@Override
            protected void afterExecute(Runnable r, Throwable t) {
                super.afterExecute(r, t);
                if (t == null && r instanceof Future<?>) {
                    try {
                        Future<?> future = (Future<?>) r;
                        if (future.isDone())
                            future.get();
                    } catch (CancellationException ce) {
                        t = ce;
                    } catch (ExecutionException ee) {
                        t = ee.getCause();
                    } catch (InterruptedException ie) {
                        Thread.currentThread().interrupt(); // ignore/reset
                    }
                }
                if (t != null)
                    t.printStackTrace();
            }
This will print out any exceptions from the thread
jme3-terrain/src/main/java/com/jme3/terrain/geomipmap/TerrainGrid.java