# Extending OpenAPI { #extending-openapi }

There are some cases where you might need to modify the generated OpenAPI schema.

In this section you will see how.

## The normal process { #the-normal-process }

The normal (default) process, is as follows.

A `FastAPI` application (instance) has an `.openapi()` method that is expected to return the OpenAPI schema.

          canUseHeaderValue = false
        } else {
          headerValue = value
        }
      }
      name.equals("Pragma", ignoreCase = true) -> {
        // Might specify additional cache-control params. We invalidate just in case.
        canUseHeaderValue = false
      }
      else -> {
        continue@loop
      }
    }

    var pos = 0

For example, if you want to read or manipulate the request body before it is processed by your application.

/// danger

This is an "advanced" feature.

If you are just starting with **FastAPI** you might want to skip this section.

///

## Use cases { #use-cases }

Some use cases include:

    #  If the parameter of condition-bean or parameter-bean has short size,
    #   NONE - Do nothing. (default)
    #   EXCEPTION - It throws an exception.
    #   RFILL - It fills the parameter by right spaces.
    #   LFILL - It fills the parameter by left spaces.
    #
    #; shortCharHandlingMode = NONE
    # - - - - - - - - - -/

    # /- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    // Test that listeners added both before and after the value is available
    // get called correctly.
    for (int i = 0; i < 20; i++) {

      // Right in the middle start up a thread to close the latch.
      if (i == 10) {
        new Thread(() -> latch.countDown()).start();
      }

      future.addListener(listenerLatch::countDown, exec);
    }

  }

  private static final class NullFriendlyComparator<T> implements Comparator<T>, Serializable {
    @Override
    public int compare(T left, T right) {
      return String.valueOf(left).compareTo(String.valueOf(right));
    }
  }

* `description`: A long description that can contain Markdown text.
* `value`: This is the actual example shown, e.g. a `dict`.
* `externalValue`: alternative to `value`, a URL pointing to the example. Although this might not be supported by as many tools as `value`.

You can use it like this:

{* ../../docs_src/schema_extra_example/tutorial005_an_py310.py hl[23:49] *}

        // that. If any of the BROWSER_COMPATIBLE_DATE_FORMAT_STRINGS ended in "'GMT'" we'd have
        // to also check that position.getIndex() == value.length() otherwise parsing might have
        // terminated early, ignoring things like "+01:00". Leaving this as != 0 means that any
        // trailing junk is ignored.
        return result
      }
    }
  }
  return null
}

import junit.framework.TestCase;
import org.jspecify.annotations.NullUnmarked;

/**
 * Tests that all package-private {@code writeReplace} methods are overridden in any existing
 * subclasses. Without such overrides, optimizers might put a {@code writeReplace}-containing class
 * and its subclass in different packages, causing the serialization system to fail to invoke {@code

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