Annotation not working

I'm working on an annotation which aims to it mandatory for a class to be immutable. Here the code of the processor:

@SupportedAnnotationTypes("archipel.immutability.IsImmutable")
@SupportedSourceVersion(SourceVersion.RELEASE_6)
public class IsImmutableProcessor extends AbstractProcessor {

    @Override
    public boolean process(Set<? extends TypeElement> annotations,
            RoundEnvironment roundEnv) {
        for (TypeElement type : annotations) {
            processMustBeImmutable(roundEnv, type);
        }
        return true;
    }

    private void processMustBeImmutable(RoundEnvironment env, TypeElement type) {
        for (Element element : env.getElementsAnnotatedWith(type)) {
            processClass(element);
        }
    }

    private void processClass(Element element) {
        boolean isFinal=false;

        for(Modifier modifier : element.getModifiers()) {
            if (modifier.equals(Modifier.FINAL)) {
                isFinal=true;
                break;
            }
        }

        if (!isFinal) {
            processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR, "Class "+element+" is not immutable because it is not final");
        } else {
            for (Element subElement : element.getEnclosedElements()) {
                if (subElement.getKind()==ElementKind.FIELD) {
                    isFinal=false;

                    for(Modifier modifier : subElement.getModifiers()) {
                        if (modifier.equals(Modifier.FINAL)) {
                            isFinal=true;
                            break;
                        }
                    }
                    if (!isFinal) {
                        processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR, "Field "+element+" is not immutable because it is not final");
                    } else {
                        Element superElement = subElement.getEnclosingElement();
                        // TODO
                    }
                }
            }
        }
    }

}

The annotation itself is trivial, of course:

@Inherited
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.SOURCE)
public @interface IsImmutable {
}

And I compile it with a Ant script:

<project name="immutability" basedir="." default="main">

    <property name="lib.dir" value="lib"/>

    <property name="src.dir" value="src"/>

    <property name="build.dir" value="build"/>
    <property name="classes.dir" value="${build.dir}/classes"/>
    <property name="meta.dir" value="${build.dir}/META-INF"/>
    <property name="jar.dir" value="${build.dir}/jar"/>

    <property name="processor-package"
        value="archipel.immutability" />

    <property name="processor" value="${processor-package}.IsImmutableProcessor"/>

    <path id="classpath">
        <fileset dir="${lib.dir}" includes="**/*.jar"/>
        <fileset dir="${classes.dir}"/>
    </path>

    <target name="clean">
    <delete dir="${build.dir}"/>
    </target>

    <target name="compile" description="Compiles the code.">
        <mkdir dir="${classes.dir}"/>
        <javac srcdir="${src.dir}" destdir="${classes.dir}" classpathref="classpath" />
    </target>

    <target name="jar" depends="compile">
        <mkdir dir="${jar.dir}"/>
        <jar destfile="${jar.dir}/${ant.project.name}.jar">
            <fileset dir="${classes.dir}"/>
            <service type="javax.annotation.processing.Processor" provider="archipel.immutability.IsImmutableProcessor"/>
        </jar>
    </target>

    <target name="main" depends="clean,jar"/>

</project>

Problem is, something must be missing, because when I try to use the resulting annotation, provided by the resulting jar file, like the following, nothing happens:

@IsImmutable
public class Immut {

    private int toto;

    public int getToto() {
        return toto;
    }

    public void setToto(int toto) {
        this.toto = toto;
    }

    public final static void main(String args[]) {
        Immut truc = new Immut();
        truc.setToto(5);
        truc.setToto(6);
    }

}

Obviousy, this class is not final, and the class should signal an error in Eclipse. But it's not.

Any idea?

Edit: The jar file I built with my build.xml seems correct: It contains the class files, and also a META-INF/services/javax.annotation.processing.Processor file, which contains archipel.immutability.IsImmutableProcessor. I import this jar file in my test project, and when I use the annotation in my Immut class (which is only a rough test), nothing happens.