Paludis meets Java, part I

Some days ago, the Wearer of the shiny hat started tinkering with JNI bindings for Paludis.

Even if we all think Java is perverse and should be avoided at all costs. Having used Java for the last four years and having studied part of the Java5 code I thought I could use everything I learnt and took the task of developing and maintaining the JNI bindings.

It has all been relatively easy. It’s good that Ciaran started them; because his experience with the Ruby bindings gave the key to developing the Java bindings cleanly.

What we are currently doing is storing a C++ pointer in Java classes and passing it around in native methods. Then we delete it when Java’s garbage collector decides it is good to call finalize.

So it mostly looks like this:

package paludis;
import paludis.Paludis;

public class Foo {
    private static Paludis _load_the_frickin_paludis_library = new Paludis();
    private long _ptr;

    private static native long _construct_string(String s);
    private static native void _destruct(long ptr);

    private static native boolean _is_foo(long ptr);

    protected void finalize() throws Throwable {

    public Foo(String s) {
        _ptr = _construct_string(s);

    public boolean isFoo() {
        return _is_foo(_ptr);

Then the C++ part looks like this:

/* vim: set sw=4 sts=4 et foldmethod=syntax : */

#include "foo.hh"
#include "paludis_java.hh"
#include <jni.h>
#include <paludis/foo.hh>

using namespace paludis;
using namespace paludis::java;

Java_paludis_Foo__1construct_1string(JNIEnv * env, jclass, jstring s)
    return to_java_ptr(new Foo(from_java_string(env, s)));

Java_paludis_Foo__1destruct(JNIEnv *, jclass, jlong ptr)
    delete from_java_ptr<Foo>(ptr);

Java_paludis_Foo__1is_1foo(JNIEnv *, jclass, jlong ptr)
    return to_java_boolean(from_java_ptr<Foo>(ptr)->is_foo());

Here, all the magic is happening behind the scenes in the paludis_java.hh header which I’ll show in a future post of the Paludis meets Java series.

— ferdy

One thought on “Paludis meets Java, part I”

  1. If you haven’t already I would suggest you take a look at JNA instead of JNI. It does it’s binding dynamically instead of having to write native “glue”.

    P.S. Your comment form won’t take my real email address for some reason.

Comments are closed.