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Groovy example source code file (Lazy.java)

This example Groovy source code file (Lazy.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Groovy tags/keywords

annotation, groovyasttransformationclass, groovyasttransformationclass, lazy, lazy, target, target

The Groovy Lazy.java source code

/*
 * 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.
 * <p>
 * Example usage without any special modifiers just defers initialization until the first call but is not thread-safe:
 * <pre>
 * {@code @Lazy} T x
 * </pre>
 * becomes
 * <pre>
 * private T $x
 *
 * T getX() {
 *    if ($x != null)
 *       return $x
 *    else {
 *       $x = new T()
 *       return $x
 *    }
 * }
 * </pre>
 *
 * If the field is declared volatile then initialization will be synchronized using
 * the <a href="http://en.wikipedia.org/wiki/Double-checked_locking">double-checked locking pattern as shown here:
 *
 * <pre>
 * {@code @Lazy} volatile T x
 * </pre>
 * becomes
 * <pre>
 * 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
 *       }
 *    }
 * }
 * </pre>
 *
 * 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 <code>{ ... } () syntax as follows:
 *
 * <pre>
 * {@code @Lazy} T x = { [1, 2, 3] } ()
 * </pre>
 * becomes
 * <pre>
 * 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
 *       }
 *    }
 * }
 * </pre>
 * <p>
 * <code>@Lazy(soft=true) will use a soft reference instead of the field and use the above rules each time re-initialization is required.
 * <p>
 * If the <code>soft flag for the annotation is not set but the field is static, then
 * the <a href="http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom">initialization on demand holder idiom is
 * used as follows:
 * <pre>
 * {@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
 * </pre>
 * becomes these methods and inners classes within the class containing the above definitions:
 * <pre>
 * 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
 * }
 * </pre>
 *
 * @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;
}

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