groovy.lang.Lazy Maven / Gradle / Ivy
/*
* Copyright 2008-2010 the original author or authors.
*
* 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 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;
/**
* Field annotation to simplify lazy initialization.
*
* Example usage without any special modifiers just defers initialization until the first call but is not thread-safe:
*
* {@code @Lazy} T x
*
* becomes
*
* private T $x
*
* T getX() {
* if ($x != null)
* return $x
* else {
* $x = new T()
* return $x
* }
* }
*
*
* If the field is declared volatile then initialization will be synchronized using
* the double-checked locking pattern as shown here:
*
*
* {@code @Lazy} volatile T x
*
* becomes
*
* private volatile T $x
*
* T getX() {
* T $x_local = $x
* if ($x_local != null)
* return $x_local
* else {
* synchronized(this) {
* if ($x == null) {
* $x = new T()
* }
* return $x
* }
* }
* }
*
*
* By default a field will be initialized by calling its default constructor.
*
* If the field has an initial value expression then this expression will be used instead of calling the default constructor.
* In particular, it is possible to use closure { ... } () syntax as follows:
*
*
* {@code @Lazy} T x = { [1, 2, 3] } ()
*
* becomes
*
* private T $x
*
* T getX() {
* T $x_local = $x
* if ($x_local != null)
* return $x_local
* else {
* synchronized(this) {
* if ($x == null) {
* $x = { [1, 2, 3] } ()
* }
* return $x
* }
* }
* }
*
*
* @Lazy(soft=true) will use a soft reference instead of the field and use the above rules each time re-initialization is required.
*
* If the soft flag for the annotation is not set but the field is static, then
* the initialization on demand holder idiom is
* used as follows:
*
* {@code @Lazy} static FieldType field
* {@code @Lazy} static Date date1
* {@code @Lazy} static Date date2 = { new Date().updated(year: 2000) }()
* {@code @Lazy} static Date date3 = new GregorianCalendar(2009, Calendar.JANUARY, 1).time
*
* becomes these methods and inners classes within the class containing the above definitions:
*
* private static class FieldTypeHolder_field {
* private static final FieldType INSTANCE = new FieldType()
* }
*
* private static class DateHolder_date1 {
* private static final Date INSTANCE = new Date()
* }
*
* private static class DateHolder_date2 {
* private static final Date INSTANCE = { new Date().updated(year: 2000) }()
* }
*
* private static class DateHolder_date3 {
* private static final Date INSTANCE = new GregorianCalendar(2009, Calendar.JANUARY, 1).time
* }
*
* static FieldType getField() {
* return FieldTypeHolder_field.INSTANCE
* }
*
* static Date getDate1() {
* return DateHolder_date1.INSTANCE
* }
*
* static Date getDate2() {
* return DateHolder_date2.INSTANCE
* }
*
* static Date getDate3() {
* return DateHolder_date3.INSTANCE
* }
*
*
* @author Alex Tkachman
* @author Paul King
*/
@java.lang.annotation.Documented
@Retention(RetentionPolicy.SOURCE)
@Target({ElementType.FIELD})
@GroovyASTTransformationClass("org.codehaus.groovy.transform.LazyASTTransformation")
public @interface Lazy {
/**
* @return if field should be soft referenced instead of hard referenced
*/
boolean soft () default false;
}