Allow configurable static asset cache control

thingifier/src/main/java/uk/co/compendiumdev/thingifier/adapter/javalin/JavalinHttpServer.java

@@ -27,6 +27,9 @@
 import uk.co.compendiumdev.thingifier.api.response.ApiResponseError;
 
 public final class JavalinHttpServer implements AutoCloseable {
+    static final String STATIC_CACHE_CONTROL_PROPERTY = "thingifier.static.cache-control";
+    private static final String STATIC_CACHE_CONTROL_ENV = "THINGIFIER_STATIC_CACHE_CONTROL";
+    private static final String DEFAULT_STATIC_CACHE_CONTROL = "max-age=0";
     private static final String[] STATIC_ASSET_PREFIXES = {
         "/css/", "/js/", "/favicon/", "/images/"
     };
@@ -116,7 +119,7 @@ private void serveClasspathStaticAsset(final Context ctx) throws Exception {
         }
 
         ctx.status(200);
-        ctx.header("Cache-Control", "max-age=0");
+        ctx.header("Cache-Control", staticCacheControl());
         String contentType = contentTypeFor(path);
         if (contentType != null) {
             ctx.contentType(contentType);
@@ -144,6 +147,24 @@ private boolean isStaticAssetPath(final String path) {
         return false;
     }
 
+    static String staticCacheControl() {
+        final String configured = System.getProperty(STATIC_CACHE_CONTROL_PROPERTY);
+        if (hasText(configured)) {
+            return configured.trim();
+        }
+
+        final String environment = System.getenv(STATIC_CACHE_CONTROL_ENV);
+        if (hasText(environment)) {
+            return environment.trim();
+        }
+
+        return DEFAULT_STATIC_CACHE_CONTROL;
+    }
+
+    private static boolean hasText(final String value) {
+        return value != null && !value.trim().isEmpty();
+    }
+
     private String classpathStaticResource(final String path) {
         String base = staticFilePath == null ? "" : staticFilePath.trim();
         while (base.startsWith("/")) {

thingifier/src/test/java/uk/co/compendiumdev/thingifier/adapter/javalin/JavalinHttpServerTest.java

@@ -21,6 +21,57 @@ void duplicateLegacyPathParamNamesAreMadeUniqueForJavalin() {
                 JavalinHttpServer.javalinPath("todos/:id/tasksof/:id"));
     }
 
+    @Test
+    void classpathStaticAssetsUseDefaultNoCacheHeader() throws Exception {
+        final String originalValue =
+                System.getProperty(JavalinHttpServer.STATIC_CACHE_CONTROL_PROPERTY);
+        System.clearProperty(JavalinHttpServer.STATIC_CACHE_CONTROL_PROPERTY);
+
+        try {
+            int port = availablePort();
+            HttpRouteRegistry registry = new HttpRouteRegistry();
+
+            try (JavalinHttpServer server = new JavalinHttpServer(port, "/public", registry)) {
+                server.start();
+
+                HttpResponse<String> response =
+                        get("http://localhost:" + port + "/css/default.css");
+
+                Assertions.assertEquals(200, response.statusCode());
+                Assertions.assertEquals(
+                        "max-age=0", response.headers().firstValue("Cache-Control").orElse(""));
+            }
+        } finally {
+            restoreStaticCacheControlProperty(originalValue);
+        }
+    }
+
+    @Test
+    void classpathStaticAssetsCanUseConfiguredCacheHeader() throws Exception {
+        final String originalValue =
+                System.getProperty(JavalinHttpServer.STATIC_CACHE_CONTROL_PROPERTY);
+        final String cacheControl = "public, max-age=31536000, immutable";
+        System.setProperty(JavalinHttpServer.STATIC_CACHE_CONTROL_PROPERTY, cacheControl);
+
+        try {
+            int port = availablePort();
+            HttpRouteRegistry registry = new HttpRouteRegistry();
+
+            try (JavalinHttpServer server = new JavalinHttpServer(port, "/public", registry)) {
+                server.start();
+
+                HttpResponse<String> response =
+                        get("http://localhost:" + port + "/css/default.css");
+
+                Assertions.assertEquals(200, response.statusCode());
+                Assertions.assertEquals(
+                        cacheControl, response.headers().firstValue("Cache-Control").orElse(""));
+            }
+        } finally {
+            restoreStaticCacheControlProperty(originalValue);
+        }
+    }
+
     @Test
     void emptyNoContentDoesNotReturnContentTypeHeader() throws Exception {
         int port = availablePort();
@@ -180,6 +231,14 @@ private HttpResponse<String> get(final String url) throws Exception {
                         HttpResponse.BodyHandlers.ofString());
     }
 
+    private void restoreStaticCacheControlProperty(final String originalValue) {
+        if (originalValue == null) {
+            System.clearProperty(JavalinHttpServer.STATIC_CACHE_CONTROL_PROPERTY);
+        } else {
+            System.setProperty(JavalinHttpServer.STATIC_CACHE_CONTROL_PROPERTY, originalValue);
+        }
+    }
+
     private int availablePort() throws Exception {
         try (ServerSocket socket = new ServerSocket(0)) {
             return socket.getLocalPort();