pdf417 java decoder Combining entity updates into a single transaction in Java

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5.7 Combining entity updates into a single transaction
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You want to update more than one entity bean in a single transaction.
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You have two or more entity beans that represent data in a single step of a transaction. For example, from the account transfer example in the chapter introduction,
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you might have entity beans that represent each of the two accounts. In this case, you want to perform both updates in single transaction.
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Develop a session bean that contains a business method that updates the entity beans within its transaction. In addition, each entity bean s transaction attribute should not be set to RequiresNew, Never, or NotSupported ; you must set it to Requires , Mandatory, or Supports . The SampleBean session bean shown in listing 5.5 updates two entity beans in a single transaction.
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Listing 5.5 SampleBean.java
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public class SampleBean implements SessionBean { private SampleEntityBean bean1; private SampleEntityBean bean2; /** * Sample business method executing in a single transaction. */ public void commitMultipleEntities() { bean1.setMyAttribute( "abc" ); bean2.setMyAttribute( "def" ); } //session bean methods below }
The transaction attribute for the session bean should be set to Required, Mandatory, or RequiresNew.
Discussion
Using session beans to encapsulate access to entity beans is an accepted practice by all EJB developers. The session bean uses its transaction for the commitMultipleEntities() business method to contain the two updates of the entity beans. In order to keep the multiple entity bean updates within a single transaction, make sure that each entity bean s transaction attribute is not set to RequiresNew. If you use RequiresNew, the EJB container will create a separate transaction for each entity bean update, breaking the single-transaction requirement. By propagating the business method s transaction to the entity beans, all the updates can be rolled back together. This ensures that all entity beans are updated if and only if they all succeed in the update.
Managing EJB state at transaction boundaries
See also
5.1 Tuning the container transaction control for your EJB 5.16 Updating multiple databases in one transaction
5.8 Managing EJB state at transaction boundaries
Problem
In order to manage their state, you want your EJBs to know when a transaction starts, completes, and/or rolls back.
Background
You don t want to use bean-managed transactions, but you do want to be notified when a transaction starts and when one completes. Being notified of transaction events allows your beans to manage their member state variables appropriately. For instance, when a transaction starts, you want to make sure the EJB has initialized its state variables. When the transaction completes, you want to make sure you are preparing state variables for the next transaction. This doesn t apply to stateless session beans, because they should not contain any state that needs to be managed at transaction boundaries.
Recipe
To get information about transaction status for stateful session beans, implement the javax.ejb.SessionSynchronization interface. The example session bean in listing 5.6 receives a callback for the beginning, successful completion, and postcompletion of a transaction.
Listing 5.6 SampleBean.java
public class SampleBean implements SessionBean, SessionSynchronization { private Object stateVariable; public void afterBegin() throws RemoteException { //Transaction has started stateVariable = initializeState(); } public void beforeCompletion() throws RemoteException { //Transaction is about to complete
Transactions
saveState(); } public void afterCompletion( boolean committed ) throws RemoteException { //test for rollback if( !committed ) stateVariable = initializeState(); } //implementation of the EJB below }
For entity beans, the ejbLoad() and ejbStore() methods are the beginning and end of a transaction (also ejbCreate() and ejbRemove() basically where data is loaded or stored). If a rollback occurs, ejbLoad() will be invoked to restore the bean s state.
Discussion
The SessionSynchronization interface should be used to prepare a stateful session bean before specific transaction boundaries. It provides callback methods to the bean occurring after the transaction begins, before the transaction commits, and after the transaction completes. The afterCompletion() callback provides a boolean parameter indicating whether the transaction has been marked for a rollback. In addition, the beforeCompletion() callback will be invoked only if the transaction is about to perform a commit. The container will invoke all the methods of this interface at the correct times. The first callback method, afterBegin(), should be used to prepare data for use inside a transaction. This process might include reading from a database. The second callback should be used to save the state of data that was created from the business method and needs to be preserved. The final method should be used to test for a rollback condition. If a rollback has occurred, use this method to restore the session bean s data to its original state before the transaction started.
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