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124 Chapter 9. Developing Message-Driven Beans
configuration (for example, distribution, multi-servers), refer to the JORAM documentation on
http://joram.objectweb.org.
9.5. Transactional Aspects
Because a transactional context cannot be carried by a message (according to the EJB 2.1 speci-
fication), an MDB will never execute within an existing transaction. However, a transaction may
be started during the
onMessage
method execution, either due to a “required” transaction attribute
(container-managed transaction) or because it is explicitly started within the method (if the MDB is
bean-managed transacted). In the second case, the message receipt will not be part of the transac-
tion. In the case of the container-managed transaction, the container starts a new transaction before
de-queuing the JMS message (the receipt of which will, thus, be part of the started transaction), then
enlist the resource manager associated with the arriving message and all the resource managers ac-
cessed by the
onMessage
method. If the
onMessage
method invokes other Enterprise Beans, the
container passes the transaction context with the invocation. Therefore, the transaction started at the
onMessage
method execution may involve several operations, such as accessing a database (via a call
to an Entity Bean, or by using a “datasource” resource), or sending messages (by using a “connection
factory” resource).
9.6. Message-Driven Beans Example
JOnAS provides examples that are located in the
examples/src/mdb
install directory.
samplemdb
is a very simple example, the code of which is used in the previous topics for illustrating
how to use Message-Driven Beans.
sampleappli
is a more complex example that shows how the sending of JMS messages and updates
in a database via JDBC may be involved in the same distributed transaction.
The following figure illustrates the architecture of this example application.