public void setDocumentId(DocumentId documentId) {
        if (documentId == null) {
            throw new IllegalArgumentException("Document id can not be null.");
        }
        this.documentId = documentId;
    }
Sets the identifier of the document to retrieve.

@param documentId The document id to set.
documentapi/src/main/java/com/yahoo/documentapi/messagebus/protocol/GetDocumentMessage.java