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public interface: DeliveryMode [javadoc | source] The delivery modes supported by the JMS API are PERSISTENT and NON_PERSISTENT.

A client marks a message as persistent if it feels that the application will have problems if the message is lost in transit. A client marks a message as non-persistent if an occasional lost message is tolerable. Clients use delivery mode to tell a JMS provider how to balance message transport reliability with throughput.

Delivery mode covers only the transport of the message to its destination. Retention of a message at the destination until its receipt is acknowledged is not guaranteed by a PERSISTENT delivery mode. Clients should assume that message retention policies are set administratively. Message retention policy governs the reliability of message delivery from destination to message consumer. For example, if a client's message storage space is exhausted, some messages may be dropped in accordance with a site-specific message retention policy.

A message is guaranteed to be delivered once and only once by a JMS provider if the delivery mode of the message is PERSISTENT and if the destination has a sufficient message retention policy.
Field Summary
static final  int NON_PERSISTENT    This is the lowest-overhead delivery mode because it does not require that the message be logged to stable storage. The level of JMS provider failure that causes a NON_PERSISTENT message to be lost is not defined.

A JMS provider must deliver a NON_PERSISTENT message with an at-most-once guarantee. This means that it may lose the message, but it must not deliver it twice. 

static final  int PERSISTENT    This delivery mode instructs the JMS provider to log the message to stable storage as part of the client's send operation. Only a hard media failure should cause a PERSISTENT message to be lost.