com.google.inject.multibindings.MapKey Maven / Gradle / Ivy

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package com.google.inject.multibindings;

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

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

/**
 * 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}.
 *
 * 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:
 *
 * 
 * {@literal @}MapKey(unwrapValue = true)
 * {@literal @}Retention(RUNTIME)
 * public {@literal @}interface MyCustomEnumKey {
 *   MyCustomEnum value();
 * }
 * 
 *
 * 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.
 */
@Documented
@Target(ANNOTATION_TYPE)
@Retention(RUNTIME)
public @interface MapKey {
    /**
     * 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;
}