001    // Copyright 2006, 2009, 2010 The Apache Software Foundation
002    //
003    // Licensed under the Apache License, Version 2.0 (the "License");
004    // you may not use this file except in compliance with the License.
005    // You may obtain a copy of the License at
006    //
007    // http://www.apache.org/licenses/LICENSE-2.0
008    //
009    // Unless required by applicable law or agreed to in writing, software
010    // distributed under the License is distributed on an "AS IS" BASIS,
011    // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012    // See the License for the specific language governing permissions and
013    // limitations under the License.
014    
015    package org.apache.tapestry5.annotations;
016    
017    import java.lang.annotation.Documented;
018    import static java.lang.annotation.ElementType.FIELD;
019    import java.lang.annotation.Retention;
020    import static java.lang.annotation.RetentionPolicy.RUNTIME;
021    import java.lang.annotation.Target;
022    
023    import static org.apache.tapestry5.ioc.annotations.AnnotationUseContext.*;
024    import org.apache.tapestry5.ioc.annotations.UseWith;
025    
026    /**
027     * Marker annotation placed on fields whose value should be retained past the end of the request. This is most often
028     * associated with fields that are <em>lazily loaded</em>. By marking such fields with the Retain annotation, the fields
029     * will <em>not</em> be discarded at the end of the request.
030     * <p/>
031     * This is quite different from {@link Persist}, because the value that's allowed to be retained is not stored
032     * persistently; it is simply not cleared out. A subsequent request, even from the same user, may be processed by a
033     * different instance of the page where the value is still null.
034     * <p/>
035     * This annotation should only be used with lazily-evaluated objects that contain no client-specific information.
036     * 
037     * @deprecated This rarely used annotation is likely to cause threading issues starting in Tapestry 5.2 (which no longer
038     *             pools pages
039     *             but uses shared instances with externalized mutable state)
040     */
041    @Target(FIELD)
042    @Retention(RUNTIME)
043    @Documented
044    @UseWith(
045    { COMPONENT, MIXIN, PAGE })
046    public @interface Retain
047    {
048    
049    }