[rest] Add caching for UoM info (#3838)

Signed-off-by: Florian Hotze <florianh_dev@icloud.com>

bundles/org.openhab.core.io.rest/src/main/java/org/openhab/core/io/rest/internal/resources/SystemInfoResource.java

@@ -12,16 +12,24 @@
  */
 package org.openhab.core.io.rest.internal.resources;
 
+import java.time.Instant;
+import java.time.temporal.ChronoUnit;
+import java.util.Date;
+import java.util.Objects;
+
 import javax.annotation.security.RolesAllowed;
 import javax.ws.rs.GET;
 import javax.ws.rs.Path;
 import javax.ws.rs.Produces;
+import javax.ws.rs.core.CacheControl;
 import javax.ws.rs.core.Context;
 import javax.ws.rs.core.MediaType;
+import javax.ws.rs.core.Request;
 import javax.ws.rs.core.Response;
 import javax.ws.rs.core.UriInfo;
 
 import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
 import org.openhab.core.auth.Role;
 import org.openhab.core.i18n.UnitProvider;
 import org.openhab.core.io.rest.RESTConstants;
@@ -29,6 +37,8 @@ import org.openhab.core.io.rest.RESTResource;
 import org.openhab.core.io.rest.internal.resources.beans.SystemInfoBean;
 import org.openhab.core.io.rest.internal.resources.beans.UoMInfoBean;
 import org.openhab.core.service.StartLevelService;
+import org.osgi.service.cm.ConfigurationEvent;
+import org.osgi.service.cm.ConfigurationListener;
 import org.osgi.service.component.annotations.Activate;
 import org.osgi.service.component.annotations.Component;
 import org.osgi.service.component.annotations.Reference;
@@ -60,13 +70,14 @@ import io.swagger.v3.oas.annotations.tags.Tag;
 @SecurityRequirement(name = "oauth2", scopes = { "admin" })
 @Tag(name = SystemInfoResource.PATH_SYSTEMINFO)
 @NonNullByDefault
-public class SystemInfoResource implements RESTResource {
+public class SystemInfoResource implements RESTResource, ConfigurationListener {
 
     /** The URI path to this resource */
     public static final String PATH_SYSTEMINFO = "systeminfo";
 
     private final StartLevelService startLevelService;
     private final UnitProvider unitProvider;
+    private @Nullable Date lastModified = null;
 
     @Activate
     public SystemInfoResource(@Reference StartLevelService startLevelService, @Reference UnitProvider unitProvider) {
@@ -74,6 +85,13 @@ public class SystemInfoResource implements RESTResource {
         this.unitProvider = unitProvider;
     }
 
+    @Override
+    public void configurationEvent(@Nullable ConfigurationEvent event) {
+        if (Objects.equals(event.getPid(), "org.openhab.i18n")) {
+            lastModified = null;
+        }
+    }
+
     @GET
     @Path("/")
     @Produces(MediaType.APPLICATION_JSON)
@@ -89,8 +107,22 @@ public class SystemInfoResource implements RESTResource {
     @Produces(MediaType.APPLICATION_JSON)
     @Operation(operationId = "getUoMInformation", summary = "Get all supported dimensions and their system units.", responses = {
             @ApiResponse(responseCode = "200", description = "OK", content = @Content(schema = @Schema(implementation = UoMInfoBean.class))) })
-    public Response getUoMInfo(@Context UriInfo uriInfo) {
+    public Response getUoMInfo(final @Context Request request, final @Context UriInfo uriInfo) {
+        if (lastModified != null) {
+            Response.ResponseBuilder responseBuilder = request.evaluatePreconditions(lastModified);
+            if (responseBuilder != null) {
+                // send 304 Not Modified
+                return responseBuilder.build();
+            }
+        } else {
+            lastModified = Date.from(Instant.now().truncatedTo(ChronoUnit.SECONDS));
+        }
+
+        CacheControl cc = new CacheControl();
+        cc.setMustRevalidate(true);
+        cc.setPrivate(true);
+
         final UoMInfoBean bean = new UoMInfoBean(unitProvider);
-        return Response.ok(bean).build();
+        return Response.ok(bean).lastModified(lastModified).cacheControl(cc).build();
     }
 }