Swift offers Java interoperability using the SwiftJava package, package plugins, and also the swift-java command line tool for source generation (similar to OpenJDK jextract, if you are familiar with that concept).
Most developers should prever the use of that package, as it features bullet proof ways to make your inter-op code safe and performant, by using either source generation (swift-java) when calling Swift from Java, or Swift macros (@JavaMethod, ...) when calling Java from Swift. Multiple safeguards and optimizations are built into the SwiftJava library and we highly recommend using it instead of this low level "raw JNI" package.
The swift-java-jni-core package presents a low-level Swift-friendly interface to the Java Native Interface (JNI) specification, which is the universal set of data types and functions for interacting with a Java Virtual Machine and compatible derivatives, such as the Android Runtime (ART). You can view it as a thin layer on top of the jni.h along with pre-packaged type conversions and ways to interact with the JVM from Swift.
This package is designed to offer low-level zero-dependency support for higher-level modules, such as SwiftJava and other projects.
A JavaValue describes a type that can be bridged with Java. JavaValue is the base protocol for bridging between Swift types and their Java counterparts via the Java Native Interface (JNI). It is suitable for describing both value types (such as Int32 or Bool) and object types.
The JavaVirtualMachine provides access to a Java Virtual Machine (JVM), which can either be loaded from within a Swift process (via JNI_CreateJavaVM), or accessed from a pre-existing in-process handle (JNI_GetCreatedJavaVMs). The JavaVirtualMachine is the entry point to interfacing with the JVM, and handles finding and loading classes, looking up and invoking methods, and handling details like locking, threads, and references.
This C module provides the standardized and implementation-agnostic headers for the Java Native Interface specification. The shape of these structures and symbols are guaranteed to be ABI stable between any compatible Java implementation.
Contributions are more than welcome, and you can read more about the process in CONTRIBUTING.md.