Added CacheControl annotation and interceptor handler

This should enable annotation based cache-control header values.
Added a cache max age of 60 seconds to statistics resource. Proxy-Servers
should cache a statistics resource for about 60 seconds.

src/main/java/de/thm/arsnova/controller/StatisticsController.java

@@ -11,6 +11,7 @@ import org.springframework.web.bind.annotation.RestController;
 
 import de.thm.arsnova.entities.Statistics;
 import de.thm.arsnova.services.IStatisticsService;
+import de.thm.arsnova.web.CacheControl;
 
 @RestController
 public class StatisticsController extends AbstractController {
@@ -19,6 +20,7 @@ public class StatisticsController extends AbstractController {
 	private IStatisticsService statisticsService;
 
 	@RequestMapping(method = RequestMethod.GET, value = "/statistics")
+	@CacheControl(maxAge = 60, policy = CacheControl.Policy.PUBLIC)
 	public final Statistics getStatistics() {
 		return statisticsService.getStatistics();
 	}

src/main/java/de/thm/arsnova/web/CacheControl.java

+package de.thm.arsnova.web;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+@Target({ ElementType.TYPE, ElementType.METHOD })
+@Retention(RetentionPolicy.RUNTIME)
+public @interface CacheControl {
+	enum Policy {
+		NO_CACHE("no-cache"),
+		NO_STORE("no-store"),
+		PRIVATE("private"),
+		PUBLIC("public");
+
+		private Policy() {
+			this.policyString = null;
+		}
+
+		private Policy(String policyString) {
+			this.policyString = policyString;
+		}
+
+		public String getPolicyString() {
+			return this.policyString;
+		}
+
+		private final String policyString;
+	}
+
+	boolean noCache() default false;
+
+	int maxAge() default 0;
+	Policy[] policy() default {};
+}

src/main/java/de/thm/arsnova/web/CacheControlInterceptorHandler.java

+package de.thm.arsnova.web;
+
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+import org.springframework.stereotype.Component;
+import org.springframework.web.method.HandlerMethod;
+import org.springframework.web.servlet.handler.HandlerInterceptorAdapter;
+
+@Component
+public class CacheControlInterceptorHandler extends HandlerInterceptorAdapter {
+
+	@Override
+	public boolean preHandle(
+			HttpServletRequest request,
+			HttpServletResponse response,
+			Object handler) throws Exception {
+
+		setCacheControlResponseHeader(request, response, handler);
+		return super.preHandle(request, response, handler);
+	}
+
+	private void setCacheControlResponseHeader(
+			HttpServletRequest request,
+			HttpServletResponse response,
+			Object handler) {
+
+		CacheControl cacheControl = getCacheControlAnnotation(request, response, handler);
+
+		if (cacheControl == null) {
+			return;
+		}
+
+		StringBuilder headerValue = new StringBuilder();
+
+		if(cacheControl.policy().length > 0) {
+			for (CacheControl.Policy policy : cacheControl.policy()) {
+				if (headerValue.length() > 0) {
+					headerValue.append(", ");
+				}
+				headerValue.append(policy.getPolicyString());
+			}
+		}
+
+		if (cacheControl.noCache()) {
+			if (headerValue.length() > 0) {
+				headerValue.append(", ");
+			}
+			headerValue.append("max-age=0, no-cache");
+			response.setHeader("cache-control", headerValue.toString());
+		}
+
+		if(cacheControl.maxAge() >= 0) {
+			if (headerValue.length() > 0) {
+				headerValue.append(", ");
+			}
+			headerValue.append("max-age=").append(cacheControl.maxAge());
+		}
+
+		response.setHeader("cache-control", headerValue.toString());
+	}
+
+	private CacheControl getCacheControlAnnotation(
+			HttpServletRequest request,
+			HttpServletResponse response,
+			Object handler
+			) {
+		if (handler == null || !(handler instanceof HandlerMethod)) {
+			return null;
+		}
+
+		final HandlerMethod handlerMethod = (HandlerMethod) handler;
+		return handlerMethod.getMethodAnnotation(CacheControl.class);
+	}
+}

src/main/webapp/WEB-INF/spring/arsnova-servlet.xml

@@ -13,6 +13,10 @@
 	<mvc:annotation-driven
 		content-negotiation-manager="contentNegotiationManager" />
 
+	<mvc:interceptors>
+		<bean class="de.thm.arsnova.web.CacheControlInterceptorHandler" />
+	</mvc:interceptors>
+
 	<bean id="contentNegotiationManager"
 		class="org.springframework.web.accept.ContentNegotiationManagerFactoryBean">
 		<property name="favorPathExtension" value="false" />

src/test/java/de/thm/arsnova/controller/StatisticsControllerTest.java

@@ -2,6 +2,7 @@ package de.thm.arsnova.controller;
 
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.header;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
 
 import org.junit.Before;
@@ -59,4 +60,11 @@ public class StatisticsControllerTest {
 		.andExpect(status().isOk())
 		.andExpect(content().string("{\"answers\":0,\"questions\":0,\"openSessions\":3,\"closedSessions\":0,\"activeUsers\":0}"));
 	}
+
+	@Test
+	public final void testShouldGetCacheControlHeaderForStatistics() throws Exception {
+		mockMvc.perform(get("/statistics"))
+		.andExpect(status().isOk())
+		.andExpect(header().string("cache-control", "public, max-age=60"));
+	}
 }