groovy.lang.Lazy Maven / Gradle / Ivy
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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
* 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().copyWith(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().copyWith(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
* }
*
*/
@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;
}