The architecture of JMS assumes an existence of an enterprise messaging service provider, which needs to be connected to before it can be used. The act of connecting can be as simple as initializing a local library, or as complex as connecting to a remote enterprise messaging service provider. The details are hidden from the client, who simply creates a connection using a connection factory obtained from JNDI.

// Get an initial naming context
Context initialContext = new InitialContext ();

// Look up the connection factory using
// a well known name in the initial context
ConnectionFactory connectionFactory;
connectionFactory = (ConnectionFactory) initialContext.lookup ("ConnectionFactory");

// Create a connection using the factory
Connection connection;
connection = ConnectionFactory.createConnection ();

// A connection only delivers messages
// once it is explicitly started
connection.start ();

All enterprise messaging communication takes place within the context of a session. The session context keeps track of things such as ordering, listeners and transactions. A session and its resources - producers and consumers but not destinations - are restricted for use by a single thread at any particular time. Multiple sessions can be used to allow multiple threads to communicate concurrently, however, there is no support for concurrent processing of messages delivered to a single consumer.

// Create a session for a connection, requesting
// no transaction support and automatic message
// acknowledgement
Session session;
session = connection.createSession (false, Session.AUTO_ACKNOWLEDGE);

The simplified API (JMS 2.0 and above) introduces context objects, which represent a single session in a single connection. The threading model restrictions still apply.

// Create a context that includes a connection and a session.
// Use try with resources to close the context when done.
try (JMSContext context = connectionFactory.createContext ();)
{
  // Create another context reusing the same connection.
  try (JMSContext another = context.createContext ();)
  {
    ...
  } catch (JMSRuntimeException ex) { ... }
} catch (JMSRuntimeException ex) { ... }