https://openjdk.java.net/ 
/*
 * Copyright (c) 2005, 2020, 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 javax.lang.model.element;


import java.util.List;
import javax.lang.model.type.TypeMirror;
import javax.lang.model.util.*;


A visitor of the values of annotation type elements, using a variant of the visitor design pattern. Unlike a standard visitor which dispatches based on the concrete type of a member of a type hierarchy, this visitor dispatches based on the type of data stored; there are no distinct subclasses for storing, for example, boolean values versus int values. Classes implementing this interface are used to operate on a value when the type of that value is unknown at compile time. When a visitor is passed to a value's accept method, the visitXyz method applicable to that value is
invoked.

 Classes implementing this interface may or may not throw a NullPointerException if the additional parameter p is null; see documentation of the implementing class for details. 
Author:Joseph D. Darcy, Scott Seligman, Peter von der Ahé
Type parameters:
  • <R> – the return type of this visitor's methods
  • <P> – the type of the additional parameter to this visitor's methods.
API Note:
WARNING: It is possible that methods will be added
to this interface to accommodate new, currently unknown, language
structures added to future versions of the Java programming
language.
Such additions have already occurred in another visitor interface in
this package to support language features added after this API was
introduced.
Visitor classes directly implementing this interface may be source
incompatible with future versions of the platform.  To avoid this
source incompatibility, visitor implementations are encouraged to
instead extend the appropriate abstract visitor class that
implements this interface.  However, an API should generally use
this visitor interface as the type for parameters, return type,
etc. rather than one of the abstract classes.

Methods to accommodate new language constructs are expected to be added as default methods to provide strong source compatibility, as done for visitModule in ElementVisitor. The implementations of the default methods in this interface will in turn call 
visitUnknown, behavior that will be overridden in concrete visitors supporting the source version with the new language construct. 
There are several families of classes implementing this visitor interface in the util
package. The families follow a naming pattern along the lines of FooVisitorN where N indicates the source version the visitor is appropriate for. In particular, a FooVisitorN is expected to handle
all language constructs present in source version N. If
there are no new language constructs added in version
N + 1 (or subsequent releases), 
FooVisitorN may also handle that later source version; in that case, the 
SupportedSourceVersion annotation on the 
FooVisitorN class will indicate a later version.
When visiting an annotation value representing a language construct
introduced after source version N, a 
FooVisitorN will throw an UnknownAnnotationValueException unless that behavior is overridden. 
When choosing which member of a visitor family to subclass,
subclassing the most recent one increases the range of source
versions covered. When choosing which visitor family to subclass,
consider their built-in capabilities:

Since:1.6
/**
 * A visitor of the values of annotation type elements, using a
 * variant of the visitor design pattern.  Unlike a standard visitor
 * which dispatches based on the concrete type of a member of a type
 * hierarchy, this visitor dispatches based on the type of data
 * stored; there are no distinct subclasses for storing, for example,
 * {@code boolean} values versus {@code int} values.  Classes
 * implementing this interface are used to operate on a value when the
 * type of that value is unknown at compile time.  When a visitor is
 * passed to a value's {@link AnnotationValue#accept accept} method,
 * the <code>visit<i>Xyz</i></code> method applicable to that value is
 * invoked.
 *
 * <p> Classes implementing this interface may or may not throw a
 * {@code NullPointerException} if the additional parameter {@code p}
 * is {@code null}; see documentation of the implementing class for
 * details.
 *
 * @apiNote
 * <strong>WARNING:</strong> It is possible that methods will be added
 * to this interface to accommodate new, currently unknown, language
 * structures added to future versions of the Java programming
 * language.
 *
 * Such additions have already occurred in another visitor interface in
 * this package to support language features added after this API was
 * introduced.
 *
 * Visitor classes directly implementing this interface may be source
 * incompatible with future versions of the platform.  To avoid this
 * source incompatibility, visitor implementations are encouraged to
 * instead extend the appropriate abstract visitor class that
 * implements this interface.  However, an API should generally use
 * this visitor interface as the type for parameters, return type,
 * etc. rather than one of the abstract classes.
 *
 * <p>Methods to accommodate new language constructs are expected to
 * be added as default methods to provide strong source compatibility,
 * as done for {@link ElementVisitor#visitModule visitModule} in
 * {@code ElementVisitor}. The implementations of the default methods
 * in this interface will in turn call {@link visitUnknown
 * visitUnknown}, behavior that will be overridden in concrete
 * visitors supporting the source version with the new language
 * construct.
 *
 * <p>There are several families of classes implementing this visitor
 * interface in the {@linkplain javax.lang.model.util util
 * package}. The families follow a naming pattern along the lines of
 * {@code FooVisitor}<i>N</i> where <i>N</i> indicates the
 * {@linkplain javax.lang.model.SourceVersion source version} the
 * visitor is appropriate for.
 *
 * In particular, a {@code FooVisitor}<i>N</i> is expected to handle
 * all language constructs present in source version <i>N</i>. If
 * there are no new language constructs added in version
 * <i>N</i>&nbsp;+&nbsp;1 (or subsequent releases), {@code
 * FooVisitor}<i>N</i> may also handle that later source version; in
 * that case, the {@link
 * javax.annotation.processing.SupportedSourceVersion
 * SupportedSourceVersion} annotation on the {@code
 * FooVisitor}<i>N</i> class will indicate a later version.
 *
 * When visiting an annotation value representing a language construct
 * introduced <strong>after</strong> source version <i>N</i>, a {@code
 * FooVisitor}<i>N</i> will throw an {@link
 * UnknownAnnotationValueException} unless that behavior is overridden.
 *
 * <p>When choosing which member of a visitor family to subclass,
 * subclassing the most recent one increases the range of source
 * versions covered. When choosing which visitor family to subclass,
 * consider their built-in capabilities:
 *
 * <ul>
 *
 * <li>{@link AbstractAnnotationValueVisitor6
 * AbstractAnnotationValueVisitor}s: Skeletal visitor implementations.
 *
 * <li>{@link SimpleAnnotationValueVisitor6
 * SimpleAnnotationValueVisitor}s: Support default actions and a
 * default return value.
 *
 * </ul>
 *
 * @param <R> the return type of this visitor's methods
 * @param <P> the type of the additional parameter to this visitor's methods.
 * @author Joseph D. Darcy
 * @author Scott Seligman
 * @author Peter von der Ah&eacute;
 * @since 1.6
 */
public interface AnnotationValueVisitor<R, P> {
    
Visits an annotation value.

Params:
  • av – the value to visit
  • p – a visitor-specified parameter
Returns: a visitor-specified result
/**
     * Visits an annotation value.
     * @param av the value to visit
     * @param p a visitor-specified parameter
     * @return  a visitor-specified result
     */
    R visit(AnnotationValue av, P p);

    
A convenience method equivalent to visit(av, null). 
Params:
  • av – the value to visit
Implementation Requirements:The default implementation is visit(av, null).
Returns: a visitor-specified result
/**
     * A convenience method equivalent to {@code visit(av, null)}.
     *
     * @implSpec The default implementation is {@code visit(av, null)}.
     *
     * @param av the value to visit
     * @return  a visitor-specified result
     */
    default R visit(AnnotationValue av) {
        return visit(av, null);
    }

    
Visits a boolean value in an annotation. 
Params:
  • b – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits a {@code boolean} value in an annotation.
     * @param b the value being visited
     * @param p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitBoolean(boolean b, P p);

    
Visits a byte value in an annotation. 
Params:
  • b – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits a {@code byte} value in an annotation.
     * @param  b the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitByte(byte b, P p);

    
Visits a char value in an annotation. 
Params:
  • c – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits a {@code char} value in an annotation.
     * @param  c the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitChar(char c, P p);

    
Visits a double value in an annotation. 
Params:
  • d – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits a {@code double} value in an annotation.
     * @param  d the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitDouble(double d, P p);

    
Visits a float value in an annotation. 
Params:
  • f – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits a {@code float} value in an annotation.
     * @param  f the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitFloat(float f, P p);

    
Visits an int value in an annotation. 
Params:
  • i – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits an {@code int} value in an annotation.
     * @param  i the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitInt(int i, P p);

    
Visits a long value in an annotation. 
Params:
  • i – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits a {@code long} value in an annotation.
     * @param  i the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitLong(long i, P p);

    
Visits a short value in an annotation. 
Params:
  • s – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits a {@code short} value in an annotation.
     * @param  s the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitShort(short s, P p);

    
Visits a string value in an annotation.

Params:
  • s – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits a string value in an annotation.
     * @param  s the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitString(String s, P p);

    
Visits a type value in an annotation.

Params:
  • t – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits a type value in an annotation.
     * @param  t the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitType(TypeMirror t, P p);

    
Visits an enum value in an annotation. 
Params:
  • c – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits an {@code enum} value in an annotation.
     * @param  c the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitEnumConstant(VariableElement c, P p);

    
Visits an annotation value in an annotation.

Params:
  • a – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits an annotation value in an annotation.
     * @param  a the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitAnnotation(AnnotationMirror a, P p);

    
Visits an array value in an annotation.

Params:
  • vals – the value being visited
  • p – a visitor-specified parameter
Returns:the result of the visit
/**
     * Visits an array value in an annotation.
     * @param  vals the value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     */
    R visitArray(List<? extends AnnotationValue> vals, P p);

    
Visits an unknown kind of annotation value.
This can occur if the language evolves and new kinds
of value can be stored in an annotation.

Params:
  • av – the unknown value being visited
  • p – a visitor-specified parameter
Throws:
Returns:the result of the visit
/**
     * Visits an unknown kind of annotation value.
     * This can occur if the language evolves and new kinds
     * of value can be stored in an annotation.
     * @param  av the unknown value being visited
     * @param  p a visitor-specified parameter
     * @return the result of the visit
     * @throws UnknownAnnotationValueException
     *  a visitor implementation may optionally throw this exception
     */
    R visitUnknown(AnnotationValue av, P p);
}