The Proxy Inter-Type Declaration

Aspect-oriented programming [16] with its support for modularizing crosscutting concerns opens up the vision that component middleware can be replaced by sets of collaborating aspects that implement the infrastructural services. In this way, application servers could be customized on a perproject basis, as they are no longer monolithic applications. In order for this vision to become reality it should be possible to implement services offered by current containers as aspects. In particular services targeting scalability and performance issues, such as passivation and instance pooling must be available. Such services build upon the concept of virtual instances [25]. They are realized in the current application servers by means of the (virtual) proxy pattern [11]. Implementing virtual instances represents a crosscutting concern, as we will discuss in the paper. Unfortunately, no current AOP language or framework has explicit support for modularizing this concern. We propose to support the generation of proxies as an inter-type declaration and argue that it is essential to allow the development of scalable and efficient component containers. It would be a significant improvement when compared with the current offered possibilities.