherServlet.class })
@@ -239,28 +234,6 @@ public class EndpointWebMvcAutoConfiguration
 	protected static class EndpointWebMvcConfiguration {
 	}
 
-	/**
-	 * {@link OncePerRequestFilter} to add the {@literal X-Application-Context} if
-	 * required.
-	 */
-	private static class ApplicationContextHeaderFilter extends OncePerRequestFilter {
-
-		private final ApplicationContext applicationContext;
-
-		ApplicationContextHeaderFilter(ApplicationContext applicationContext) {
-			this.applicationContext = applicationContext;
-		}
-
-		@Override
-		protected void doFilterInternal(HttpServletRequest request,
-				HttpServletResponse response, FilterChain filterChain)
-						throws ServletException, IOException {
-			response.addHeader("X-Application-Context", this.applicationContext.getId());
-			filterChain.doFilter(request, response);
-		}
-
-	}
-
 	/**
 	 * {@link ApplicationListener} to propagate the {@link ContextClosedEvent} from a
 	 * parent to a child.

spring-boot/src/main/java/org/springframework/boot/context/web/ApplicationContextHeaderFilter.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright 2012-2016 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.boot.context.web;
+
+import java.io.IOException;
+
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+import org.springframework.context.ApplicationContext;
+import org.springframework.web.filter.OncePerRequestFilter;
+
+/**
+ * {@link OncePerRequestFilter} to add a {@literal X-Application-Context} header that
+ * contains the {@link ApplicationContext#getId() ApplicationContext ID}.
+ *
+ * @author Phillip Webb
+ * @author Venil Noronha
+ * @since 1.4.0
+ */
+public class ApplicationContextHeaderFilter extends OncePerRequestFilter {
+
+	/**
+	 * Public constant for {@literal X-Application-Context}.
+	 */
+	public static final String HEADER_NAME = "X-Application-Context";
+
+	private final ApplicationContext applicationContext;
+
+	public ApplicationContextHeaderFilter(ApplicationContext context) {
+		this.applicationContext = context;
+	}
+
+	@Override
+	protected void doFilterInternal(HttpServletRequest request,
+			HttpServletResponse response, FilterChain filterChain)
+					throws ServletException, IOException {
+		response.addHeader(HEADER_NAME, this.applicationContext.getId());
+		filterChain.doFilter(request, response);
+	}
+
+}