为何Redis选择单线程而非多线程架构？

In this blog, let us talk about Around Advice. There are some few points about it.

• Around Advice is combination of Before Advice and After Advice.
• In a single Around Advice we can implement both before and after services.
• Note, Around Advice is not given by spring framework, it is from Open Source implementation called AOP alliance.
• Around Advice can be used by any framework which supports AOP.
• Around Advice can access the return value of business method and it can modify the value and it can return a different value back to the client, as return type is Object, but in the After Advice its not possible right, as its return type is void.

In order to create Around Advice, we should implement an interface called MethodInterceptor and override the method called invoke which has a prarameter MethodInvocation type. We can use this parameter to invoke our business logic method like methodInvoation.proceed(). And we can add our before services before this code execution and put after services after this code.

Let’s see an instance.

package spring.test.aop;

import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;

public class MyAroundAdvice implements MethodInterceptor{

	@Override
	public Object invoke(MethodInvocation invocation) throws Throwable {
		// Adding before service in here.
		
		invocation.proceed();
		
		// Adding after service in here.
		// we can change the return value we want via below return codes.
		
		return null;
	}

}

In above codes, we can see that we put the before services before the proceed() method and put the after services after it. and we can change the return value of the logic method.

So now let us to see a full expample.

MyLogicInterface.java

package spring.test.aop;

public interface MyLogicInterface {
	int add(int a, int b);
}

MyLogicClass.java

package spring.test.aop;

public class MyLogicClass implements MyLogicInterface {

	@Override
	public int add(int a, int b) {
		System.out.println(String.format("The internal result of %s + %s is: %s", a, b, a + b));
		return a + b;
	}

}

In our logic class, we print the result value before the return of method.

MyAroundAdvice.java

package spring.test.aop;

import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;

public class MyAroundAdvice implements MethodInterceptor {

	@Override
	public Object invoke(MethodInvocation invocation) throws Throwable {
		// Adding before service
		methodDescription(invocation);

		Object currentResult = invocation.proceed();

		// Adding after service.
		Object changedResult = changedReturnValue(invocation);

		return changedResult == null ? currentResult : changedResult;
	}

	private void methodDescription(MethodInvocation invocation) {
		String methodName = invocation.getMethod().getName();
		Object[] args = invocation.getArguments();

		String description = String.format("Before Services : %s(%s, %s)",
				methodName, args[0], args[1]);
		System.out.println(description);
	}

	private Object changedReturnValue(MethodInvocation invocation) {
		Object[] args = invocation.getArguments();
		int secondParam = (int) args[1];

		// changed the return value to 0 when the second parameter is less than 0.
		if (secondParam < 0)
			return 0;

		return null;
	}

}

In this our around advice. we print the method description in our before service and changed the return value to 0 when the second parameter is less than 0 in after service.

springConfig.xml

<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN"
			"http://www.springframework.org/dtd/spring-beans-2.0.dtd">

<beans>
	<bean id="myAroundAdvice" class="spring.test.aop.MyAroundAdvice"/>
	
	<bean id="myLogicClass" class="spring.test.aop.MyLogicClass"/>
	
	<bean id="proxyFactoryBean" class="org.springframework.aop.framework.ProxyFactoryBean">
		<property name="proxyInterfaces" value="spring.test.aop.MyLogicInterface"/>
		<property name="interceptorNames">
			<list>
				<value>myAroundAdvice</value>
			</list>
		</property>
		<property name="target" ref="myLogicClass"/>
	</bean>
</beans>

The spring config xml is same with before except the interceptorNames.

App.java

package spring.test.aop;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class App {

	private static ApplicationContext applicationContext;

	public static void main(String[] args) {
		applicationContext = new ClassPathXmlApplicationContext("springConfig.xml");
		MyLogicInterface logicInterface = (MyLogicInterface)applicationContext.getBean("proxyFactoryBean");
		
		int result1 = logicInterface.add(1, 10);
		System.out.println(String.format("After Advice - The result is: %s \n", result1));
		
		int result2 = logicInterface.add(1, -10);
		System.out.println(String.format("After Advice - The result is: %s", result2));
	}

}

You will notice that we pass -10 to second parameter in second times method called. Let’s see the result:

Before Services : add(1, 10)
The internal result of 1 + 10 is: 11
After Advice - The result is: 11 

Before Services : add(1, -10)
The internal result of 1 + -10 is: -9
After Advice - The result is: 0

You see, the internal result is -9, but after service, the result is changed to 0. So the return value is changed by our Around Advice.

Note again: The Around Advice can change the return value of business logic method, but After Advice cannot.