Programming WebLogic JTA

      

Introducing Transactions

This section discusses the following topics:

 


Overview of Transactions in WebLogic Server Applications

This section includes the following sections:

 

ACID Properties of Transactions

One of the most fundamental features of WebLogic Server is transaction management. Transactions are a means to guarantee that database changes are completed accurately and that they take on all the ACID properties of a high-performance transaction, including:

WebLogic Server protects the integrity of your transactions by providing a complete infrastructure for ensuring that database updates are done accurately, even across a variety of resource managers. If any one of the operations fails, the entire set of operations is rolled back.

 

Supported Programming Model

WebLogic Server supports transactions in the Sun Microsystems, Inc., Java Platform, Enterprise Edition (Java EE) programming model. WebLogic Server provides full support for transactions in Java applications that use Enterprise JavaBeans, in compliance with the Enterprise JavaBeans Specification 3.0, published by Sun Microsystems, Inc. WebLogic Server also supports the Java Transaction API (JTA) Specification 1.1, also published by Sun Microsystems, Inc.

 

Supported API Models

WebLogic Server supports the Sun Microsystems, Inc. Java Transaction API (JTA), which is used by:

For information about JTA, see the following sources:

 

Distributed Transactions and the Two-Phase Commit Protocol

WebLogic Server supports distributed transactions and the two-phase commit protocol for enterprise applications. A distributed transaction is a transaction that updates multiple resource managers (such as databases) in a coordinated manner. In contrast, a local transaction begins and commits the transaction to a single resource manager that internally coordinates API calls; there is no transaction manager. The two-phase commit protocol is a method of coordinating a single transaction across two or more resource managers. It guarantees data integrity by ensuring that transactional updates are committed in all of the participating databases, or are fully rolled back out of all the databases, reverting to the state prior to the start of the transaction. In other words, either all the participating databases are updated, or none of them are updated.

Distributed transactions involve the following participants:

The first phase of the two-phase commit protocol is called the prepare phase. The required updates are recorded in a transaction log file, and the resource must indicate, through a resource manager, that it is ready to make the changes. Resources can either vote to commit the updates or to roll back to the previous state. What happens in the second phase depends on how the resources vote. If all resources vote to commit, all the resources participating in the transaction are updated. If one or more of the resources vote to roll back, then all the resources participating in the transaction are rolled back to their previous state.

 

Support for Business Transactions

WebLogic JTA provides the following support for your business transactions:

 


When to Use Transactions

Transactions are appropriate in the situations described in the following list. Each situation describes a transaction model supported by the WebLogic Server system. Keep in mind that distributed transactions should not span more than a single user input screen; more complex, higher level transactions are best implemented with a series of distributed transactions.

 


What Happens During a Transaction

This topic includes the following sections:

 

Transactions in WebLogic Server EJB Applications

Figure 2-1 illustrates how transactions work in a WebLogic Server EJB application. Figure 2-1 How Transactions Work in a WebLogic Server EJB Application

How Transactions Work in a WebLogic Server EJB Application

WebLogic Server supports two types of transactions in WebLogic Server EJB applications:

The sequence of transaction events differs between container-managed and bean-managed transactions.

Container-managed Transactions

For EJB applications with container-managed transactions, a basic transaction works in the following way:

  1. In the EJB's deployment descriptor, the Bean Provider or Application Assembler specifies the transaction type (transaction-type element) for container-managed demarcation (Container).

  2. In the EJB's deployment descriptor, the Bean Provider or Application Assembler specifies the default transaction attribute (trans-attribute element) for the EJB, which is one of the following settings: NotSupported, Required, Supports, RequiresNew, Mandatory, or Never. For a detailed description of these settings, see Section 17.6.2 in the Enterprise JavaBeans Specification 2.0, published by Sun Microsystems, Inc.

  3. Optionally, in the EJB's deployment descriptor, the Bean Provider or Application Assembler specifies the trans-attribute for one or more methods.

  4. When a client application invokes a method in the EJB, the EJB container checks the trans-attribute setting in the deployment descriptor for that method. If no setting is specified for the method, the EJB uses the default trans-attribute setting for that EJB.

  5. The EJB container takes the appropriate action depending on the applicable trans-attribute setting.

    • For example, if the trans-attribute setting is Required, the EJB container invokes the method within the existing transaction context or, if the client called without a transaction context, the EJB container begins a new transaction before executing the method.

    • In another example, if the trans-attribute setting is Mandatory, the EJB container invokes the method within the existing transaction context. If the client called without a transaction context, the EJB container throws the javax.transaction.TransactionRequiredException exception.

  6. During invocation of the business method, if it is determined that a rollback is required, the business method calls the EJBContext.setRollbackOnly method, which notifies the EJB container that the transaction is to be rolled back at the end of the method invocation.

    Calling the EJBContext.setRollbackOnly method is allowed only for methods that have a meaningful transaction context.

  7. At the end of the method execution and before the result is sent to the client, the EJB container completes the transaction, either by committing the transaction or rolling it back (if the EJBContext.setRollbackOnly method was called).

Bean-managed Transactions

For EJB applications with bean-managed transaction demarcations, a basic transaction works in the following way:

  1. In the EJB's deployment descriptor, the Bean Provider or Application Assembler specifies the transaction type (transaction-type element) for container-managed demarcation (Bean).

  2. The client application uses JNDI to obtain an object reference to the UserTransaction object for the WebLogic Server domain.

  3. The client application begins a transaction using the UserTransaction.begin method, and issues a request to the EJB through the EJB container. All operations on the EJB execute within the scope of a transaction.

    • If a call to any of these operations raises an exception (either explicitly or as a result of a communication failure), the exception can be caught and the transaction can be rolled back using the UserTransaction.rollback method.

    • If no exceptions occur, the client application commits the current transaction using the UserTransaction.commit method. This method ends the transaction and starts the processing of the operation. The transaction is committed only if all of the participants in the transaction agree to commit.

  4. The UserTransaction.commit method causes the EJB container to call the transaction manager to complete the transaction.

  5. The transaction manager is responsible for coordinating with the resource managers to update any databases.

 

Transactions in WebLogic Server RMI Applications

Figure 2-2 illustrates how transactions work in a WebLogic Server RMI application. Figure 2-2 How Transactions Work in a WebLogic Server RMI Application

How Transactions Work in a WebLogic Server RMI Application

For RMI client and server applications, a basic transaction works in the following way:

  1. The application uses JNDI to return an object reference to the UserTransaction object for the WebLogic Server domain.

    Obtaining the object reference begins a conversational state between the application and that object. The conversational state continues until the transaction is completed (committed or rolled back). Once instantiated, RMI objects remain active in memory until they are released (typically during server shutdown). For the duration of the transaction, the WebLogic Server infrastructure does not perform any deactivation or activation.

  2. The client application begins a transaction using the UserTransaction.begin method, and issues a request to the server application. All operations on the server application execute within the scope of a transaction.

    • If a call to any of these operations raises an exception (either explicitly or as a result of a communication failure), the exception can be caught and the transaction can be rolled back using the UserTransaction.rollback method.

    • If no exceptions occur, the client application commits the current transaction using the UserTransaction.commit method. This method ends the transaction and starts the processing of the operation. The transaction is committed only if all of the participants in the transaction agree to commit.

  3. The UserTransaction.commit method causes WebLogic Server to call the transaction manager to complete the transaction.

  4. The transaction manager is responsible for coordinating with the resource managers to update any databases.

For more information, see Transactions in RMI Applications.

 


Transactions Sample Code

This section includes the following sections:

 

Transactions Sample EJB Code

This section provides a walkthrough of sample code fragments from a class in an EJB application. This topic includes the following sections:

The code fragments demonstrate using the UserTransaction object for bean-managed transaction demarcation. The deployment descriptor for this bean specifies the transaction type (transaction-type element) for transaction demarcation (Bean).

Notes: In a global transaction, use a database connection from a local JDBC data source—on the WebLogic Server instance on which the EJB is running. Do not use a connection from a JDBC data source on a remote WebLogic Server instance.

Note: These code fragments do not derive from any of the sample applications that ship with WebLogic Server. They merely illustrate the use of the UserTransaction object within an EJB application.

Importing Packages

Listing 2-1 shows importing the necessary packages for transactions, including:

Using JNDI to Return an Object Reference

Listing 2-2 shows how look up an object on the JNDI tree. Listing 2-2 Performing a JNDI Lookup

Context ctx = null;

Hashtable env = new Hashtable();

env.put(Context.INITIAL_CONTEXT_FACTORY,
"weblogic.jndi.WLInitialContextFactory");

// Parameters for the WebLogic Server.
// Substitute the correct hostname, port number
// user name, and password for your environment:
env.put(Context.PROVIDER_URL, "t3://localhost:7001");
env.put(Context.SECURITY_PRINCIPAL, "Fred");
env.put(Context.SECURITY_CREDENTIALS, "secret");

ctx = new InitialContext(env);

UserTransaction tx = (UserTransaction)
ctx.lookup("javax.transaction.UserTransaction");

Starting a Transaction

Listing 2-3 shows starting a transaction by getting a UserTransaction object and calling the javax.transaction.UserTransaction.begin() method. Database operations that occur after this method invocation and prior to completing the transaction exist within the scope of this transaction. Listing 2-3 Starting a Transaction

UserTransaction tx = (UserTransaction)

ctx.lookup("javax.transaction.UserTransaction");
tx.begin();

Completing a Transaction

Listing 2-4 shows completing the transaction depending on whether an exception was thrown during any of the database operations that were attempted within the scope of this transaction:

 

Transactions Sample RMI Code

This topic provides a walkthrough of sample code fragments from a class in an RMI application. This topic includes the following sections:

The code fragments demonstrate using the UserTransaction object for RMI transactions. For guidelines on using transactions in RMI applications, see Transactions in RMI Applications.

These code fragments do not derive from any of the sample applications that ship with WebLogic Server. They merely illustrate the use of the UserTransaction object within an RMI application.

Importing Packages

Listing 2-5 shows importing the necessary packages, including the following packages used to handle transactions:

After importing these classes, initialize an instance of the UserTransaction object to null.

Using JNDI to Return an Object Reference to the UserTransaction Object

Listing 2-6 shows searching the JNDI tree to return an object reference to the UserTransaction object for the appropriate WebLogic Server domain.

Obtaining the object reference begins a conversational state between the application and that object. The conversational state continues until the transaction is completed (committed or rolled back). Once instantiated, RMI objects remain active in memory until they are released (typically during server shutdown). For the duration of the transaction, the WebLogic Server infrastructure does not perform any deactivation or activation. Listing 2-6 Performing a JNDI Lookup

Context ctx = null;

Hashtable env = new Hashtable();

env.put(Context.INITIAL_CONTEXT_FACTORY,
"weblogic.jndi.WLInitialContextFactory");

// Parameters for the WebLogic Server.
// Substitute the correct hostname, port number
// user name, and password for your environment:
env.put(Context.PROVIDER_URL, "t3://localhost:7001");
env.put(Context.SECURITY_PRINCIPAL, "Fred");
env.put(Context.SECURITY_CREDENTIALS, "secret");

ctx = new InitialContext(env);

UserTransaction tx = (UserTransaction)
ctx.lookup("javax.transaction.UserTransaction");

Starting a Transaction

Listing 2-7 shows starting a transaction by calling the javax.transaction.UserTransaction.begin() method. Database operations that occur after this method invocation and prior to completing the transaction exist within the scope of this transaction. Listing 2-7 Starting a Transaction

UserTransaction tx = (UserTransaction)

ctx.lookup("javax.transaction.UserTransaction");
tx.begin();

Completing a Transaction

Listing 2-8 shows completing the transaction depending on whether an exception was thrown during any of the database operations that were attempted within the scope of this transaction: