It has been recognized in several works that a slice of behavior affecting a set of collaborating classes is a better unit of reuse than a single class. Different techniques and language extensions have been suggested to express such slices in programming languages. We propose delegation layers, an approach that scales the OO mechanisms for single objects, such as delegation, late binding, and subtype polymorphism, to sets of collaborating objects. Technically, delegation layers combine and generalize delegation and virtual class concepts. Due to their runtime semantics, delegation layers are more flexible than previous compile time approaches like mixin layers.

Current software development tools let developers model a software system and generate program code from the models to run the system. However, generating code and installing a non-trivial system induces a time delay between changing the model and executing it that makes rapid model prototyping awkward if not impossible. This paper presents the architecture of a virtual machine for UML that interprets UML models without any intermediate code-generation step. The paper shows how to embed UML in a metalevel architecture so that a key property of model-based systems, the causal connection between models and model instances, is guaranteed. With this architecture, changes to a model have immediate effects on its execution, providing users with rapid feedback about the model's structure and behavior. This approach supports model innovation better than today's codegeneration approaches.

Modeling object-oriented applications using collaborations and roles is now well accepted. Collaboration-based or role-based designs decompose an application into tasks performed by a subset of the applications' classes. Collaborations provide a larger unit of understanding and reuse than classes, and are an important aid in the maintenance and evolution of the software. This kind of design information is lost, however, at the implementation level, making it hard to maintain and evolve an existing software application. The extraction of collaborations from code is therefore an important issue in design recovery. In this paper we propose an iterative approach which uses dynamic information to support the recovery and understanding of collaborations. We describe a tool we have developed to support our approach and demonstrate its use on a case study.

the xoRBAC component that provides a flexible RBAC service. The xoRBAC implementation conforms to level 4a of the unified NIST model for RBAC and can be reused for arbitrary applications on Unix or Windows with a C or Tcl linkage. xoRBAC runtime elements can be serialized and recreated from RDF data models conforming to a welldefined RDF schema. Furthermore we present our experiences with xoRBAC for the deployment within the HTTP environment for a web-based mobile code system.

Specifying, enforcing and evolving access control policies is essential to prevent security breaches and unavailability of resources. These access control design concerns impose requirements that allow only authorized users to access protected computer-based resources. Addressing these concerns in a design results in the spreading of access control functionality across several design modules. The pervasive nature of access control functionality makes it difficult to evolve, analyze, and enforce access control policies. To tackle this problem, we propose using an aspect-oriented modeling(AOM) approach for addressing access control concerns. In the AOM approach, functionality that addresses a pervasive access control concern is localized in an aspect. Other functional design concerns are addressed in a model of the application referred to as a primary model. Composing access control aspects with a primary model results in an application model that addresses access control concerns. We illustrate our approach using a form of Role-Based Access Control.

The development of reusable requirements and design artifacts often requires one to characterize families of problem and solution models. This paper presents a metamodeling approach to characterizing a family of models. A characterization is expressed as a Role Model that consists of roles that can be played by UML model elemets. In this paper we describe how a family of UML static structural diagrams that have the structural properties defined by a pattern can be characterized by a Static Role Model (SRM). The Abstract Factory pattern is used to illustrate how SRMs can be used to specify reusable designs expressed as patterns.

classes. An abstract class is an incomplete implementation of one or more interfaces. It can be used to define behavior that is common for a group of components implementing a group of interfaces.

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To achieve the goal of realizing object adaptationto environments, a new role-based model Epsilon and a language EpsilonJ is proposed. In Epsilon, an environment is defined as a field of collaboration between roles and an object adapts to the environment assuming one of the roles. Objects can freely enter or leave environments and belong to multiple environments at a time so that dynamic adaptation or evolution of objects is realized. Environments and roles are the first class constructs at runtime as well as at model description time so that separation of concerns is not only materialized as a static structure but also observed as behaviors. Environments encapsulating collaboration are independent reuse components to be deployed separately from objects. In this paper, the Epsilon model and the language are explained with some examples. The effectiveness of the model is illustrated by a case study on the problem of integrated systems. Implementation of the language is also reported. 1.