十、适配器模式（Adapter Pattern）

10.1 介绍

适配器模式属于结构性模式，它为两个不同接口之间互通提供了一种手段。

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10.2 Spring中MethodInterceptor适配器

在Spring Aop框架中，MethodInterceptor接口被用来拦截指定的方法，对方法进行增强。

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大家都知道在Aop中每个advistor 里面会有一个advice具体做切面动作，Spring提供了AspectJAfterReturningAdvice，AspectJMethodBeforeAdvice，AspectJAroundAdvice，AspectJAfterAdvice这几个advice,在XML 配置aop时候会指定<aop:after-returning/>,<aop:before/>,<aop:around/>,<aop:after/>,其实内部就是创建上面对应的这些advice。

从图知道AspectJAfterReturningAdvice和AspectJMethodBeforeAdvice没有实现MethodInterceptor接口，其他两者则实现了该接口。而Spring Aop的方法拦截器却必须是实现了MethodInterceptor的，所以Spring提供了对应的适配器来适配这个问题,分别是MethodBeforeAdviceAdapter和AfterReturningAdviceAdapter和ThrowsAdviceAdapter。

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看下DefaultAdvisorAdapterRegistry的 getInterceptors方法：

public MethodInterceptor[] getInterceptors(Advisor advisor) throws UnknownAdviceTypeException {
    List<MethodInterceptor> interceptors = new ArrayList<MethodInterceptor>(3);

    //从advistor中获取advice
    Advice advice = advisor.getAdvice();

    //如果实现了MethodInterceptor则直接加入，比如AspectJAroundAdvice，AspectJAfterAdvice
    if (advice instanceof MethodInterceptor) {
        interceptors.add((MethodInterceptor) advice);
    }

    //否者看是否有当前advice的适配器，首先检验是否支持，支持则返回对应的适配器
    for (AdvisorAdapter adapter : this.adapters) {
        if (adapter.supportsAdvice(advice)) {
            interceptors.add(adapter.getInterceptor(advisor));
        }
    }
    if (interceptors.isEmpty()) {
        throw new UnknownAdviceTypeException(advisor.getAdvice());
    }
    return interceptors.toArray(new MethodInterceptor[interceptors.size()]);
}

以MethodBeforeAdviceAdapter为例子看下：

class MethodBeforeAdviceAdapter implements AdvisorAdapter, Serializable {

    public boolean supportsAdvice(Advice advice) {
        return (advice instanceof MethodBeforeAdvice);
    }

    public MethodInterceptor getInterceptor(Advisor advisor) {
        MethodBeforeAdvice advice = (MethodBeforeAdvice) advisor.getAdvice();
        return new MethodBeforeAdviceInterceptor(advice);
    }

}

public class MethodBeforeAdviceInterceptor implements MethodInterceptor, Serializable {

    private MethodBeforeAdvice advice;


    /**
     * Create a new MethodBeforeAdviceInterceptor for the given advice.
     * @param advice the MethodBeforeAdvice to wrap
     */
    public MethodBeforeAdviceInterceptor(MethodBeforeAdvice advice) {
        Assert.notNull(advice, "Advice must not be null");
        this.advice = advice;
    }

    public Object invoke(MethodInvocation mi) throws Throwable {
        this.advice.before(mi.getMethod(), mi.getArguments(), mi.getThis() );
        return mi.proceed();
    }

}

可知MethodBeforeAdviceInterceptor继承了MethodInterceptor作为了一个适配器内部委托请求给MethodBeforeAdvice。