mongodb 自訂遞增主鍵

private final static String COLUMN_SEQ = "seq";    private final static String COLUMN_CATEGORY = "category";    private Mongo mongo;    private String dbName;    private String collName;    public void setMongo(Mongo mongo) {        this.mongo = mongo;    }    public void setDbName(String dbName) {        this.dbName = dbName;    }    public void setCollName(String collName) {        this.collName = collName;    }    @Override    public long nextSeq(String category) {        if (category == null || "".equals(category)) {            throw new IllegalArgumentException("category can't be null or empty");        }        DBObject update = new BasicDBObject("$inc", new BasicDBObject(COLUMN_SEQ, 1L));        DBObject query = new BasicDBObject(COLUMN_CATEGORY, category);        return (Long) mongo.getDB(dbName).getCollection(collName).findAndModify(query, null, null, false, update, true,                true).get(COLUMN_SEQ);    }