注解卫雪峰7 MinutesMay 18, 2019

注解

注解（元数据）为我们在代码中添加信息提供了一种形式化的方法，使我们可以在稍后某个时刻非常方便的使用这些数据。

三种标准注解：@Override, @Deprecated, @SuppressWarnings

四种元注解: @Target, @Rentetion, @Documented, @Inherited(允许子类继承父类中的注解)

1 基本语法

1.1 定义注解

@Target(ElementType.METHOD)
@Rentention(RententionPolicy.RUNTIME)
public @interface Test{}

• @Target 定义注解应用于什么地方
• @Retention 定义注解在哪一个级别使用

案例：使用注解掌握项目进度

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface UseCase {
    public int id();
    public String desc() default "no description";
}

public class PasswordUtil {

    @UseCase(id = 1, desc = "Password must contain at least one numeric")
    public boolean validatePassword(String password){
        return password.matches("\\w*\\d\\w*");
    }

    @UseCase(id = 2)
    public String encryptPassword(String password){
        return new StringBuffer(password).reverse().toString();
    }

    @UseCase(id = 3, desc = "new password can not equal previously used ones")
    public boolean checkForNewPassword(List<String> prePasswords, String newPassword){
        return !prePasswords.contains(newPassword);
    }
}

1.2 编写注解处理器

 public static void trackUseCases(List<Integer> useCases, Class<?> clz) {
        for (Method method : clz.getDeclaredMethods()){
            UseCase usecase = method.getAnnotation(UseCase.class);
            if(usecase != null) {
                System.out.println("Found use case:" + usecase.id() +" " + usecase.desc());
                useCases.remove(new Integer(usecase.id()));
            }
        }
        for (int i : useCases) {
            System.out.println("Warning: Missing use case-" + i);
        }
    }

    public static void main(String args[]) {
        List<Integer> useCases = new ArrayList<>();
        Collections.addAll(useCases, 1, 2, 3, 4, 5);
        trackUseCases(useCases, PasswordUtil.class);
    }
    
    ----------
Found use case:2 no description
Found use case:1 Password must contain at least one numeric
Found use case:3 new password can not equal previously used ones
Warning: Missing use case-4
Warning: Missing use case-5

1.2.1 生成外部文件