# 5.4.5. 切面说明

切面说明（在 AspectJ 中称为类型间声明）使切面能够声明通知对象实现给定接口，并代表这些对象提供该接口的实现。

您可以使用`@DeclareParents`注解进行介绍。此注解用于声明匹配类型有一个新的父级（因此得名）。例如，给定一个名为`UsageTracked` 的接口和一个名为`DefaultUsageTracked` 的接口的实现， 以下切面声明服务接口的所有实现者也实现`UsageTracked`接口（例如，通过 JMX 进行统计）：

```java
@Aspect
public class UsageTracking {

    @DeclareParents(value="com.xzy.myapp.service.*+", defaultImpl=DefaultUsageTracked.class)
    public static UsageTracked mixin;

    @Before("com.xyz.myapp.CommonPointcuts.businessService() && this(usageTracked)")
    public void recordUsage(UsageTracked usageTracked) {
        usageTracked.incrementUseCount();
    }

}
```

要实现的接口由注解字段的类型决定。注解`@DeclareParents`的 `value`属性是 AspectJ 类型模式。任何匹配类型的 bean 都会实现`UsageTracked`接口。请注意，在前面示例的之前通知中，服务 bean 可以直接用作`UsageTracked`接口的实现。如果以编程方式访问 bean，您将编写以下内容：

```java
UsageTracked usageTracked = (UsageTracked) context.getBean("myService");
```


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