Current smart grid is undergoing a drastic change in order to deal with increasingly diversified and various service requests from the huge number of the users. The next-generation smart grid, characterized by the service-oriented middleware, will be upgraded by jointly employing the technologies in the areas of communications, control, and computing. To design a general middleware, an efficient design principle plays a fundamental role, while reliable communication infrastructure and heterogeneous applications lead to sustainability and stability. In this paper, we propose an integrated and efficient middleware for heterogeneous services in smart grid. Specifically, we first develop a mutual application access control principle that keeps the users can obtain a satisfying assignment. Next, a collaborative and dynamic information exchange infrastructure is proposed for different smart meters, and a local information collector is designed to implement the communication and computing through power management messages. Finally, we exactly present four steps to design a flexible service-oriented middleware for heterogeneous applications. Numerical simulation results are provided to demonstrate the efficiency of the proposed middleware.