02.学习java中的@interface

• 注解是Java对于其他语言对Java造成的语言特性压力做出的一种回应，是众多引入到Java SE5中的重要的语言变化之一。

• Java1.6内置了三种标准注解和四种元注解（元注解专职负责注解其他注解）。

• 大多数时候我们都是定义自己的注解，并编写自己的处理器来处理它们。

  
  内置的三种标准注解
  元注解

• @Target(ElementType.METHOD)

package java.lang.annotation;

/**
 * @author  Joshua Bloch
 * @since 1.5
 * @jls 9.6.4.1 @Target
 * @jls 4.1 The Kinds of Types and Values
 */
public enum ElementType {
    /** Class, interface (including annotation type), or enum declaration */
    TYPE,

    /** Field declaration (includes enum constants) */
    FIELD,

    /** Method declaration */
    METHOD,

    /** Formal parameter declaration */
    PARAMETER,

    /** Constructor declaration */
    CONSTRUCTOR,

    /** Local variable declaration */
    LOCAL_VARIABLE,

    /** Annotation type declaration */
    ANNOTATION_TYPE,

    /** Package declaration */
    PACKAGE,

    /**
     * Type parameter declaration
     *
     * @since 1.8
     */
    TYPE_PARAMETER,

    /**
     * Use of a type
     *
     * @since 1.8
     */
    TYPE_USE
}

• @Retention(RetentionPolicy.RUNTIME)

package java.lang.annotation;

/**
 * @author  Joshua Bloch
 * @since 1.5
 */
public enum RetentionPolicy {
    /**
     * Annotations are to be discarded by the compiler.
     */
    SOURCE,

    /**
     * Annotations are to be recorded in the class file by the compiler
     * but need not be retained by the VM at run time.  This is the default
     * behavior.
     */
    CLASS,

    /**
     * Annotations are to be recorded in the class file by the compiler and
     * retained by the VM at run time, so they may be read reflectively.
     *
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME
}

定义注解

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface UseCase {
    public int id();

    public String description() default "no description";
}

使用注解

public class PasswordUtils {
    @UseCase(id = 47, description =
            "Passwords must contain at least one numeric")
    public boolean validatePassword(String password) {
        return (password.matches("\\w*\\d\\w*"));
    }

    @UseCase(id = 48)
    public String encryptPassword(String password) {
        return new StringBuilder(password).reverse().toString();
    }

    @UseCase(id = 49, description =
            "New passwords can't equal previously used ones")
    public boolean checkForNewPassword(
            List<String> prevPasswords, String password) {
        return !prevPasswords.contains(password);
    }
}

编写注解处理器

public class UseCaseTracker {
    public static void trackUseCases(List<Integer> useCases, Class<?> cl) {
        for (Method m : cl.getDeclaredMethods()) {
            UseCase uc = m.getAnnotation(UseCase.class);
            if (uc != null) {
                System.out.println("Found Use Case: " + uc.id() + " " + uc.description());
                useCases.remove(new Integer(uc.id()));
            }
        }
        for (int i : useCases) {
            System.out.println("Warning: Missing use case-" + i);
        }
    }
    public static void main(String[] args) {
        List<Integer> useCases = new ArrayList<Integer>();
        Collections.addAll(useCases, 47, 48, 49, 50);
        trackUseCases(useCases, PasswordUtils.class);
    }
}

• 运行结果：

Found Use Case: 47 Passwords must contain at least one numeric
Found Use Case: 48 no description
Found Use Case: 49 New passwords can't equal previously used ones
Warning: Missing use case-50

几个注意点：

• 标签@UseCase由UseCase.java定义，其中包括int元素id,以及一个String元素description。

• 注解元素可用的类型如下（否则编译器报错）：

  • 所有基本类型（int, float, boolean等）
  • String
  • Class
  • enum
  • Annotation
  • 以上类型的数组

• 注解元素必须要么具有默认值，要么在使用注解时提供元素的值。

• 注解不支持继承。（见Thinking in Java Fourth Edition 20.2.4）

参考：

• Thinking in Java Fourth Edition (Bruce Eckel)