Java 8对注解处理提供了两点改进：可重复的注解及可用于类型的注解。

    写一个注解，如下

    1. @Retention(RetentionPolicy.RUNTIME)
    2. @Target({TYPE, METHOD, FIELD, PARAMETER, CONSTRUCTOR, 
    3.          LOCAL_VARIABLE, ANNOTATION_TYPE, PACKAGE, TYPE_PARAMETER, TYPE_USE})
    4. public @interface MyAnnotation {
    5.     String value() default "编程指南";
    6. }
    8. //其中RetentionPolicy 
    9. public enum RetentionPolicy {
    10.     /**
    11.      * Annotations are to be discarded by the compiler.
    12.      */
    13.     SOURCE,
    15.     /**
    16.      * Annotations are to be recorded in the class file by the compiler
    17.      * but need not be retained by the VM at run time.  This is the default
    18.      * behavior.
    19.      */
    20.     CLASS,
    22.     /**
    23.      * Annotations are to be recorded in the class file by the compiler and
    24.      * retained by the VM at run time, so they may be read reflectively.
    25.      *
    26.      * @see java.lang.reflect.AnnotatedElement
    27.      */
    28.     RUNTIME
    29. }
    31. //ElementType如下
    32. public enum ElementType {
    33.     /** Class, interface (including annotation type), or enum declaration */
    34.     TYPE,
    36.     /** Field declaration (includes enum constants) */
    37.     FIELD,
    39.     /** Method declaration */
    40.     METHOD,
    42.     /** Formal parameter declaration */
    43.     PARAMETER,
    45.     /** Constructor declaration */
    46.     CONSTRUCTOR,
    48.     /** Local variable declaration */
    49.     LOCAL_VARIABLE,
    51.     /** Annotation type declaration */
    52.     ANNOTATION_TYPE,
    54.     /** Package declaration */
    55.     PACKAGE,
    57.     /**
    58.      * Type parameter declaration
    59.      *
    60.      * @since 1.8
    61.      */
    62.     TYPE_PARAMETER,
    64.     /**
    65.      * Use of a type
    66.      *
    67.      * @since 1.8
    68.      */
    69.     TYPE_USE
    70. }

    jdk8以前想实现可重复的注解需要这样

    1. //1. 写一个注解
    2. @Retention(RetentionPolicy.RUNTIME)
    3. @Target({TYPE, METHOD, FIELD, PARAMETER, CONSTRUCTOR,
    4.          LOCAL_VARIABLE, ANNOTATION_TYPE, PACKAGE, TYPE_PARAMETER, TYPE_USE})
    5. public @interface MyAnnotation {
    6.     String value() default "编程指南";
    7. }
    8. //2. 写一个数组类型的注解
    9. @Retention(RetentionPolicy.RUNTIME)
    10. @Target({TYPE, METHOD, FIELD, PARAMETER, CONSTRUCTOR,
    11.          LOCAL_VARIABLE, ANNOTATION_TYPE, PACKAGE, TYPE_PARAMETER, TYPE_USE})
    12. public @interface MyAnnotations {
    13.     MyAnnotation[] value() default {};
    14. }
    15. //3.使用示例
    16. //重复注解报错  //Duplicate annotation. The declaration of 
    17. //'com.initit.java8.annotation.MyAnnotation' 
    18. //does not have a valid java.lang.annotation.Repeatable annotation
    19. //@MyAnnotation
    20. @MyAnnotation
    21. //重复注解这样用
    22. @MyAnnotations({
    23.         @MyAnnotation("注解1"),
    24.         @MyAnnotation("注解2"),
    25.         @MyAnnotation("注解3"),
    26. })
    27. public class AnnotationDemo {
    29. }

    jdk8提供的可重复注解

    • 增加元注解，标注为可以重复的注解@Repeatable(MyAnnotations.class)，其中参数是一个数组类型的注解
    1. //1.写一个注解 并增加@Repeatable(MyAnnotations.class)
    2. @Repeatable(MyAnnotations.class)
    3. @Retention(RetentionPolicy.RUNTIME)
    4. @Target({TYPE, METHOD, FIELD, PARAMETER, CONSTRUCTOR,
    5.          LOCAL_VARIABLE, ANNOTATION_TYPE, PACKAGE, TYPE_PARAMETER, TYPE_USE})
    6. public @interface MyAnnotation {
    7.     String value() default "编程指南";
    8. }
    9. //2.建MyAnnotations.class
    10. @Retention(RetentionPolicy.RUNTIME)
    11. @Target({TYPE, METHOD, FIELD, PARAMETER, CONSTRUCTOR, 
    12.          LOCAL_VARIABLE, ANNOTATION_TYPE, PACKAGE, TYPE_PARAMETER, TYPE_USE})
    13. public @interface MyAnnotations {
    14.     MyAnnotation[] value() default {};
    15. }
    16. //3. 使用
    17. @MyAnnotation("注解1"),
    18. @MyAnnotation("注解2"),
    19. @MyAnnotation("注解3"),
    20. public class AnnotationDemo {
    21.     //jdk8还增加了 类型注解
    22.     private @MyAnnotation String name;
    23. }