注解（也被称为元数据）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地使用这些数据。

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1.定义注解：

package com.test;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

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

2.在下面的类中，有三个方法被注解为用例：

package com.test;

import java.util.List;

public class PasswordUtils {
	@UseCase(id = 47, description = "Passwords must contain at lease 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 password can't equal previously used ones")
	public boolean checkForNewPassword(List<String> prevPasswords, String password) {
		return !prevPasswords.contains(password);
	}
}

package com.test;

import java.lang.reflect.*;
import java.util.*;

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);
	}
}

Found Use Case:49 New password can‘t equal previously used ones
Found Use Case:47 Passwords must contain at lease one numeric
Found Use Case:48 no description
Warning: Missing use case-50

Java注解(入门)