JMaker was born from frustration with using Gradle for managing Java projects with nonstandard setups. Gradle's design prioritizes sane defaults, and build files should only need to describe deviations from the default project structure. This philosophy works excellently for pure Java projects, but runs into problems if the project mixes in other languages or dynamically generated resources. It also has a hard time running arbitrary code as part of its build files. Although it uses a programming language to define them, it only exposes an extremely small, minimal API. Each plugin has an API, but the details are often undocumented. Finally, it can have difficulties building an accurate dependency graph for unusual project setups.
GNU Make has the opposite philosophy; it offers some defaults, but assumes very little about how it's being used. Make excels at managing complex build processes by using very robust bash integration and a well-defined dependency graph. However, makefiles are described using a very old language with strange corner cases and limitations. Notably, it has extreme difficulties handling commands, variable values, and bash commands that include colons. The Java command-line compiler (javac) makes use of colons in many of its commands, so Make has a hard time with Java.
JMaker was an effort to make a tool that could leverage the strengths of each, with fewer downsides. It used Lua to define its configuration and commands, and uses a rule-based production system like Make. That would allow it to have a clear and well-defined dependency graph, with all the power of using an actual programming language for build files.
I finished a fully functioning Lua parser and interpreter with custom syntax for recipe definition. But work on the project stalled when writing the code to handle external scripts. I learned how difficult it is to correctly start processes, capture their input and output, manage environment variables, and handle crashed or unresponsive commands. Make has built up a very robust system for calling executables and running bash scripts that is extremely difficult to port to Java. It handles many corner-cases and works around issues that I didn't even know existed until I tried to implement a similar system.
I started JMaker as part of my Blue Lagoon project. Gradle wasn't able to understand some asset-generating scripts for tileset management. Unfortunately, I wasn't able to get it working well enough to replace Gradle on that project.