# Middleware { #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.

    /** String object indicating that {@link #line} is empty */
    protected static final String EMPTY = new String();

    /** {@link BufferedReader} */
    protected final BufferedReader reader;

    /** The line that has been read */
    protected String line = EMPTY;

    /**
     * Returns an {@link Iterable} that wraps a {@link LineIterator} for use in enhanced for-loops.
     *

 * <ul>
 * <li>{@link Experimental}: Indicates that this interface or its implementation
 * may still be evolving and could change in future versions.</li>
 * <li>{@link FunctionalInterface}: Denotes that this is a functional interface,
 * allowing implementations as lambda expressions or method references.</li>
 * <li>{@link Consumer}: Signifies that this type is intended to be implemented

For example, let's say that you want to have a *path operation* that allows to update items, and returns HTTP status codes of 200 "OK" when successful.

But you also want it to accept new items. And when the items didn't exist before, it creates them, and returns an HTTP status code of 201 "Created".

To achieve that, import `JSONResponse`, and return your content there directly, setting the `status_code` that you want:

        String actualUserName = ntlmRequest.getRemoteUser();

        // Assert: Verify that the returned user name is the expected one
        assertEquals(expectedUserName, actualUserName, "getRemoteUser should return the principal's name.");
    }

    /**
     * Test method for {@link NtlmHttpServletRequest#getUserPrincipal()}.
     * Verifies that the method returns the correct principal object.
     */
    @Test

They're also concrete: each URL identifies a specific path (like `/square/okhttp`) and query (like `?q=sharks&lang=en`). Each webserver hosts many URLs.

### [Addresses](https://square.github.io/okhttp/4.x/okhttp/okhttp3/-address/)

            // Do NOT call setRootLoggerLevel - this is the fix!
        }

        // Verify that setRootLoggerLevel was not called
        assertEquals(0, slf4jConfiguration.setRootLoggerLevelCallCount);

        // Verify that system property was not set
        assertNull(System.getProperty(Constants.MAVEN_LOGGER_DEFAULT_LOG_LEVEL));
    }

    @Test

## Wildcards { #wildcards }

It's also possible to declare the list as `"*"` (a "wildcard") to say that all are allowed.

But that will only allow certain types of communication, excluding everything that involves credentials: Cookies, Authorization headers like those used with Bearer Tokens, etc.

* Why is this change done? What's the use case?
* For user-facing features, what will the API look like?
* What test cases should it have? What could go wrong?
* How will it roughly be implemented? We'll happily provide code pointers to save you time.

We may ask you to answer these questions directly in the GitHub issue or (for large changes) in a shared Google Doc.

  })
  private void waitForInterrupt(Thread currentThread) {
    /*
     * If someone called cancel(true), it is possible that the interrupted bit hasn't been set yet.
     * Wait for the interrupting thread to set DONE. (See interruptTask().) We want to wait so that
     * the interrupting thread doesn't interrupt the _next_ thing to run on this thread.
     *