What is ezContract?

ezContract is a Java implementation of Design By Contract (DBC) for the Java language. Through the use of markers which are predefined dummy methods and attributes, ezContract proposes a new way to annotate contracts. The annotated programs can be directly compiled by standard Java compilers. A bytecode instrumentor is developed to manipulate the bytecode to inject contract evaluation instructions and make the contracts executable at runtime. The marker approach avoids two primary problems found in the existing practices: source compatibility that depends on language extension and symbolic barrier that leaves contracts and their targets unrelated. It also facilitates streamlined integration with IDEs and improves readability as well as writability of the contract-annotated programs.



Motivation and background

DBC has been generally acknowledged as a useful and pragmatic technique to build reliable software systems. However, its use has so far been restricted due to the lack of support by the mainstream programming languages such as Java and C++. Currently, the Eiffel language is the only commercial language that fully supports DBC. The situation has not escaped the attentions from the research community; numerous researches on porting DBC into other languages have been conducted. However, the results so far fall short in making DBC a standard practice by software developers on real-world projects. The main reasons include the following:


1.      DBC is a paradigm shift in programming and the design of programming languages. The existing researches mainly focus on implanting DBC from Eiffel into new languages. The language compatibility issues have been largely overlooked, which make the use of DBC awkward or even incorrect.

2.      The concept of DBC includes both software correctness and robustness. Most of the existing researches merely focus on software correctness and rarely touch the robustness issue. In the DBC context robustness is related to throwing, catching, and handling exception caused by contract violation.

3.      A DBC-enabled programming language alone will not be sufficient to support DBC-based software development in the real world. Tool support in the integrated development environments (IDE) is a critical issue for a DBC-enabled programming language to be widely adopted. In the existing researches to porting DBC to other languages, tool support issues have been largely ignored.


Founded by the National Science Council of Taiwan, we execute a research project to investigate and resolve the above issues. In particular, our research focuses on the support of DBC in the Java programming language, which is platform independent and widely used. To achieve this, we propose solutions that make using a new object-oriented library, a bytecode instrumentor, a Java exception handling architecture, and supporting tools.


Scope of ezContract

DBC intends to build reliable software by improving the two quality attributes: correctness and robustness. In DBC, correctness is addressed by contracts and robustness is tackled by exception handling. Each of the issues is complicated on its own. To adopt DBC in Java, we established two projects, ezContract and ezException, to tackle contracts and exception handling issues, respectively. People who are interested in exception handling are encouraged to visit the exException project: https://sourceforge.net/projects/ezexception/.


Current status of ezContract

ezContract has been developed since March 2005. Currently, ezContract supports the following features of DBC:


1.          Specifying preconditions, postconditions, and class invariants for Java classes and interfaces, with and without source code.

2.          Evaluating contracts defined in classes.

3.          The ability to use old expressions (the value before entering a method) and to access the result (i.e., the return value of a method) in postconditions.

4.          Subcontracting support. That is, preconditions are or-ed, and postconditions and class invariants are and-ed with those of the parent class, respectively.

5.          The ability to show correct line number in source code when contracts is violated (facilitating debugging).


Development plan

1.          Evaluating contracts defined in interfaces. Current version of ezContract can only evaluate contracts defined in classes. Although specifying contracts in interfaces is supported, the bytecode instrumentor is yet to produce contract evaluation code for interfaces.

2.          Instrumenting programs at bytecode loading time. Currently, the instrumented programs must be written to disk and can be loaded with a default Java class loader. A specific class loader will be developed to support instrumentation on the fly (i.e., at class loading time).

3.          Integrated with Eclipse. Developing an Eclipse plug-in to check the usage of ezContract. For example, using old expresses and accessing the result in precondition should not be allowed. The plug-in will check any violation of the usage of ezContract at compile time and generate warnings and suggestions for developers.


The history of ezContract

ezContractor was introduced the paper titled " ezContract: Using Marker Library and Bytecode Instrumentation to Support Design by Contract in Java ", by Yu Chin Cheng, Chien-Tsun Chen, and Chin-Yun Hsieh at the year 2007. However, ezContractor has been design and developed since 2005. In the end of 2007, we decided to release ezContract (and ezException) as open source software.



ezContract is released under the Eclipse Public License.


ezContract at SourceForge

ezContract is hosted in https://sourceforge.net/projects/ezcontract/.