@BeforeClass
    public static void setup() throws IOException {
        http = new SparkTestUtil(4567);

        get(HELLO, (q, a) -> HELLO_WORLD);

        after(HELLO, (q, a) -> {
            String body = a.body();
            assertEquals(HELLO_WORLD, body);
        });

        get(SPECIAL, (q, a) -> {
            a.body(XIDXUS);
            return "";
        });

        after(SPECIAL, (q, a) -> {
            String body = a.body();
            assertEquals(XIDXUS, body);
        });

        get(PORAKATIKAOKAO, (q, a) -> {
            a.body(GALLUS_SCANDALUM);
            return null;
        });

        after(PORAKATIKAOKAO, (q, a) -> {
            String body = a.body();
            assertEquals(GALLUS_SCANDALUM, body);
        });

        get(MAXIME, (q, a) -> {
            a.body(XIDXUS);
            return $11AB;
        });

        after(MAXIME, (q, a) -> {
            String body = a.body();
            assertEquals($11AB, body);
        });

        Spark.awaitInitialization();
    }
Validates and shows the "rules" for how response "body" is set.
src/test/java/spark/ResponseBodyTest.java