Members is a blocking method and should not be called in the UI thread. public Object[] getChildren(Object parent) { if(parent == null) return null; if( parent instanceof List) return ((List)parent).toArray(); if( parent instanceof String ) return new Object[0]; if( parent instanceof IResolve ) try { return ((IResolve)parent ).members(null).toArray(); } catch (IOException e) { CorePlugin.log(CatalogUIPlugin.getDefault(), e); } return null; }