Closing MongoDB Java Connection

Question

I am trying to design a Mongo Db connection class where I am maintaning MongoClient as static.

private static MongoClient client = null;

public static DB connectToMongo() throws Exception {
    if (null != client) {
        return client.getDB(DBNAME);
    }       
    client = new MongoClient(HOST,PORT);                
    return client.getDB(DBNAME);    
}

My whole web application uses the above method to connect to Mongo as follows:

db = MongoDBConnection.connectToMongo();
collection = db.getCollection("collectionName");

After performing DB operations I never call the close connection for MongoClient. The connection class would always return the same instance of MongoClient which is never closed.The only thing I close is cursors.

• Is it necessary to close the MongoClient every time we query the database? Is my above design valid?

Answer 1

You should definitely not close the MongoClient every time you query the database. The MongoClient maintains a connection pool, which is relatively expensive to set up, so you'll want to re-use the MongoClient instance across the lifetime of your web application.

A couple of other things to point out:

• 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.
• If you ever re-deploy your web application without first restarting your application server, you must ensure that the MongoClient is closed when your web application is shutdown. You can do that, for example, with a ServletContextListener.