Java tutorial
/* * Copyright (c) 2003, 2018, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.lang.reflect; import java.lang.annotation.Annotation; import java.lang.annotation.AnnotationFormatError; import java.lang.annotation.Repeatable; import java.util.Arrays; import java.util.LinkedHashMap; import java.util.Map; import java.util.Objects; import java.util.function.Function; import java.util.stream.Collectors; import sun.reflect.annotation.AnnotationSupport; import sun.reflect.annotation.AnnotationType; /** * Represents an annotated element of the program currently running in this * VM. This interface allows annotations to be read reflectively. All * annotations returned by methods in this interface are immutable and * serializable. The arrays returned by methods of this interface may be modified * by callers without affecting the arrays returned to other callers. * * <p>The {@link #getAnnotationsByType(Class)} and {@link * #getDeclaredAnnotationsByType(Class)} methods support multiple * annotations of the same type on an element. If the argument to * either method is a repeatable annotation type (JLS 9.6), then the * method will "look through" a container annotation (JLS 9.7), if * present, and return any annotations inside the container. Container * annotations may be generated at compile-time to wrap multiple * annotations of the argument type. * * <p>The terms <em>directly present</em>, <em>indirectly present</em>, * <em>present</em>, and <em>associated</em> are used throughout this * interface to describe precisely which annotations are returned by * methods: * * <ul> * * <li> An annotation <i>A</i> is <em>directly present</em> on an * element <i>E</i> if <i>E</i> has a {@code * RuntimeVisibleAnnotations} or {@code * RuntimeVisibleParameterAnnotations} or {@code * RuntimeVisibleTypeAnnotations} attribute, and the attribute * contains <i>A</i>. * * <li>An annotation <i>A</i> is <em>indirectly present</em> on an * element <i>E</i> if <i>E</i> has a {@code RuntimeVisibleAnnotations} or * {@code RuntimeVisibleParameterAnnotations} or {@code RuntimeVisibleTypeAnnotations} * attribute, and <i>A</i> 's type is repeatable, and the attribute contains * exactly one annotation whose value element contains <i>A</i> and whose * type is the containing annotation type of <i>A</i> 's type. * * <li>An annotation <i>A</i> is <em>present</em> on an element <i>E</i> if either: * * <ul> * * <li><i>A</i> is directly present on <i>E</i>; or * * <li>No annotation of <i>A</i> 's type is directly present on * <i>E</i>, and <i>E</i> is a class, and <i>A</i> 's type is * inheritable, and <i>A</i> is present on the superclass of <i>E</i>. * * </ul> * * <li>An annotation <i>A</i> is <em>associated</em> with an element <i>E</i> * if either: * * <ul> * * <li><i>A</i> is directly or indirectly present on <i>E</i>; or * * <li>No annotation of <i>A</i> 's type is directly or indirectly * present on <i>E</i>, and <i>E</i> is a class, and <i>A</i>'s type * is inheritable, and <i>A</i> is associated with the superclass of * <i>E</i>. * * </ul> * * </ul> * * <p>The table below summarizes which kind of annotation presence * different methods in this interface examine. * * <table class="plain"> * <caption>Overview of kind of presence detected by different AnnotatedElement methods</caption> * <thead> * <tr><th colspan=2 scope="col">Method</th> * <th colspan=4 scope="col">Kind of Presence</th> * <tr><th scope="col">Return Type</th> * <th scope="col">Signature</th> * <th scope="col">Directly Present</th> * <th scope="col">Indirectly Present</th> * <th scope="col">Present</th> * <th scope="col">Associated</th> * </thead> * <tbody> * <tr><td style="text-align:right">{@code T}</td> * <th scope="row" style="font-weight:normal; text-align:left">{@link #getAnnotation(Class) getAnnotation(Class<T>)} * <td></td><td></td><td style="text-align:center">X</td><td></td> * </tr> * <tr><td style="text-align:right">{@code Annotation[]}</td> * <th scope="row" style="font-weight:normal; text-align:left">{@link #getAnnotations getAnnotations()} * <td></td><td></td><td style="text-align:center">X</td><td></td> * </tr> * <tr><td style="text-align:right">{@code T[]}</td> * <th scope="row" style="font-weight:normal; text-align:left">{@link #getAnnotationsByType(Class) getAnnotationsByType(Class<T>)} * <td></td><td></td><td></td><td style="text-align:center">X</td> * </tr> * <tr><td style="text-align:right">{@code T}</td> * <th scope="row" style="font-weight:normal; text-align:left">{@link #getDeclaredAnnotation(Class) getDeclaredAnnotation(Class<T>)} * <td style="text-align:center">X</td><td></td><td></td><td></td> * </tr> * <tr><td style="text-align:right">{@code Annotation[]}</td> * <th scope="row" style="font-weight:normal; text-align:left">{@link #getDeclaredAnnotations getDeclaredAnnotations()} * <td style="text-align:center">X</td><td></td><td></td><td></td> * </tr> * <tr><td style="text-align:right">{@code T[]}</td> * <th scope="row" style="font-weight:normal; text-align:left">{@link #getDeclaredAnnotationsByType(Class) getDeclaredAnnotationsByType(Class<T>)} * <td style="text-align:center">X</td><td style="text-align:center">X</td><td></td><td></td> * </tr> * </tbody> * </table> * * <p>For an invocation of {@code get[Declared]AnnotationsByType(Class <T>)}, * the order of annotations which are directly or indirectly * present on an element <i>E</i> is computed as if indirectly present * annotations on <i>E</i> are directly present on <i>E</i> in place * of their container annotation, in the order in which they appear in * the value element of the container annotation. * * <p>There are several compatibility concerns to keep in mind if an * annotation type <i>T</i> is originally <em>not</em> repeatable and * later modified to be repeatable. * * The containing annotation type for <i>T</i> is <i>TC</i>. * * <ul> * * <li>Modifying <i>T</i> to be repeatable is source and binary * compatible with existing uses of <i>T</i> and with existing uses * of <i>TC</i>. * * That is, for source compatibility, source code with annotations of * type <i>T</i> or of type <i>TC</i> will still compile. For binary * compatibility, class files with annotations of type <i>T</i> or of * type <i>TC</i> (or with other kinds of uses of type <i>T</i> or of * type <i>TC</i>) will link against the modified version of <i>T</i> * if they linked against the earlier version. * * (An annotation type <i>TC</i> may informally serve as an acting * containing annotation type before <i>T</i> is modified to be * formally repeatable. Alternatively, when <i>T</i> is made * repeatable, <i>TC</i> can be introduced as a new type.) * * <li>If an annotation type <i>TC</i> is present on an element, and * <i>T</i> is modified to be repeatable with <i>TC</i> as its * containing annotation type then: * * <ul> * * <li>The change to <i>T</i> is behaviorally compatible with respect * to the {@code get[Declared]Annotation(Class<T>)} (called with an * argument of <i>T</i> or <i>TC</i>) and {@code * get[Declared]Annotations()} methods because the results of the * methods will not change due to <i>TC</i> becoming the containing * annotation type for <i>T</i>. * * <li>The change to <i>T</i> changes the results of the {@code * get[Declared]AnnotationsByType(Class<T>)} methods called with an * argument of <i>T</i>, because those methods will now recognize an * annotation of type <i>TC</i> as a container annotation for <i>T</i> * and will "look through" it to expose annotations of type <i>T</i>. * * </ul> * * <li>If an annotation of type <i>T</i> is present on an * element and <i>T</i> is made repeatable and more annotations of * type <i>T</i> are added to the element: * * <ul> * * <li> The addition of the annotations of type <i>T</i> is both * source compatible and binary compatible. * * <li>The addition of the annotations of type <i>T</i> changes the results * of the {@code get[Declared]Annotation(Class<T>)} methods and {@code * get[Declared]Annotations()} methods, because those methods will now * only see a container annotation on the element and not see an * annotation of type <i>T</i>. * * <li>The addition of the annotations of type <i>T</i> changes the * results of the {@code get[Declared]AnnotationsByType(Class<T>)} * methods, because their results will expose the additional * annotations of type <i>T</i> whereas previously they exposed only a * single annotation of type <i>T</i>. * * </ul> * * </ul> * * <p>If an annotation returned by a method in this interface contains * (directly or indirectly) a {@link Class}-valued member referring to * a class that is not accessible in this VM, attempting to read the class * by calling the relevant Class-returning method on the returned annotation * will result in a {@link TypeNotPresentException}. * * <p>Similarly, attempting to read an enum-valued member will result in * a {@link EnumConstantNotPresentException} if the enum constant in the * annotation is no longer present in the enum type. * * <p>If an annotation type <i>T</i> is (meta-)annotated with an * {@code @Repeatable} annotation whose value element indicates a type * <i>TC</i>, but <i>TC</i> does not declare a {@code value()} method * with a return type of <i>T</i>{@code []}, then an exception of type * {@link java.lang.annotation.AnnotationFormatError} is thrown. * * <p>Finally, attempting to read a member whose definition has evolved * incompatibly will result in a {@link * java.lang.annotation.AnnotationTypeMismatchException} or an * {@link java.lang.annotation.IncompleteAnnotationException}. * * @see java.lang.EnumConstantNotPresentException * @see java.lang.TypeNotPresentException * @see AnnotationFormatError * @see java.lang.annotation.AnnotationTypeMismatchException * @see java.lang.annotation.IncompleteAnnotationException * @since 1.5 * @author Josh Bloch */ public interface AnnotatedElement { /** * Returns true if an annotation for the specified type * is <em>present</em> on this element, else false. This method * is designed primarily for convenient access to marker annotations. * * <p>The truth value returned by this method is equivalent to: * {@code getAnnotation(annotationClass) != null} * * <p>The body of the default method is specified to be the code * above. * * @param annotationClass the Class object corresponding to the * annotation type * @return true if an annotation for the specified annotation * type is present on this element, else false * @throws NullPointerException if the given annotation class is null * @since 1.5 */ default boolean isAnnotationPresent(Class<? extends Annotation> annotationClass) { return getAnnotation(annotationClass) != null; } /** * Returns this element's annotation for the specified type if * such an annotation is <em>present</em>, else null. * * @param <T> the type of the annotation to query for and return if present * @param annotationClass the Class object corresponding to the * annotation type * @return this element's annotation for the specified annotation type if * present on this element, else null * @throws NullPointerException if the given annotation class is null * @since 1.5 */ <T extends Annotation> T getAnnotation(Class<T> annotationClass); /** * Returns annotations that are <em>present</em> on this element. * * If there are no annotations <em>present</em> on this element, the return * value is an array of length 0. * * The caller of this method is free to modify the returned array; it will * have no effect on the arrays returned to other callers. * * @return annotations present on this element * @since 1.5 */ Annotation[] getAnnotations(); /** * Returns annotations that are <em>associated</em> with this element. * * If there are no annotations <em>associated</em> with this element, the return * value is an array of length 0. * * The difference between this method and {@link #getAnnotation(Class)} * is that this method detects if its argument is a <em>repeatable * annotation type</em> (JLS 9.6), and if so, attempts to find one or * more annotations of that type by "looking through" a container * annotation. * * The caller of this method is free to modify the returned array; it will * have no effect on the arrays returned to other callers. * * @implSpec The default implementation first calls {@link * #getDeclaredAnnotationsByType(Class)} passing {@code * annotationClass} as the argument. If the returned array has * length greater than zero, the array is returned. If the returned * array is zero-length and this {@code AnnotatedElement} is a * class and the argument type is an inheritable annotation type, * and the superclass of this {@code AnnotatedElement} is non-null, * then the returned result is the result of calling {@link * #getAnnotationsByType(Class)} on the superclass with {@code * annotationClass} as the argument. Otherwise, a zero-length * array is returned. * * @param <T> the type of the annotation to query for and return if present * @param annotationClass the Class object corresponding to the * annotation type * @return all this element's annotations for the specified annotation type if * associated with this element, else an array of length zero * @throws NullPointerException if the given annotation class is null * @since 1.8 */ default <T extends Annotation> T[] getAnnotationsByType(Class<T> annotationClass) { /* * Definition of associated: directly or indirectly present OR * neither directly nor indirectly present AND the element is * a Class, the annotation type is inheritable, and the * annotation type is associated with the superclass of the * element. */ T[] result = getDeclaredAnnotationsByType(annotationClass); if (result.length == 0 && // Neither directly nor indirectly present this instanceof Class && // the element is a class AnnotationType.getInstance(annotationClass).isInherited()) { // Inheritable Class<?> superClass = ((Class<?>) this).getSuperclass(); if (superClass != null) { // Determine if the annotation is associated with the // superclass result = superClass.getAnnotationsByType(annotationClass); } } return result; } /** * Returns this element's annotation for the specified type if * such an annotation is <em>directly present</em>, else null. * * This method ignores inherited annotations. (Returns null if no * annotations are directly present on this element.) * * @implSpec The default implementation first performs a null check * and then loops over the results of {@link * #getDeclaredAnnotations} returning the first annotation whose * annotation type matches the argument type. * * @param <T> the type of the annotation to query for and return if directly present * @param annotationClass the Class object corresponding to the * annotation type * @return this element's annotation for the specified annotation type if * directly present on this element, else null * @throws NullPointerException if the given annotation class is null * @since 1.8 */ default <T extends Annotation> T getDeclaredAnnotation(Class<T> annotationClass) { Objects.requireNonNull(annotationClass); // Loop over all directly-present annotations looking for a matching one for (Annotation annotation : getDeclaredAnnotations()) { if (annotationClass.equals(annotation.annotationType())) { // More robust to do a dynamic cast at runtime instead // of compile-time only. return annotationClass.cast(annotation); } } return null; } /** * Returns this element's annotation(s) for the specified type if * such annotations are either <em>directly present</em> or * <em>indirectly present</em>. This method ignores inherited * annotations. * * If there are no specified annotations directly or indirectly * present on this element, the return value is an array of length * 0. * * The difference between this method and {@link * #getDeclaredAnnotation(Class)} is that this method detects if its * argument is a <em>repeatable annotation type</em> (JLS 9.6), and if so, * attempts to find one or more annotations of that type by "looking * through" a container annotation if one is present. * * The caller of this method is free to modify the returned array; it will * have no effect on the arrays returned to other callers. * * @implSpec The default implementation may call {@link * #getDeclaredAnnotation(Class)} one or more times to find a * directly present annotation and, if the annotation type is * repeatable, to find a container annotation. If annotations of * the annotation type {@code annotationClass} are found to be both * directly and indirectly present, then {@link * #getDeclaredAnnotations()} will get called to determine the * order of the elements in the returned array. * * <p>Alternatively, the default implementation may call {@link * #getDeclaredAnnotations()} a single time and the returned array * examined for both directly and indirectly present * annotations. The results of calling {@link * #getDeclaredAnnotations()} are assumed to be consistent with the * results of calling {@link #getDeclaredAnnotation(Class)}. * * @param <T> the type of the annotation to query for and return * if directly or indirectly present * @param annotationClass the Class object corresponding to the * annotation type * @return all this element's annotations for the specified annotation type if * directly or indirectly present on this element, else an array of length zero * @throws NullPointerException if the given annotation class is null * @since 1.8 */ default <T extends Annotation> T[] getDeclaredAnnotationsByType(Class<T> annotationClass) { Objects.requireNonNull(annotationClass); return AnnotationSupport .getDirectlyAndIndirectlyPresent( Arrays.stream(getDeclaredAnnotations()).collect(Collectors.toMap(Annotation::annotationType, Function.identity(), ((first, second) -> first), LinkedHashMap::new)), annotationClass); } /** * Returns annotations that are <em>directly present</em> on this element. * This method ignores inherited annotations. * * If there are no annotations <em>directly present</em> on this element, * the return value is an array of length 0. * * The caller of this method is free to modify the returned array; it will * have no effect on the arrays returned to other callers. * * @return annotations directly present on this element * @since 1.5 */ Annotation[] getDeclaredAnnotations(); }