The default Transaction Manager implementation.
A CRM is responsible for converting local transactions from/to a form that can be serialized so it can be passed over the wire to another Java VM.
The JTA specification does not define the CRM, so we have implemented our own.
The J2EE specification suggests that the CRM used is the OTS CosTSPortability CRM, but that depends heavily on CORBA, and JBoss is not dependent on any particular object request broker technology, so we do not use that.
The advantage of using our own CRM implementation instead of CosTSPortablilty is twofold:
In JBoss everything is a MBean. Implementing your JTA implementation as an MBean makes it simpler to manage it in a standardized way.
Since your JTA implementation is a service you probably want to make your MBean implemementation extend fromorg.jboss.system.ServiceMBeanSupport.
    When running in the server, your MBean should publish three objects
    in JNDI:
    javax.transaction.TransactionManager.
        This is the standard JTA transaction manager that the JBoss server
        uses to control transactions.
      org.jboss.tm.TransactionPropagationContextImporter.
        This is used by the JBoss communication layers to convert the wire
        representation of a transaction to a
        javax.transaction.Transaction instance that can be used
        locally.
      org.jboss.tm.TransactionPropagationContextFactory.
        This is used by the JBoss communication layers to convert the wire
        representation of a javax.transaction.Transaction
        instance to a form that can be passed over the wire to another JBoss
        server.
      See the implementation of
    org.jboss.tm.TransactionManagerService for an example of
    such a MBean. Please note that the three names your MBean should publish
    in JNDI need not refer to the same object instance as the
    TransactionManagerService does.
Please note that the transaction propagation context (TPC) used by
    the CRM interfaces TransactionPropagationContextImporter
    and TransactionPropagationContextFactory is of type
    java.lang.Object. This allows for the widest range of
    possible TPC implementations. For your TPC to work with the JBoss
    JRMP transport, it should be Java-serializable at runtime.