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The standard reflection API of Java provides the ability to introspect a program but not to alter program behavior. This paper presents an extension to the reflection API for addressing this limitation. Unlike other extensions enabling behavioral reflection, our extension called Javassist enables structural reflection in Java. For using a standard Java virtual machine (JVM) and avoiding a performance problem, Javassist allows structural reflection only before a class is loaded into the JVM. However, Javassist still covers various applications including a language extension emulating behavioral reflection. This paper also presents the design principles of Javassist, which distinguish Javassist from related work. 1 Introduction Java is a programming language supporting reflection. The reflective ability of Java is called the reflection API. However, it is almost restricted to introspection, which is the ability to introspect data structures used in a program such as a class. The...

While the availability of platform-independent code on the Internet is increasing, third-party code rarely exhibits all of the features desired by end users. Unfortunately, developers cannot foresee and provide for all possible extensions. In this paper, we describe load-time transformation, a stage in the program development lifecycle in which classes are modified at load time according to usersupplied directives. This allows the users to select transformations that add new features, customize the implementation of existing features, and apply the changes to all classes in the environment. The Java Object Instrumentation Environment (JOIE) is a toolkit for constructing transformations of Java classes. An enhanced class loader calls user-supplied transformers that specify rules for transforming target classes. We describe some applications of load-time transformation, including extending the Java environment, integrating classes with specialized environments, and adding functionality...

Aspect-oriented programming (AOP) controls tangling of concerns by isolating aspects that cross-cut each other into building blocks. Component-based programming (CBP) supports software development by isolating reusable building blocks that can be assembled and connected in many different ways. We show how AOP and CBP can be integrated by introducing a new component construct for programming class collaborations, called aspectual component. Aspectual components extend adaptive plug-and-play components (AP&P) with a modification interface that turns them into an effective tool for AOP. A key ingredient of aspectual components is that they are written in terms of a generic data model, called a participant graph, which is later mapped into a data model. We introduce a new property of this map, called instance-refinement, to ensure the proper deployment of components. We show how aspectual components can be implemented in Java, and demonstrate that aspectual components improve the AspectJ language for AOP from Xerox PARC.

. The aim of component technology is the replacement of large monolithic applications with sets of smaller software components, whose particular functionality and interoperation can be adapted to users' needs. However, the adaptation mechanisms of component software are still limited. Most proposals concentrate on adaptations that can be achieved either at compile time or at link time. Current support for dynamic component adaptation, i.e. unanticipated, incremental modifications of a component system at run-time, is not sufficient. This paper proposes object-based inheritance (also known as delegation) as a complement to purely forwarding-based object composition. It presents a typesafe integration of delegation into a class-based object model and shows how it overcomes the problems faced by forwarding-based component interaction, how it supports independent extensibility of components and unanticipated, dynamic component adaptation. 1 Introduction Component-oriented pro...

Modern software must evolve in response to changing conditions. In the most widely used programming environments, code is static and cannot change at runtime. This poses problems for applications, that have limited down-time. More support is needed for dynamic evolution. In this paper we present an approach for supporting dynamic evolution of Java programs. In this approach, Java programs can evolve by changing their components, namely classes, during their execution. Changes in a class lead to changes in the its instances, thereby allowing evolution of both code and state. The approach promotes compatibility with existing Java applications, and maintains the security and type safety controls imposed by Java’s dynamic linking mechanism. Experimental analyses of our implementation indicate that the implementation imposes a moderate performance penalty relative to the unmodified virtual machine.

Scala fuses object-oriented and functional programming in a statically typed programming language. It is aimed at the construction of components and component systems. This paper gives an overview of the Scala language for readers who are familar with programming methods and programming language design.

When using Aspect Oriented Programming in the development of software components, a developer must understand the program units actually changed by weaving, how they behave, and possibly correct the aspects used. Support for rapid AOP prototyping and debugging is therefore crucial in such situations. Rapid prototyping is difficult with current aspect weaving tools because they do not support dynamic changes. This paper describes PROSE (PROgrammable extenSions of sErvices), a platform based on Java which addresses dynamic AOP. Aspects are expressed in the same source language as the application (Java), and PROSE allows aspects to be woven, unwoven, or replaced at run-time.

Library developers who have to evolve a library to accommodate changing requirements often face a dilemma: Either they implement a clean, efficient solution but risk breaking client code, or they maintain compatibility with client code, but pay with increased design complexity and thus higher maintenance costs over time. We address this dilemma by presenting a lightweight approach for evolving application programming interfaces (APIs), which does not depend on version control or configuration management systems. Instead, we capture API refactoring actions as a developer evolves an API. Users of the API can then replay the refactorings to bring their client software components up to date. We present CatchUp!, an implementation of our approach that captures and replays refactoring actions within an integrated development environment semi-automatically. Our experiments suggest that our approach could be valuable in practice.

. In this paper we deal with object-oriented component integration issues. We argue that traditional framework customization techniques are inappropriate for componentbased programming since they lack support for non-invasive, encapsulated, dynamic customization. We propose a new language construct, called a pluggable composite adapter, for expressing component gluing. A pluggable composite adapter allows the separation of customization code from component implementation, resulting in better modularity, flexible extensibility, and improved maintenance and understandability. We also discuss alternative realizations of the construct. 1. Introduction Component software, i.e., software that is an assembly of individual, independently developed parts, is becoming the predominant architecture. We consider two factors as the driving force behind this development. First, as indicated in [22] component software represents a middle path between the two extremes that predominate traditional sof...