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/*
 *  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
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package groovy.lang;

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;

/**
 * Class annotation to make a singleton class. The singleton is obtained through normal property access using the singleton property (defaults to "instance").
 *
 * Such classes can be initialized during normal static initialization of the class or lazily (on first access).
 * To make the singleton lazy use {@code @Singleton(lazy=true)}.
 * Lazy singletons are implemented with double-checked locking and a volatile backing field.
 * By default, no explicit constructors are allowed. To create one or more explicit constructors
 * use {@code @Singleton(strict=false)}.
 * This could be used to:
 * 
    *
  • provide your own custom initialization logic in your own no-arg constructor - you * will be responsible for the entire code (the {@code @Singleton} annotation becomes merely documentation)
  • *
  • provide one or more constructors with arguments for a quasi-singleton - these constructors will be used * to create instances that are independent of the singleton instance returned by the singleton property
  • *
* * @author Alex Tkachman * @author Paul King */ @java.lang.annotation.Documented @Retention(RetentionPolicy.SOURCE) @Target({ElementType.TYPE}) @GroovyASTTransformationClass("org.codehaus.groovy.transform.SingletonASTTransformation") public @interface Singleton { /** * @return if this singleton should be lazy */ boolean lazy() default false; /** * @return if this singleton should have strict semantics */ boolean strict() default true; /** * @return the singleton property name */ String property() default "instance"; }




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