EJB Programmer's Guide: Transactional Behaviour

Target Audience and Content

The target audience for this guide is the Enterprise Bean provider, i.e. the person in charge of developing the software components on the server side. It describes how to define the transactional behaviour of an EJB application.

The content of this guide is the following:

1. Target Audience and Content
2. Declarative Transaction Management
3. Bean-managed Transaction
4. Distributed Transaction Management

Declarative Transaction Management

For container-managed transaction management, the transactional behaviour of an enterprise bean is defined at configuration time and is part of the assembly-descriptor element of the standard deployment descriptor. It is possible to define a common behaviour for all the methods of the bean, or to define the behaviour at the method level. This is done by specifying a transactional attribute, which can be one of the following:

• NotSupported: if the method is called within a transaction, this transaction is suspended during the time of the method execution.
• Required: if the method is called within a transaction, the method is executed in the scope of this transaction, else, a new transaction is started for the execution of the method, and committed before the method result is sent to the caller.
• RequiresNew: the method will always be executed within the scope of a new transaction. The new transaction is started for the execution of the method, and committed before the method result is sent to the caller. If the method is called within a transaction, this transaction is suspended before the new one is started and resumed when the new transaction has completed.
• Mandatory: the method should always be called within the scope of a transaction, else the container will throw the TransactionRequired exception.
• Supports: the method is invoked within the caller transaction scope; if the caller does not have an associated transaction, the method is invoked without a transaction scope.
• Never: The client is required to call the bean without any transaction context; if it is not the case, a java.rmi.RemoteException is thrown by the container.

In the deployment descriptor, the specification of the transactional attributes appears in the assembly-descriptor as follows:

  <assembly-descriptor>
    <container-transaction>
      <method>
      <ejb-name>AccountImpl</ejb-name>
      <method-name>*</method-name>
      </method>
      <trans-attribute>Supports</trans-attribute>
    </container-transaction>
    <container-transaction>
      <method>
      <ejb-name>AccountImpl</ejb-name>
      <method-name>getBalance</method-name>
      </method>
      <trans-attribute>Required</trans-attribute>
    </container-transaction>
    <container-transaction>
      <method>
      <ejb-name>AccountImpl</ejb-name>
      <method-name>setBalance</method-name>
      </method>
      <trans-attribute>Mandatory</trans-attribute>
    </container-transaction>
  </assembly-descriptor>
    

In this example, for all methods of the AccountImpl bean which are not explicitly specified in a container-transaction element, the default transactional attribute is Supports (defined at the bean-level), and the transactional attributes are Required and Mandatory (defined at the method-name level) for the methods getBalance and setBalance respectively.

Bean-managed Transaction

A bean that manages its transactions itself must set the transaction-type element in its standard deployment descriptor to:

  <transaction-type>Bean</transaction-type>
    

To demarcate the transaction boundaries in a bean with bean-managed transactions, the bean programmer should use the javax.transaction.UserTransaction interface, which is defined on an EJB server object that may be obtained using the EJBContext.getUserTransaction() method (the SessionContext object or the EntityContext object depending on whether the method is defined on a session or on an entity bean). The following example shows a session bean method "doTxJob" demarcating the transaction boundaries; the UserTransaction object is obtained from the sessionContext object, which should have been initialized in the setSessionContext method (refer to the example of the session bean).

public void doTxJob() throws  RemoteException {
     UserTransaction ut = sessionContext.getUserTransaction();
     ut.begin();
     ... // transactional operations
     ut.commit();
}

Another way to do this is to use JNDI and to retrieve UserTransaction with the name java:comp/UserTransaction in the initial context.

Distributed Transaction Management

As explained in the previous section, the transactional behaviour of an application can be defined in a declarative way or coded in the bean and/or the client itself (transaction boundaries demarcation). In any case, the distribution aspects of the transactions are completely transparent to the bean provider and to the application assembler. This means that a transaction may involve beans located on several JOnAS servers and that the platform itself will handle management of the global transaction. It will perform the two-phase commit protocol between the different servers, and the bean programmer need do nothing.

Once the beans have been developed and the application has been assembled, it is possible for the deployer and for the administrator to configure the distribution of the different beans on one or several machines, and within one or several JOnAS servers. This can be done without impacting either the beans code or their deployment descriptors. The distributed configuration is specified at launch time. In the environment properties of an EJB server, the following can be specified:

• which enterprise beans the JOnAS server will handle,
• if a Java Transaction Monitor will be located in the same Java Virtual Machine (JVM) or not.

To achieve this goal, two properties must be set in the jonas.properties file, jonas.service.ejb.descriptors and jonas.service.jtm.remote. The first one lists the beans that will be handled on this server (by specifying the name of their ejb-jar files), and the second one sets the Java Transaction Monitor (JTM) launching mode:

• if set to true, the JTM is remote, i.e. the JTM must be launched previously in another JVM,
• if set to false, the JTM is local, i.e. it will run in the same JVM as the EJB Server.

Example:

  jonas.service.ejb.descriptors       Bean1.jar, Bean2.jar
  jonas.service.jtm.remote            false
    

The Java Transaction Monitor can run outside any EJB server, in which case it can be launched in a stand-alone mode using the following command:

TMServer

Using these configuration facilities, it is possible to adapt the beans distribution to the resources (cpu and data) location, for optimizing performance.

The following figure illustrates four cases of distribution configuration for three beans.

1. Case 1: The three beans B1, B2, and B3 are located on the same JOnAS server, which embeds a Java Transaction Monitor.
2. Case 2: The three beans are located on different JOnAS servers, one of them running the Java Transaction Monitor, which manages the global transaction.
3. Case 3: The three beans are located on different JOnAS servers, the Java Transaction Monitor is running outside of any JOnAS server.
4. Case 4: The three beans are located on different JOnAS servers. Each server is running a Java Transaction Monitor. One of the JTM acts as the master monitor, while the two others are slaves.

These different configuration cases may be obtained by launching the JOnAS servers and eventually the JTM (case 3) with the adequate properties. The rational when choosing one of these configurations is resources location and load balancing. However, consider the following pointers:

• if the beans should run on the same machine, with the same server configuration, case 1 is the more appropriate;
• if the beans should run on different machines, case 4 is the more appropriate, since it favours local transaction management;
• if the beans should run on the same machine, but require different server configurations, case 2 is a good approach.