groovy.lang.Singleton Maven / Gradle / Ivy
/*
* 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 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";
}