<p>A {@link StringMapKey} and {@link ClassMapKey} are provided for convenience with maps whose
 * keys are strings or classes. For maps with enums or primitive types as keys, you must provide
 * your own MapKey annotation, such as this one for an enum:
 *
 * <pre>
 * {@literal @}MapKey(unwrapValue = true)
 * {@literal @}Retention(RUNTIME)
 * public {@literal @}interface MyCustomEnumKey {
 *   MyCustomEnum value();
 * }
 * </pre>
 *
 * You can also use the whole annotation as the key, if {@code unwrapValue=false}.
 * When unwrapValue is false, the annotation type will be the key type for the injected map and
 * the annotation instances will be the key values. If {@code unwrapValue=true}, the value() type
 * will be the key type for injected map and the value() instances will be the keys values.
 *
<p>A {@link StringMapKey} and {@link ClassMapKey} are provided for convenience with maps whose
 * keys are strings or classes. For maps with enums or primitive types as keys, you must provide
 * your own MapKey annotation, such as this one for an enum:
 *
 * <pre>
 * {@literal @}MapKey(unwrapValue = true)
 * {@literal @}Retention(RUNTIME)
 * public {@literal @}interface MyCustomEnumKey {
 *   MyCustomEnum value();
 * }
 * </pre>
 *
 * You can also use the whole annotation as the key, if {@code unwrapValue=false}.
 * When unwrapValue is false, the annotation type will be the key type for the injected map and
 * the annotation instances will be the key values. If {@code unwrapValue=true}, the value() type
 * will be the key type for injected map and the value() instances will be the keys values.
 * | MapKey.java