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Re-engineering (1): The Java Native Interface

There’s one other vitally important area of background information necessary to grok; I should have included this in the Homework email. It’s hard to contemplate a Java binding of a C library without understanding the API used to cross the boundary between Java code and C code: the Java Native Interface, or JNI.

The JNI Book

There’s a good book on the topic which at some point became available in an HTML version. You can download it from the link below and I recommend that you do. You’ll need to make a couple symlinks to make the navigation work, though.

The book does get a touch off the beaten track at some points. Without the experience we already have I’m sure it would be easy to get lost down blind alleys. You don’t need to worry about the C code creating and running a VM, for example. And their example of leveraging existing native libraries (binding, to us) is chaotic.

Despite these points (which you can ignore), the rest of the book is excellent. Certainly if you’ve done any work already with the existing java-gnome libraries it’ll be a welcome reference to help understand some of the things we’ve been doing by rote. :)

In any case, there’s one message that comes through loud and clear: while you can do manipulations equivalent to Java code on the C side, it takes MANY more lines of code to do so and is WAY more error-prone; therefore, don’t!

Obviously there are corner cases where we won’t be able to avoid using the power of JNI to cause things to happen on the Java side [raising Exceptions is the thin edge of that particular wedge] but nevertheless it is increasingly obvious to me that if we’re careful about the choices we make we can end up avoiding a huge amount of hassle. As Ismael said on #java-gnome last night: “I prefer that approach more, where you do as most as you can in the Java side. It’s faster and there are better tools to debug with”. Agreed.

The JNI Specification

The spec itself is somewhat obscurely written. As far as I can tell, everything you need is in the above book. I’d still recommend downloading a copy of the spec for reference. Doing:

$ wget -x -nc -r -I/j2se/1.4.2/docs/guide/jni/spec/

will get you a copy which you can then move somewhere convenient.


People (myself included) always express concerns about the performance of code which uses JNI.

Someone in #classpath pointed out the following link:

Reading any selected paragraph of the above reference or the JNI book would quickly give you the idea that most of JNI is the kiss of death for performance. Well, it turns out that java-gnome in its present 2.x form already does things the worst case way:

And you know what? It’s not slow! Java GTK apps are perfectly snappy. My inference is that the underlying libraries we are calling are costly enough (or, fast enough) that a few cycles for the overhead of JNI are not a significant factor. So restricting ourselves based on some paranoia about this or that way of doing things turns out to premature. (Take finalizers for example — so what if “It is typically slower to create and reclaim instances of classes with finalizers than to create and reclaim those without finalizers.” — in the present design we use them to good effect, and things work just fine. If we need them in the re-engineered bindings, so be it)

Doesn’t mean that we won’t choose better algorithms if we can, but we can first and foremost concentrate on getting the job done properly, and accept small JNI overheads should a particular approach that we choose imply those overheads.


Originally written as an email to java-gnome-hackers by Andrew Cowie on 18 Aug 06; last modified 4 Dec 06.

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