Question

Can I have an annotation for a method that when its parameters are null, just do nothing, effectively as not being invoked?

Answer 1

Probably the easiest way to do this is using interfaces and dynamic proxies. Annotations do nothing other than add metadata. You’re going to have to add code to act based on the annotation.

You’d have to do a few things —

Create an interface

public interface IService {

     @ValidateNull // Your custom annotation
     public void yourMethod(String s1);
}

When using the implementation, instantiate it as a JDK Proxy.

IService myService = (IService)java.lang.Proxy.newInstance(ServiceImpl.class.getClassLoader(),
                           ServiceImpl.class.getInterfaces(), 
                           new YourProxy(new ServiceImpl());

Now, you can via reflection, capture all invocations of your method in YourProxy class.

 public YourProxy implements InvocationHandler {
public Object invoke(Object arg0, Method method, Object[] args) throws Throwable {
    if (method.isAnnotationPresent(ValidateNull.class)) {
                   //Check args if they are null and return.
            }
     }
 }

If you dont want to do this, then you’re looking at more heavyweight frameworks such as AspectJ / Spring AOP.