001    /*
002     *  This file is part of the Jikes RVM project (http://jikesrvm.org).
003     *
004     *  This file is licensed to You under the Eclipse Public License (EPL);
005     *  You may not use this file except in compliance with the License. You
006     *  may obtain a copy of the License at
007     *
008     *      http://www.opensource.org/licenses/eclipse-1.0.php
009     *
010     *  See the COPYRIGHT.txt file distributed with this work for information
011     *  regarding copyright ownership.
012     */
013    package org.vmmagic;
014    
015    import java.lang.annotation.Retention;
016    import java.lang.annotation.RetentionPolicy;
017    import java.lang.annotation.Target;
018    import java.lang.annotation.ElementType;
019    import java.lang.annotation.Inherited;
020    
021    /**
022     * The Unboxed annotation marks a type as unboxed. Besides the primitive types, all Java values
023     * are boxed types. Conceptually, they are represented by a pointer to a heap object. However,
024     * an unboxed type is represented by the value itself. All methods on an unboxed type are
025     * {@link Intrinsic}s.
026     *
027     * <p>As an unboxed type is not a regular java object there are a few constraints on the way unboxed
028     * types are handled;</p>
029     * <ul>
030     *   <li>All methods are {@link Intrinsic} and thus there can be no virtual methods.</li>
031     *   <li>An unboxed type can not be synchronized on.</li>
032     *   <li>An unboxed type has no hashcode.</li>
033     *   <li>An unboxed type MUST NOT be passed where an object is expected or when two overloaded methods
034     *       can only be distinguished by by Object vs unboxed parameter type.</li>
035     * </ul>
036     *
037     * <p>NOTE: At the current time the Unboxed annotation is only used for documentation
038     * purposes but in the near future it is expected that the semantics of the annotation will
039     * be enforced by the compiler.</p>
040     */
041    @Retention(RetentionPolicy.RUNTIME)
042    @Target(ElementType.TYPE)
043    @Inherited
044    public @interface Unboxed { }