AspectJ-TM is a simple and practical aspect-oriented extension to Java-TM. With just a few new constructs, AspectJ provides support for modular implementation of a range of crosscutting concerns. In AspectJ's dynamic join point model, join points are well-defined points in the execution of the program

The World Wide Web has succeeded in large part because its software architecture has been designed to meet the needs of an Internet-scale distributed hypermedia system. The Web has been iteratively developed over the past ten years through a series of modifications to the standards that define its architecture. In order to identify those aspects of the Web that needed improvement and avoid undesirable modifications, a model for the modern Web architecture was needed to guide its design, definition, and deployment. Software architecture research investigates methods for determining how best to partition a system, how components identify and communicate with each other, how information is communicated, how elements of a system can evolve independently, and how all of the above can be described using formal and informal notations. My work is motivated by the desire to understand and evaluate the architectural design of network-based application software through principled use of architectural constraints, thereby obtaining the functional, performance, and social properties desired of an architecture. An architectural style is a named, coordinated set of architectural constraints. This dissertation defines a framework for understanding software architecture via architectural styles and demonstrates how styles can be used to guide the architectural design of network-based application software. A survey of architectural styles for network-based applications is used to classify styles according to the architectural properties they induce on an architecture for distributed hypermedia. I then introduce the Representational State Transfer (REST) architectural style and describe how REST has been used to guide the design and development of the architecture for the modern Web. REST emphasizes scalability of component interactions, generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems. I describe the software engineering principles guiding REST and the interaction constraints chosen to retain those principles, contrasting them to the constraints of other architectural styles. Finally, I describe the lessons learned from applying REST to the design of the Hypertext Transfer Protocol and Uniform Resource Identifier standards, and from their subsequent deployment in Web client and server software.

This paper proposes an approach to the design of configurable and open middleware platforms based on the concept of reflection. More specifically, the paper introduces a language-independent reflective architecture featuring a perobject meta-space, the use of meta-models to structure meta-space, and a consistent use of object graphs for composite components. This is complemented by a component framework supporting the construction of meta-spaces. The paper also reports on experiences of implementing the architecture (with emphasis on experiments with open bindings). Keywords Middleware, (re-)configuration, reflection, open implementation, open bindings. 1 INTRODUCTION Middleware has emerged as an important architectural component in supporting distributed applications. The role of middleware is to present a unified programming model to application writers and to mask out problems of heterogeneity and distribution. The importance of the topic is reflected in the increasing visibili...

This paper presents a methodology for designing extensible languages for distributed computing.

: A key problem in operating systems is permitting the orderly introduction of new properties and new implementation techniques. We describe a mechanism, subcontract, that within the context of an object-oriented distributed system permits application programmers control over fundamental object mechanisms. This allows programmers to define new object communication without modifying the base system. We describe how new subcontracts can be introduced as alternative communication mechanisms in the place of existing subcontracts. We also briefly descibe some of the uses we have made of the subcontract mechanism to support caching, crash recovery, and replication. email addresses: graham.hamilton@eng.sun.com michael.powell@eng.sun.com jim.mitchell@eng.sun.com A Sun Microsystems, Inc. Business M/S 29-01 2550 Garcia Avenue Mountain View, CA 94043 2 Subcontract: A Flexible Base for Distributed Programming Graham Hamilton, Michael L. Powell, James G. Mitchell Sun Microsystems Labo...

This paper identifies and analyzes the emergence of a new paradigm in software engineering, called separation of concerns, which tries to formally separate the basic algorithm from special purpose concerns such as synchronization, real-time constraints, and location control. This separation allows for the locality of different kinds of information in the programs, making them easier to write, understand, reuse, and modify. We identify the major concerns existing in today's software applications, and analyze recent proposals in the literature that address separation of single concerns. Furthermore, we study the commonalities of these proposals and discuss how they can be used to achieve separation of concerns in general. Finally, we address the problem of composition of several separated concerns.

This paper presents a scheme for coordinated error recovery between multiple interacting objects in a concurrent object-oriented system. A conceptual framework for fault tolerance is established based on a general object concurrency model that is supported by most concurrent object-oriented languages and systems. This framework integrates two complementary concepts — conversations and transactions. Conversations (associated with cooperative exception handling) are used to provide coordinated error recovery between concurrent interacting activities whilst transactions are used to maintain the consistency of shared resources in the presence of concurrent access. The serialisability property of transactions is exploited in order to help prevent unexpected information smuggling. The proposed framework is illustrated by means of a case study, and various linguistic and implementation issues are discussed.

: The FRIENDS system developed at LAAS-CNRS is a metalevel architecture providing libraries of metaobjects for fault tolerance, secure communications and group-based distributed applications. The use of metaobjects provides a nice separation of concerns between mechanisms and applications. Metaobjects can be used transparently by applications and can be composed according to the needs of a given application, a given architecture and its underlying properties. In FRIENDS, metaobjects are used recursively to add new properties to applications. They are designed using an object oriented design method and implemented on top of basic system services. This paper describes the FRIENDS software-based architecture, the objectoriented development of metaobjects, the experiments that we have done and summarises the advantages and drawbacks of a metaobject approach for building fault tolerant systems. 1 Introduction The use of a metalevel architecture to build dependable systems has emerged recen...

This paper presents OpenJava, which is a macro system that we have developed for Java. With traditional macro systems designed for non object-oriented languages, it is difficult to write a number of macros typical in object-oriented programming since they require the ability to access a logical structure of programs. One of the drawbacks of traditional macro systems is that abstract syntax trees are used for representing source programs. This paper first points out this problem and then shows how OpenJava addresses this problem. A key idea of OpenJava is to use metaobjects, which was originally developed for reflective computing, for representing source programs.

The benefits of computational reflection are the abilities to reason and alter the dynamic behavior of computation from within the language framework. This is more beneficial in concurrent/distributed computing, where the complexity of the system is much greater compared to sequential computing; we have demonstrated various benefits in our past research of Object-Oriented Concurrent Reflective (OOCR) architectures. Unfortunately, attempts to formulate reflective features provided in practical reflective systems, such as resource management, have led to some difficulties in maintaining the linguistic lucidity necessary in computational reflection. The primary reason is that previous OOCR architectures lack the ingredients for group-wide object coordination. We present a new OOCR language with a hybrid group reflective architecture, ABCL/R2, whose key features are the notion of heterogeneous object groups and coordinated management of group shared resources. We describe and gi...