Smooth Build is a build tool which provides a functional language which lets you describe a build process in a simple and readable way. A build script for a simple Java program can be as simple as "app.jar: files("src") | javac | jar;" All built artifacts are cached and indexed (hashed) using the sha1 of the expression (function name and its arguments) which returned a given artifact. This way, Smooth never builds the same artifact twice.
android-metadata is a utility library for Android designed to simplify reading metadata values from AndroidManifest.xml. It can quickly access metadata values from anywhere with a few lines of code, read individual metadata values into type-specific variables, and read one or more metadata values into annotated Java classes.
Vibur Object Pool is a general-use concurrent Java object pool which provides both validating and non-validating pool implementations. The pool is built entirely using the standard Java concurrency utilities, does not use any synchronized blocks/methods, and does not have any external dependencies. This pool is an excellent choice for pooling/caching of expensive to create Java objects such as database connections or other socket connections to remote hosts, and similar.
gradle-sablecc-plugin is a gradle plugin which creates parsers using SableCC. SableCC supports automatic CST-to-AST transformation, emits all the visitor patterns and analysis helpers you will likely ever need, and is LR, not LL(k). Many example grammars are available for modern languages; the author of this plugin has written dozens.
jdiagnostics is a library of routines for inspecting the state of a JVM and building a support bundle. It makes it easy to diagnose issues with classpaths, resources, serviceloaders, versioning, XML parsers, and so forth. It has no dependencies, and is an essential library to build into any infrastructure application.
Immutables allows you to embrace immutablity to tackle complexity and concurrency challenges. To get the benefits of immutability in Java, there's an annotation processor to easily create simple and consistent value objects. You can think of it as Guava's Immutable Collections but for regular objects. It includes support for JSON marshaling, MongoDB repositories, and JAX-RS.