{!../../docs_src/custom_response/tutorial004.py!}
```

In this example, the function `generate_html_response()` already generates and returns a `Response` instead of returning the HTML in a `str`.

By returning the result of calling `generate_html_response()`, you are already returning a `Response` that will override the default **FastAPI** behavior.

# Middleware

You can add middleware to **FastAPI** applications.

A "middleware" is a function that works with every **request** before it is processed by any specific *path operation*. And also with every **response** before returning it.

* It takes each **request** that comes to your application.
* It can then do something to that **request** or run any needed code.

              }
            },
            directExecutor()));
    thingToCancel[0] = serializer.submit(Callables.returning(null), directExecutor());
    results.add(thingToCancel[0]);
    // Enqueue more than enough tasks to force reentrancy.
    for (int i = 0; i < 5; i++) {
      results.add(serializer.submit(Callables.returning(null), directExecutor()));
    }

    manualExecutorTask[0].run();

////

FastAPI will use this return type to:

* **Validate** the returned data.
    * If the data is invalid (e.g. you are missing a field), it means that *your* app code is broken, not returning what it should, and it will return a server error instead of returning incorrect data. This way you and your clients can be certain that they will receive the data and the data shape expected.
* Add a **JSON Schema** for the response, in the OpenAPI *path operation*.

              }
            },
            directExecutor()));
    thingToCancel[0] = serializer.submit(Callables.returning(null), directExecutor());
    results.add(thingToCancel[0]);
    // Enqueue more than enough tasks to force reentrancy.
    for (int i = 0; i < 5; i++) {
      results.add(serializer.submit(Callables.returning(null), directExecutor()));
    }

    manualExecutorTask[0].run();

    assertEquals(1, prepender.times);
  }

  public void testConstructor_returning() throws Exception {
    Invokable<?, Prepender> delegate =
        Prepender.constructor(String.class, int.class).returning(Prepender.class);
    Prepender prepender = delegate.invoke(null, "a", 1);
    assertEquals("a", prepender.prefix);
    assertEquals(1, prepender.times);
  }

    assertEquals(1, prepender.times);
  }

  public void testConstructor_returning() throws Exception {
    Invokable<?, Prepender> delegate =
        Prepender.constructor(String.class, int.class).returning(Prepender.class);
    Prepender prepender = delegate.invoke(null, "a", 1);
    assertEquals("a", prepender.prefix);
    assertEquals(1, prepender.times);
  }

You can also declare the `Response` parameter in dependencies, and set cookies (and headers) in them.

## Return a `Response` directly

You can also create cookies when returning a `Response` directly in your code.

To do that, you can create a response as described in [Return a Response Directly](response-directly.md){.internal-link target=_blank}.

Then set Cookies in it, and then return it:

    assertFalse(task.isDone());
    task.run();
    assertTrue(task.isDone());
    assertFalse(task.isCancelled());
    assertEquals(2, getDone(task).intValue());
  }

  public void testCancelled() throws Exception {
    TrustedListenableFutureTask<Integer> task = TrustedListenableFutureTask.create(returning(2));
    assertFalse(task.isDone());
    task.cancel(false);

    super.setUp();
    timeLimiter = new FakeTimeLimiter();
  }

  public void testCallWithTimeout_propagatesReturnValue() throws Exception {
    String result =
        timeLimiter.callWithTimeout(Callables.returning(RETURN_VALUE), DELAY_MS, MILLISECONDS);

    assertThat(result).isEqualTo(RETURN_VALUE);
  }

  public void testCallWithTimeout_wrapsCheckedException() throws Exception {