Slides from a talk on GCJNI and Haven, given at the Haskell Implementor's Meeting in Portland, 1/15/2002.

What is GCJNI?

GCJNI is a library that allows Haskell to invoke Java code via the Java Native Interface (JNI). The implementation uses GreenCard to make the JNI (a C language interface) available to Haskell. GCJNI includes a few convenient features, such as:

• Integration of the Haskell and Java garbage collectors, so that Java objects are garbage collected when they are no longer accessible from Haskell.
• Type class based overloading, which makes it easy to pass common types (like Int, Float and String) to or from Java code.
• A tool (GenBindings) which uses Java reflection on a set of compiled Java classes to generate a Haskell module with a simple, high-level, type-safe interface to the underlying Java code.

GCJNI has been succesfully tested using both hugs98 and ghc under both Linux and Windows. Binary distributions are provided for all for of these (compiler, platform) combinations.

The Haskell types and type classes produced by the GenBindings code generator follow the same design as 'Lambada' by Erik Meijer and Sigbjorn Finne. For more details on the code produced by GenBindings, see:

• Erik Meijer and Sigbjorn Finne, Lambada, Haskell as a better Java, Haskell Workshop 2000
• Mark Shields and Simon Peyton-Jones, Object-Oriented Style Overloading for Haskell, Workshop on Multi-Language Infrastructure and Interoperability (BABEL'01)

Caveats

GCJNI is a one way interface: It enables Haskell code to invoke Java code, but does not allow callbacks from Java to Haskell. Despite this limitation, we have found GCJNI to be very useful. For example, Haven is a purely functional Haskell library for scalable vector graphics, implemented using the Java2D renderer accessed via GCJNI.

The GCJNI library interface lacks substantial documentation beyond the basic release notes. However, much of the API is simply GreenCard'ed versions of the corresponding functions documented in the Java Native Interface (JNI). For more extensive documentation on the JNI, see:

• Sheng Liang, Java(TM) Native Interface: Programmer's Guide and Specification, Addison-Wesley, 1999.

Prerequisites

• An installation of Hugs98 or ghc 5.02.2.
• Java 2 SDK version 1.4.0 standard edition.
  
• GreenCard 2.03
  (I have also tested with GreenCard 2.01, but I strongly recommend 2.03, as the installation and build process has improved considerably.)

Downloading

GCJNI is currently available as a source release. We have worked hard to make it easy to configure, compile and install from sources for any platform.

• x86/Linux Source Release: gcjni-1.2-src.tgz
• Windows Source Release: gcjni-1.2-win32-src.tgz
• Release Notes

Related Projects

Haven is a purely functional Haskell library for scalable vector graphics, implemented using the Java2D renderer accessed via GCJNI. Haven is also the best demonstration of GCJNI and the GenBindings tool. If you download GCJNI, you are encouraged to download and try Haven, to get a feel for what GCJNI can really do.

Feedback

I would be very interested in getting feedback on GCJNI. Please send me email if you have any problems using GCJNI, or if you find GCJNI useful.