A method, computer program product, and data processing system for detecting and identifying data crossover errors in servlet code are disclosed. According to a preferred embodiment, techniques of aspect-oriented programming (AOP) are used to instrument JAVA Servlet code to detect potential data crossover errors. Specifically, pointcuts are defined to intercept both the association follow an object with a particular session and the “getting” and “setting” of such objects. Advice code associated with these pointcuts is used to update and/or consult a “collator” data structure, which is used to determine if an object or variable associated with one session is being accessed by a different session. In a preferred embodiment, the AspectJ aspect-oriented programming language/system is used to define the pointcuts and advice code.