The `callback_url` query parameter uses a Pydantic [Url](https://docs.pydantic.dev/latest/api/networks/) type.

///

The only new thing is the `callbacks=invoices_callback_router.routes` as an argument to the *path operation decorator*. We'll see what that is next.

## Documenting the callback { #documenting-the-callback }

The actual callback code will depend heavily on your own API app.

And it will probably vary a lot from one app to the next.

import com.google.common.annotations.GwtCompatible;
import java.io.Serializable;

/**
 * An abstract base class for implementing the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>. The {@link
 * #delegate()} method must be overridden to return the instance being decorated.
 *
 * <p>This class does <i>not</i> forward the {@code hashCode} and {@code equals} methods through to

 * 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
 * default} methods. Instead, it inherits their default implementations. When those implementations

/**
 * 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
 * methods of the delegate. For example, overriding {@link #add} alone <b>will not</b> change the

 * 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
 * default} methods. Instead, it inherits their default implementations. When those implementations

    return (list instanceof RandomAccess)
        ? new RandomAccessWrappedList(key, list, ancestor)
        : new WrappedList(key, list, ancestor);
  }

  /**
   * Collection decorator that stays in sync with the multimap values for a key. There are two kinds
   * of wrapped collections: full and subcollections. Both have a delegate pointing to the
   * underlying collection class.
   *

 * A table which forwards all its method calls to another table. Subclasses should override one or
 * more methods to modify the behavior of the backing map as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * @author Gregory Kick
 * @since 7.0
 */
@GwtCompatible
public abstract class ForwardingTable<
        R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>

 * more methods to modify the behavior of the backing cache as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * @author Charles Fry
 * @since 10.0
 */
@GwtIncompatible
public abstract class ForwardingCache<K, V> extends ForwardingObject implements Cache<K, V> {

 * A table which forwards all its method calls to another table. Subclasses should override one or
 * more methods to modify the behavior of the backing map as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * @author Gregory Kick
 * @since 7.0
 */
@GwtCompatible
public abstract class ForwardingTable<
        R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>

 * override one or more methods to modify the behavior of the backing future as desired per the <a
 * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p><b>Warning:</b> The methods of {@code ForwardingFuture} forward <b>indiscriminately</b> to the
 * methods of the delegate. For example, overriding {@link #get(long, TimeUnit)} alone <b>will