https://github.com/google/guice/guice: Guice is a lightweight dependency injection framework for Java 6 and above (Google, Inc.) 
/*
 * Copyright (C) 2015 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.inject.multibindings;

import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;


Allows users define customized key type annotations for map bindings by annotating an annotation of a Map's key type. The custom key annotation can be applied to methods also annotated with @ProvidesIntoMap. 
A StringMapKey and 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: 
@MapKey(unwrapValue = true) @Retention(RUNTIME) public @interface MyCustomEnumKey { MyCustomEnum value(); } 
 You can also use the whole annotation as the key, if 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 unwrapValue=true, the value() type will be the key type for injected map and the value() instances will be the keys values. 
Since:4.0
/**
 * Allows users define customized key type annotations for map bindings by annotating an annotation
 * of a {@code Map}'s key type. The custom key annotation can be applied to methods also annotated
 * with {@literal @}{@link ProvidesIntoMap}.
 *
 * <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.
 *
 * @since 4.0
 */
@Documented
@Target(ANNOTATION_TYPE)
@Retention(RUNTIME)
public @interface MapKey {
  
if unwrapValue is false, then the whole annotation will be the type and annotation instances will be the keys. If unwrapValue is true, the value() type of key type annotation will be the key type for injected map and the value instances will be the keys. 
/**
   * if {@code unwrapValue} is false, then the whole annotation will be the type and annotation
   * instances will be the keys. If {@code unwrapValue} is true, the value() type of key type
   * annotation will be the key type for injected map and the value instances will be the keys.
   */
  boolean unwrapValue() default true;
}