A computer system configured to intercept method calls placed on an object. By intercepting method calls, processing may be performed on a method call-by-method call basis. As part of the processing, metadata for an object is consulted to partition the methods for that object into subsets. The processing performed in response to any specific method call may be based on the subset to which the method belongs. The type of metadata may depend on the desired operation of the computer system. Metadata representing a white list or black list may be used in a computer system that implements a security zone that allows access to methods deemed to be safe or to deny access to methods deemed to be unsafe. In a performance monitoring system, metadata may identify methods to be logged or methods for which execution cost are to be measured. Values returned by a method call may also be processed when the method calls are intercepted. The returned value may be wrapped with a wrapper that holds data useful in implementing a security zone or for performing other processing.