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Implement Design by Contract for Java using dynamic proxies

Write bug-free code with the DBCProxy framework

By Anders Eliasson, JavaWorld.com, 02/15/02

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While developing software and its subsequent versions, developers often enhance functionality by modifying existing code. However, developers who modify or enhance that code are usually not its original authors. Depending on the complexity of code and its supporting documentation, and the clarity of requirements, enhancements may present a difficult problem.

For instance, suppose you could enhance an existing application by performing the following tasks: extend a class and override a method in the extended class. Questions arising during the design and implementation phase would most likely include: Should the super class method be called? If the super class method is called, should the overriding method call the super class method at the beginning or end of the overriding method?

Many developers experience this situation everyday as they maintain and enhance existing software. As a result, they are at the mercy of requirements specifications and existing software documentation to understand software's complexity. If either artifact is too vague, the developer may find designing and implementing the necessary changes difficult.

However, software that includes assertions, based on the Design by Contract (DBC) theory, combats that problem. You could clearly design and subsequently enhance that software because those assertions would advertise class, method, and interface contractual obligations. The assertions would provide a clear enforceable picture of the existing software's state, behavior, and interface. Thus, they make enhancing existing software much easier for developers.

In this article, I present a DBC for Java framework called DBCProxy. This framework uses dynamic proxy classes, which were introduced in Sun Microsystems's JDK 1.3. I'll begin by briefly introducing DBC, and then I'll describe DBCProxy's architecture and how you can use it in your development process.

Introducing DBC

When software developers perform object-oriented analysis and design (OOAD), they strive to identify appropriate abstractions within a problem domain. Additionally, they label these abstractions with a specific name and usually provide some additional text to further clarify it. In the software construction and implementation phase, developers utilize Java to transform these abstractions into Java interfaces, classes, and methods. These constructs let you advertise an abstraction's purpose and functionality.

Descriptive text and identifying names may offer a great deal of information about an abstraction. However, rarely do they provide a precise specification. Descriptive text may contain enough information to make it sound like a detailed specification, but plain text is often ambiguous and not enforceable.

DBC provides a formal way of writing detailed specifications with assertions; these can be viewed as enforceable contracts between the designer/developer of a class or interface (abstraction) and the class or interface client. To accomplish this, DBC uses three assertion types:

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