Try to fix issue 534

src/main/java/spark/ResponseTransformerExtend.java

@@ -0,0 +1,18 @@
+package spark;
+
+
+@FunctionalInterface
+public interface ResponseTransformerExtend {
+
+    /**
+     * Method called for rendering the output.
+     *
+     * @param model object used to render output.
+     * @param req request object to render output.
+     * @param res response object to render output.
+     * @return message that it is sent to client.
+     * @throws java.lang.Exception when render fails
+     */
+    String render(Object model, Request req, Response res) throws Exception;
+
+}

src/test/java/spark/TransformerExtendTest.java

@@ -0,0 +1,56 @@
+package spark;
+
+import org.junit.Before;
+import org.junit.Test;
+import spark.examples.transformer.MyMessage;
+import spark.routematch.RouteMatch;
+
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+//CS304 Issue link: https://github.com/perwendel/spark/issues/534
+public class TransformerExtendTest {
+
+    HttpServletRequest servletRequest;
+    HttpServletResponse servletResponse;
+    Request request;
+    Response response;
+    RouteMatch match = new RouteMatch(null, "/hi", "/hi", "text/html");
+    TransformerExtend transformer;
+
+    @Before
+    public void setup() {
+        servletRequest = mock(HttpServletRequest.class);
+        request = new Request(match, servletRequest);
+
+
+        servletResponse = mock(HttpServletResponse.class);
+        response = new Response(servletResponse);
+
+        transformer = new TransformerExtend();
+    }
+
+    @Test
+    public void testJsonRequest() throws Exception {
+        final String contentType = "application/json";
+
+        when(servletRequest.getContentType()).thenReturn(contentType);
+
+        assertEquals("{\"message\":\"Hello World\"}", transformer.render(new MyMessage("Hello World"), request, response));
+
+    }
+
+    @Test
+    public void testNotJsonRequest() throws Exception {
+        final String contentType = "text/xml";
+
+        when(servletRequest.getContentType()).thenReturn(contentType);
+
+        assertEquals("Not Json type message. Other types are not implemented.", transformer.render(new MyMessage("Hello World"), request, response));
+
+    }
+}

src/test/java/spark/examples/transformer/TransformerExtend.java

@@ -0,0 +1,18 @@
+package spark;
+
+import com.google.gson.Gson;
+
+//CS304 Issue link: https://github.com/perwendel/spark/issues/534
+public class TransformerExtend implements ResponseTransformerExtend {
+
+    @Override
+    public String render(Object model, Request req, Response res) throws Exception {
+        if(req.contentType().equals("application/json")) {
+            Gson gson = new Gson();
+            res.type("application/json");
+            System.out.println(res.type());
+            return gson.toJson(model);
+        }
+        return "Not Json type message. Other types are not implemented.";
+    }
+}