Programming Posts: April 2017

This is a short tutorial that explains how to write an advice to change value of method arguments.

I have implemented a MutateArgsAspect which is applied on methods annotated with @MutateArgs

The advice converts the string argument passed to the annotated method from "BEFORE" to "AFTER"

Steps 1: Implement the annotation

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package com.waseem.foundation;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Retention;

@Retention(RUNTIME)

public @interface MutateArgs {

}

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Steps 2: Implement the Aspect

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package com.waseem.foundation;

import org.aspectj.lang.ProceedingJoinPoint;

import org.aspectj.lang.annotation.Around;

import org.aspectj.lang.annotation.Aspect;

import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

import org.springframework.stereotype.Component;

@Aspect

@Component

public class MutateArgsAspect {

 private static final Logger LOGGER = LoggerFactory.getLogger(MutateArgsAspect.class);

 @Around("@annotation(MutateArgs) && execution(* *(..))")

 public Object before(ProceedingJoinPoint pjp) throws Throwable {

  LOGGER.debug(pjp.getSignature().toLongString());

  Object[] args = pjp.getArgs();

  for (int i = 0; i < args.length; i++) {

         if ("BEFORE".equals(args[i])) {

          LOGGER.info("args[{}] : {}", i, args[i]);

             args[i] = "AFTER";

}

}

     Object output = pjp.proceed(args);

  return output;

}

}

Note the @Around annotation.

1- @annotation(MutateArgs) means the Aspect is applied on all the methods annotated with MutateArgs annotation

2- execution(* *(..)) means the execution of any method regardless of return or parameter types

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Step 3: Apply annotation on the methods for the Advice to work.

Here is an example where this annotation is applied on an API method

@GET

@Path("/mutateInController")

@Produces(MediaType.APPLICATION_JSON)

@MutateArgs

public Response mutateArgsOfThisMethod(@QueryParam("name")  String name) {

 LOGGER.info("Name in Controller : {}", name);

 return Response.ok(name).build();

}

Here is another example were this annotation is applied on another simple method

@MutateArgs

public String mutateArgsOfThisMethod(String name) {

 LOGGER.info("Name in Manager : {}", name);

 return name;

}

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