Dynamic updates to running programs improve development productivity and reduce downtime of long-running applications. This feature is however severely limited in current virtual machines for object-oriented languages. In particular, changes to classes often apply only to methods invoked after a class change, but not to active methods on the call stack of threads. Additionally, adding and removing methods as well as fields is often not supported. We present a novel programming model for safe and atomic code updates of Java programs that also updates methods that are currently executed. We introduce safe update regions and pause threads only there before an update. We automatically convert the stack frames to suit the new versions of the methods. Our implementation is based on a production-quality Java virtual machine. Additionally, we present SafeWeave, a dynamic aspect-oriented programming system that exposes the atomic code updates through a high-level programming model. AspectJ advice can be added to and removed from a running application.Changes are atomic and correctness is guaranteed even though weaving happens in parallel to program execution, and the system fully supports the dynamic class loading of Java. We show that the enhanced evolution features do not incur any performance penalty before and after version changes.