Connection Settings

The Java driver has two ways of specifying the settings of a connection to a MongoDB server deployment.

Connection String

The connection string is the simplest way to specify the properties of a connection. . A connection string mostly follows RFC 3986, with the exception of the domain name. For MongoDB, it is possible to list multiple domain names separated by a comma. Below are some example connection strings.

• For a standalone mongod, mongos, or a direct connection to a member of a replica set:

mongodb://host:27017

• To connect to multiple mongos or a replica set:

mongodb://host1:27017,host2:27017

The authentication guide contains information on how to provide credentials in the connection string.

The Database Component

The database component is optional and is used to indicate which database to authenticate against. When the database component is not provided, the “admin” database is used.

mongodb://host:27017/mydb

Above, the database by the name of “mydb” is where the credentials are stored for the application.

Options

Many options can be provided via the connection string. The ones that cannot may be provided in a MongoClientSettings instance. To provide an option, append a ? to the connection string and separate options by an &.

mongodb://host:27017/?replicaSet=rs0&maxPoolSize=200

The above connection string sets the “replicaSet” value to “rs0” and the “maxPoolSize” to “200”.

For a comprehensive list of the available options, see the ConnectionString documentation.

MongoClient

A MongoClient instance will be the root object for all interaction with MongoDB. It is all that is needed to handle connecting to servers, monitoring servers, and performing operations against those servers.

To create a MongoClient use the MongoClients.create() static helper. Without any arguments MongoClients.create() will return a MongoClient instance will connect to “localhost” port 27017.

MongoClient client = MongoClients.create();

Alternatively, a connection string may be provided:

MongoClient client = MongoClients.create(new ConnectionString("mongodb://host:27017,host2:27017/?replicaSet=rs0"));

Finally, the MongoClientSettings class provides an in-code way to set the same options from a connection string. This is sometimes necessary, as the connection string does not allow an application to configure as many properties of the connection as MongoClientSettings.
MongoClientSettings instances are immutable, so to create one an application uses a builder:

ClusterSettings clusterSettings = ClusterSettings.builder().hosts(asList(new ServerAddress("localhost"))).description("Local Server").build();
MongoClientSettings settings = MongoClientSettings.builder().clusterSettings(clusterSettings).build();
MongoClient client = MongoClients.create(settings);

Netty Configuration

By default, the async driver relies on the AsynchronousSocketChannel class, introduced in Java 7. If configured properly, the driver will use Netty instead. An application must use Netty for the following reasons:

• The application is configured to use SSL to communicate with the MongoDB server.
• The application runs on Java 6.

To configure the driver to use Netty, the application must configure the MongoClientSettings appropriately:

MongoClient client = MongoClients.create(MongoClientSettings.builder()
                                                 .clusterSettings(ClusterSettings.builder()
                                                                          .hosts(Arrays.asList(new ServerAddress()))
                                                                          .build())
                                                 .streamFactoryFactory(NettyStreamFactoryFactory.builder().build())
                                                 .build());

or via connection string:

MongoClient client = MongoClients.create("mongodb://localhost/?streamType=netty");

By default the Netty-based streams will use the NioEventLoopGroup and Netty’s default ByteBufAllocator, but these are configurable via the NettyStreamFactoryFactory constructor.