Mongo java 程序中 空指针

public synchronized DBCollection getDBCollection(String collection_name){
        DBCollection dbColl = null;
        System.out.println("poolSize:"+dbPool.size());
        while(dbPool.size() == 0){
            try {
                System.out.println("sleep------------------------------------");
                java.lang.Thread.sleep(100);
            } catch (InterruptedException e) {
                System.out.println("连接全部用光,这里sleep出错了.");
            }
        }
        System.out.println(collection_name+"-----------");
        //NullPointException
       dbColl = dbPool.remove(0).getCollection(collection_name);
        return dbColl;
    }