@Override
        public void write(JmeExporter je) throws IOException {
            OutputCapsule capsule = je.getCapsule(this);

            Vector3f v1 = new Vector3f(1f, 2f, 3f);
            Vector3f notV1 = v1.clone();

            capsule.write(v1, "v1", null);
            capsule.write(v1, "also_v1", null);
            capsule.write(notV1, "not_v1", null);

            // testing reference loop.  this used to cause infinite recursion.
            Node n1 = new Node("node_1");
            Node n2 = new Node("node_2");

            n1.setUserData("node_2", n2);
            n2.setUserData("node_1", n1);

            capsule.write(n1, "node_1", null);
            capsule.write(n2, "node_2", null);
        }
Test suite for implementations of the JmeExporter and JmeImporter interfaces.
There are tests here for all write* and read* methods of the OutputCapsule and InputCapsule interfaces respectively.
jme3-plugins/src/test/java/com/jme3/export/InputOutputCapsuleTest.java