Alfresco Javascript API has a search.findNode("workspace://SpacesStore/" + documentguid) method, that giving a guid of a document (node), one can get the noderef of the document.
How to get this "org.alfresco.service.cmr.repository.NodeRef" using java (backend java), having only the document/node guid?
You can use the NodeRef constructor :
Construct a Node Reference from a string representation of a Node Reference. NodeRef(String protocol, String identifier, String id)
Example :
NodeRef myNoderef = new NodeRef("workspace","SpacesStore",documentguid)
If you want to get more information of your document, or do some modification on it, you can use the NodeService of FileFolderService (it depends of your needs) :
fileFolderService.getFileInfo(myNoderef)