Contract4J

Contract4J is a tool that supports Design by Contract® (DbC) programming in Java 5. Contract tests are defined using Java 5 annotations and aspects written in AspectJ evaluate the test expressions at runtime and handle failures.

In DbC, the designer considers the conditions required for a module or method to successfully perform its intended services, the preconditions, and what results it can guarantee to deliver, the postconditions, if the preconditions are satisfied. The designer can also specify invariants that must be satisfied.

DbC is an underused technique for improving software quality, by thinking through design issues more carefully and testing compliance during development and testing. It also complements other techniques very well, such as Test-Driven Development (here and here).

Contract4J, like the original implementation of DbC in Eiffel and the XDoclet-based Barter for Java 1.4 and earlier, provides a standard way to define DbC tests and to evaluate them at runtime. Barter uses XDoclet tags and generates AspectJ test code. Contract4J uses Java 5 annotations to define the condition tests and it uses "prebuilt" AspectJ aspects to advise the execution points ("join points") where the tests should be evaluated. If the tests fail, both tools terminate program execution.

Annotations have a few advantages over javadoc-style tags: they can be included in the generated Javadocs and the JVM can be aware of the annotations at runtime. The runtime awareness can support other aspect behaviors.

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