| With the explosive development of Internet, software platforms for Web applications have been the mainstream category in software industry.The core and key element of the platform is Web Application Server(WAS),which resides at the middle tier in the three-tier client/server architecture.WAS solves the problems in applying traditional middlewares to Web computing environment and provides a series of run-time services such as naming, load balancing,transaction,failover,fault tolerance and security, to support for the deployment, integration, and execution of Web applications.High scalability,availability, reliability, and performance are the advanced features of WASs. WAS has a huge potential market, and it can be used to to build enterprise management system, e-commerce, and e-government system.Attention to the new category of middleware has been drawn from the academia and software industry after it was born in!998. It is considered to be one of the most exciting milestones of enterprise software technology since the relational database and has become very popular in the last few years. To meet the urgent need for WAS and cater for the mainstream trend of Web applications, this thesis outlines some key issues in developing WAS, and their solutions are applied to design and implement J2EE application server.Many international IT companies have released their WAS product, and have become mainstream WAS product providers. The fact makes it necessary to develope WAS whose copyright is reserved for our country. In this dissertation, the current state-of-the-art of Web application servers is outlined. We come to the conclusion that support for openness, flexibility, customizability, portability, and client transparency is not available in the current mainstream WASs,which typically adopt a monolithic architecture and inherent black box philosophy to their design. A comparative study among some leading Web application servers is presented by evaluating their functions and performance.To cater for an increasingly diverse range of QoS demands, a dynamic reconfiguration model for WASs, referred to as OnceDR, is presented, and a run-time reconfiguration algorithm is designed to drive the system to be reconfiguration-safe state for the purpose of preserving the reconfiguration consistency. A multi-phasereconfigurable architecture is introduced on the base of OnceDR model, supplemented by the marriage of microkernel technology and reflection technology. In the architecute, WAS is very open and flexible to support deployment-time configurability and run-time reconfigurability.A reflective container model for EJB components is proposed. In this model multiple meta-objects are composed to represent the functionality of an EJB component. The AOP technology is introduced to the design of containers, and an AOP mechanism is implemented via reflection. OnceAOC is designed.The container can be capable of adapting their behavior to the changing environments though dynamically weaving multiple aspects at run-time.Process-collocation and host-collocation is supported to improve the performance of communication in OnceAOC.To improve the dependability and scalability of WASs, a middleware-based adaptive load balancing service is designed. We also describe the key design challenges including hot plug-in, customizable load balancing strategy, adaptive control, state migration and fault tolerance etc., and outline the technical solutions.Moreover, a fuzzy and positive load balancing algorithm is also proposed with combination the advantages of both sender-initiated algorithm and receiver-initiated algorithm.The adaptive algorithm is charactered by quick matching and stability.The most challenging problem is to maintain component consistency during component dynamic redistribution. To resolve this inconsistency problem, some kinds of component migration constrains (CMC) are defined basically. A component migration model for J2EE application servers is proposed, and SLB_Copy, SFB_Copy and EB_Copy component migration algo...
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