One of my favourite uses of AspectJ is to generate compile-time error messages.  This allows you to provide guidance in the IDE for developers writing new code within a framework or library. 

Here’s a quick example. BaseDTO is a base class that developers will extend. It’s used with a framework that requires a no-arg constructor (Jackson in this case, but it’s a common requirement), but when constructed explicitly, the UriInfo parameter is mandatory.

	// No-arg constructor for unmarshalling, but otherwise don't call this one
	public BaseDTO() {}
	
	public BaseDTO(UriInfo uriInfo) {
		this._links = new Links(uriInfo);
	}

We can’t express that requirement in normal Java. As a result, developers can waste a lot of time debugging a new subclass. AspectJ to the rescue!

public aspect DTOChecker {
	
	pointcut dtoConstructor(): call(BaseDTO+.new(..)) 
	&& !call(BaseDTO+.new(javax.ws.rs.core.UriInfo, ..));
	
	declare error : dtoConstructor() :  "DTOChecker: Constructors for subclasses of BaseDTO must include a UriInfo parameter.";

}

Then, if we try to call a constructor for any subclass of BaseDTO without including a parameter of type UriInfo, we get a compile error. In Eclipse, that looks like this: