Using SPEL to call a method in the same class passing a variable taken from the actual method

I want to be able to call another method of my class by passing the value of a parameter of the calling method, so suppose:

public class MyClass{

   @MySpelAnnotation("#this.otherMethod()") //how to I pass "arg" as parameter?
   public void mainMethod(String arg){
        ...
   }

   public void otherMethod(String arg){

   }

How can I be able to take arg from the first method and pass it to the second one?

You need to evaluate the expression against an evaluation context that contains the argument bound as a variable under the #arg name:

@MySpelAnnotation("#this.otherMethod(#arg)")

You can use Spring's MethodBasedEvaluationContext for this:

@Aspect
static class MyAspect {
    @After("@annotation(annotation)")
    void after(JoinPoint joinPoint, MySpelAnnotation annotation) {
        MethodSignature signature = (MethodSignature) joinPoint.getSignature();

        // expression should be cached for better performance
        Expression expression = new SpelExpressionParser()
                .parseExpression(annotation.value());

        MethodBasedEvaluationContext context =
                new MethodBasedEvaluationContext(
                        joinPoint.getTarget(),
                        signature.getMethod(),
                        joinPoint.getArgs(),
                        new DefaultParameterNameDiscoverer());

        expression.getValue(context);
    }
}

Note that parsing the expression is a non-trivial operation that should ideally be cached.