Build system with lazy retrieval for Java projects

In the modern-day development, projects use Continuous Integration Services (CISs) to execute the build for every change in the source code. To ensure that the project remains correct and deployable, a CIS performs a clean build each time. In a clean environment, a build system needs to retrieve the project's dependencies (e.g., guava.jar). The retrieval, however, can be costly due to dependency bloat: despite a project using only a few files from each library, the existing build systems still eagerly retrieve all the libraries at the beginning of the build. This paper presents a novel build system, Molly, which lazily retrieves parts of libraries (i.e., files) that are needed during the execution of a build target. For example, the compilation target needs only public interfaces of classes within the libraries and the test target needs only implementation of the classes that are being invoked by the tests. Additionally, Molly generates a transfer script that retrieves parts of libraries based on prior builds. Molly's design requires that we ignore the boundaries set by the library developers and look at the files within the libraries. We implemented Molly for Java and evaluated it on 17 popular open-source projects. We show that test targets (on average) depend on only 9.97% of files in libraries. A variant of Molly speeds up retrieval by 44.28%. Furthermore, the scripts generated by Molly retrieve dependencies, on average, 93.81% faster than the Maven build system.