MongoClient维护一个连接池，您一次使用MongoClient打开一个Db连接，并在您的应用程序中重复使用它，因为建立新的TCP连接在时间和内存上都是昂贵的明智的做法是重复使用连接的原因.另外，新的连接也会导致使用Db上的内存在MongoDB上创建新的线程.

The MongoClient maintains a connection pool,You open a Db connection once with MongoClient and reuse it across your application because setting up a new TCP connection is EXPENSIVE timewise and memory wise that's why you reuse connections. Also a new connection will cause a new Thread to be created on MongoDB using memory on the Db as well.

要注意的一点是，connectToMongo方法中存在竞争条件.您需要同步对该方法的访问，以确保最多创建一个MongoClient实例.

• point to be noted that there is a race condition in the connectToMongo method. You need to synchronize access to that method to ensure that at most one instance of MongoClient is ever created.