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Groovy: A powerful, dynamic language for the JVM
There is a newer version: 3.0.23
/*
 *  Licensed to the Apache Software Foundation (ASF) under one
 *  or more contributor license agreements.  See the NOTICE file
 *  distributed with this work for additional information
 *  regarding copyright ownership.  The ASF licenses this file
 *  to you 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 groovy.lang;

import groovy.transform.Undefined;
import org.codehaus.groovy.transform.GroovyASTTransformationClass;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Annotation to automatically delegate part of the functionality of an owner class to the
 * annotated delegation target. The target can be a field (or property) or a method's return value.
 * 
 * The delegate type is either the type of the annotated field (or property) or the return type of
 * the annotated method. The method can be thought of as a getter or factory method for the delegate.
 * All public instance methods present in the delegate type and not present in the owner class
 * will be added to owner class at compile time. The implementation of such automatically added
 * methods is code which calls through to the delegate as per the normal delegate pattern.
 * 

 * As an example, consider this code:
 * 
 * class Event {
 *     {@code @Delegate} Date when
 *     String title, url
 * }
 *
 * def gr8conf = new Event(title: "GR8 Conference",
 *                           url: "http://www.gr8conf.org",
 *                          when: Date.parse("yyyy/MM/dd", "2009/05/18"))
 *
 * def javaOne = new Event(title: "JavaOne",
 *                           url: "http://java.sun.com/javaone/",
 *                          when: Date.parse("yyyy/MM/dd", "2009/06/02"))
 *
 * assert gr8conf.before(javaOne.when)
 * 
 *
 * In this example, the {@code Event} class will have a method called
 * {@code before(Date otherDate)} as well as other public methods of the
 * {@code Date} class.
 * The implementation of the {@code before()} method will look like this:
 *  *     public boolean before(Date otherDate) {
 *         return when.before(otherDate);
 *     }
 * 
 *
 * By default, the owner class will also be modified to implement any interfaces
 * implemented by the delegate type. So, in the example above, because {@code Date}
 * implements {@code Cloneable} the following will be true:
 *
 *  * assert gr8conf instanceof Cloneable
 * 
 *
 * This behavior can be disabled by setting the
 * annotation's {@code interfaces} element to false,
 * i.e. {@code @Delegate(interfaces = false)}, e.g. in the above
 * example, the delegate definition would become:
 *  *     {@code @Delegate}(interfaces = false) Date when
 * 
 * and the following would be true:
 *  * assert !(gr8conf instanceof Cloneable)
 * 
 *
 * If multiple delegation targets are used and the same method signature occurs
 * in more than one of the respective delegate types, then the delegate will be
 * made to the first defined target having that signature. If this does occur,
 * it might be regarded as a smell (or at least poor style) and it might be
 * clearer to do the delegation by long hand.
 * 
 * By default, methods of the delegate type marked as {@code @Deprecated} are
 * not automatically added to the owner class (but see the technical note
 * about interfaces below). You can force these methods to
 * be added by setting the annotation's {@code deprecated} element to true,
 * i.e. {@code @Delegate(deprecated = true)}.
 * 

 * For example, in the example above if we change the delegate definition to:
 * 
 *     {@code @Delegate}(deprecated = true) Date when
 * 
 * then the following additional lines will execute successfully (during 2009):
 *  * assert gr8conf.year + 1900 == 2009
 * assert gr8conf.toGMTString().contains(" 2009 ")
 * 
 * Otherwise these lines produce a groovy.lang.MissingPropertyException
 * or groovy.lang.MissingMethodException respectively as those two methods are
 * {@code @Deprecated} in {@code Date}.
 * 
 * Technical notes:
 * 

 * Static methods, synthetic methods or methods from the GroovyObject interface
 * are not candidates for delegation
 * Non-abstract non-static methods defined in the owner class or its superclasses take
 * precedence over methods with identical signatures from a {@code @Delegate} target
 * All methods defined in the owner class (including static, abstract or private etc.)
 * take precedence over methods with identical signatures from a {@code @Delegate} target
 * Recursive delegation to your own class is not allowed
 * Mixing of {@code @Delegate} with default method arguments is known not to work in some cases.
 * We recommend not using these features together.
 * When the delegate type is an interface, the {@code deprecated} attribute will be
 * ignored if the owner class implements that interface (i.e. you must set {@code interfaces=false}
 * if you want the {@code deprecated} attribute to be used). Otherwise, the resulting class would
 * not compile anyway without manually adding in any deprecated methods in the interface.
 * {@code @Delegate} can work in combination with {@code @Lazy} when annotating a field (or property)
 * 
 */
@java.lang.annotation.Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.METHOD})
@GroovyASTTransformationClass("org.codehaus.groovy.transform.DelegateASTTransformation")
public @interface Delegate {
    /**
     * @return true if owner class should implement interfaces implemented by delegate type
     */
    boolean interfaces() default true;

    /**
     * Whether to apply the delegate pattern to deprecated methods; to avoid compilation
     * errors, this is ignored if the type of the delegate target is an interface and
     * {@code interfaces=true}.
     *
     * @return true if owner class should delegate to methods annotated with @Deprecated
     */
    boolean deprecated() default false;

    /**
     * Whether to carry over annotations from the methods of the delegate
     * to your delegating method. Currently Closure annotation members are
     * not supported.
     *
     * @return true if generated delegate methods should keep method annotations
     */
    boolean methodAnnotations() default false;

    /**
     * Whether to carry over annotations from the parameters of delegate
     * methods to your delegating method. Currently Closure annotation members are
     * not supported.
     *
     * @return true if generated delegate methods should keep parameter annotations
     */
    boolean parameterAnnotations() default false;

    /**
     * List of method and/or property names to exclude when delegating.
     * Only one of 'includes', 'includeTypes', 'excludes' or 'excludeTypes' should be used.
     * For convenience, a String with comma separated names
     * can be used in addition to an array (using Groovy's literal list notation) of String values.
     * If interfaces is true (the default), you will need to manually supply any methods excluded
     * from delegation that are required for the interface.
     * @since 2.2.0
     */
    String[] excludes() default {};


    /**
     * List of interfaces containing method signatures to exclude when delegating.
     * Only one of 'includes', 'includeTypes', 'excludes', 'excludeTypes' should be used.
     * If interfaces is true (the default), you will need to manually supply any methods excluded
     * from delegation that are required for the interface.
     * @since 2.3.0
     */
    Class[] excludeTypes() default {};

    /**
     * List of method and/or property names to include when delegating.
     * Only one of 'includes', 'includeTypes', 'excludes' or 'excludeTypes' should be used.
     * For convenience, a String with comma separated names
     * can be used in addition to an array (using Groovy's literal list notation) of String values.
     * The default value is a special marker value indicating that no includes are defined; all fields
     * are included if 'includes' remains undefined and 'excludes' is explicitly or implicitly an empty list.
     * If interfaces is true (the default), you will need to manually supply any methods not included
     * via delegation that are required for the interface.
     * @since 2.2.0
     */
    String[] includes() default {Undefined.STRING};

    /**
     * List of interfaces containing method signatures to include when delegating.
     * Only one of 'includes', 'includeTypes', 'excludes' or 'excludeTypes' should be used.
     * The default value is a special marker value indicating that no includeTypes are defined.
     * If interfaces is true (the default), you will need to manually supply any methods excluded
     * from delegation that are required for the interface.
     * @since 2.3.0
     */
    Class[] includeTypes() default {Undefined.CLASS.class};

    /**
     * Whether to apply the delegate pattern to all methods, including those with names that are considered internal.
     *
     * @return true if owner class should delegate to methods which have internal names
     * @since 2.5.0
     */
    boolean allNames() default false;
}