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:

<div class="termy">

```console
$ <font color="#4E9A06">fastapi</font> dev <u style="text-decoration-style:solid">main.py</u>

  <span style="background-color:#009485"><font color="#D3D7CF"> FastAPI </font></span>  Starting development server 🚀

             Searching for package file structure from directories
             with <font color="#3465A4">__init__.py</font> files

<div class="termy">

```console
$ <font color="#4E9A06">fastapi</font> dev <u style="text-decoration-style:solid">main.py</u>

  <span style="background-color:#009485"><font color="#D3D7CF"> FastAPI </font></span>  Starting development server 🚀

             Searching for package file structure from directories
             with <font color="#3465A4">__init__.py</font> files

import java.util.Set;
import org.jspecify.annotations.Nullable;

/**
 * A multimap which forwards all its method calls to another multimap. Subclasses should override
 * one or more methods to modify the behavior of the backing multimap as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><b>{@code default} method warning:</b> This class does <i>not</i> forward calls to {@code

import java.util.Set;
import org.jspecify.annotations.Nullable;

/**
 * A set which forwards all its method calls to another set. Subclasses should override one or more
 * methods to modify the behavior of the backing set as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><b>Warning:</b> The methods of {@code ForwardingSet} forward <b>indiscriminately</b> to the

import java.util.Set;
import org.jspecify.annotations.Nullable;

/**
 * A multimap which forwards all its method calls to another multimap. Subclasses should override
 * one or more methods to modify the behavior of the backing multimap as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><b>{@code default} method warning:</b> This class does <i>not</i> forward calls to {@code

import java.util.Queue;
import org.jspecify.annotations.Nullable;

/**
 * A queue which forwards all its method calls to another queue. Subclasses should override one or
 * more methods to modify the behavior of the backing queue as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><b>Warning:</b> The methods of {@code ForwardingQueue} forward <b>indiscriminately</b> to the

   * backing map.
   *
   * <p>This instance will assume ownership of {@code countMap}, and other code should not maintain
   * references to the map or modify it in any way.
   *
   * <p>The returned multiset is serializable if the input map is.
   *
   * @param countMap backing map for storing the elements in the multiset and their counts. It must

//     tags:
//       - key: "name"
//         value: "pick*" # match objects with tag 'name', with all values starting with 'pick'
//     metadata:
//       - key: "content-type"
//         value: "image/*" # match objects with 'content-type', with all values starting with 'image/'
//     kmskey: "key-id" # match objects with KMS key-id (applicable only for sse-kms)
//   notify:

import org.jspecify.annotations.Nullable;

/**
 * A {@link BlockingDeque} which forwards all its method calls to another {@code BlockingDeque}.
 * Subclasses should override one or more methods to modify the behavior of the backing deque as
 * desired per the <a href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><b>Warning:</b> The methods of {@code ForwardingBlockingDeque} forward <b>indiscriminately</b>