AnnotationValueVisitor.java

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package javax.lang.model.element;

import java.util.List;

import javax.lang.model.type.TypeMirror;

/**
 * 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.
 *
 * <p> <b>WARNING:</b> 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&trade; programming
 * language.  Therefore, 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>Note that methods to accommodate new language constructs could
 * be added in a source <em>compatible</em> way if they were added as
 * <em>default methods</em>.  However, default methods are only
 * available on Java SE 8 and higher releases and the {@code
 * javax.lang.model.*} packages bundled in Java SE 8 were required to
 * also be runnable on Java SE 7.  Therefore, default methods
 * were <em>not</em> used when extending {@code javax.lang.model.*}
 * to cover Java SE 8 language features.  However, default methods
 * are used in subsequent revisions of the {@code javax.lang.model.*}
 * packages that are only required to run on Java SE 8 and higher
 * platform versions.
 *
 * @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
 */
// 注解值访问器，可以对不同类型的注解值执行相应的操作
public interface AnnotationValueVisitor<R, P> {
    /**
     * Visits an annotation value.
     * @param av the value to visit
     * @param p a visitor-specified parameter
     * @return  a visitor-specified result
     */
    // 自定义访问AnnotationValue的方式
    R visit(AnnotationValue av, P p);
    
    /**
     * 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
     */
    // 等同于visit(av, null)
    default R visit(AnnotationValue av) {
        return visit(av, null);
    }
    
    /**
     * 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
     */
    // 注解值类型为byte
    R visitByte(byte b, P p);
    
    /**
     * 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
     */
    // 注解值类型为short
    R visitShort(short s, P p);
    
    /**
     * 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
     */
    // 注解值类型为int
    R visitInt(int i, P p);
    
    /**
     * 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
     */
    // 注解值类型为long
    R visitLong(long i, P p);
    
    /**
     * 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
     */
    // 注解值类型为float
    R visitFloat(float f, P p);
    
    /**
     * 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
     */
    // 注解值类型为double
    R visitDouble(double d, P p);
    
    /**
     * 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
     */
    // 注解值类型为char
    R visitChar(char c, P p);
    
    /**
     * 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
     */
    // 注解值类型为boolean
    R visitBoolean(boolean b, P p);
    
    /**
     * 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
     */
    // 注解值类型为字符串
    R visitString(String s, P p);
    
    /**
     * 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
     */
    // 注解值类型为引用类型
    R visitType(TypeMirror t, P p);
    
    /**
     * 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
     */
    // 注解值类型为枚举
    R visitEnumConstant(VariableElement c, P p);
    
    /**
     * 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
     */
    // 注解值类型为注解
    R visitAnnotation(AnnotationMirror a, P p);
    
    /**
     * 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
     */
    // 注解值类型为数组
    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.
     * @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
     */
    // 注解值类型未知（在当前JDK版本下可能未知）
    R visitUnknown(AnnotationValue av, P p);
}