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A grid system must integrate heterogeneous resources with varying quality and availability. For example, the load on any given resource may increase during execution of a time-constrained job. This places importance on the system's ability to recognize the state of these resources. This paper presents an approach used as a basis for system adaptation in which grid jobs are maintained at runtime. A reflective technique is used to simplify the adaptation in the grid application. The design of an adaptable resource broker is described and experimentally evaluated. Reflection is incorporated into the broker to separate functional and non-functional aspects of the system and facilitate the implementation of non-functional properties such as job migration. Results indicate that this approach enhances the likelihood of timely job completion in a dynamic grid system.