Container-Managed Transactions

In an enterprise bean with container-managed transactions, the EJB container sets the boundaries of the transactions. You can use container-managed transactions with any type of enterprise bean: session, entity, or message-driven. Container-managed transactions simplify development because the enterprise bean code does not explicitly mark the transaction's boundaries. The code does not include statements that begin and end the transaction.

Typically, the container begins a transaction immediately before an enterprise bean method starts. It commits the transaction just before the method exits. Each method can be associated with a single transaction. Nested or multiple transactions are not allowed within a method.

Container-managed transactions do not require all methods to be associated with transactions. When deploying a bean, you specify which of the bean's methods are associated with transactions by setting the transaction attributes.

Transaction Attributes

A transaction attribute controls the scope of a transaction. Figure 30-1 illustrates why controlling the scope is important. In the diagram, method-A begins a transaction and then invokes method-B of Bean-2. When method-B executes, does it run within the scope of the transaction started by method-A, or does it execute with a new transaction? The answer depends on the transaction attribute of method-B.

[画像:Transaction Scope]

Figure 30-1 Transaction Scope

A transaction attribute can have one of the following values:

Required
RequiresNew
Mandatory
NotSupported
Supports
Never

Required

If the client is running within a transaction and invokes the enterprise bean's method, the method executes within the client's transaction. If the client is not associated with a transaction, the container starts a new transaction before running the method.

The Required attribute will work for most transactions. Therefore, you may want to use it as a default, at least in the early phases of development. Because transaction attributes are declarative, you can easily change them later.

RequiresNew

If the client is running within a transaction and invokes the enterprise bean's method, the container takes the following steps:

Suspends the client's transaction
Starts a new transaction
Delegates the call to the method
Resumes the client's transaction after the method completes

If the client is not associated with a transaction, the container starts a new transaction before running the method.

You should use the RequiresNew attribute when you want to ensure that the method always runs within a new transaction.

Mandatory

If the client is running within a transaction and invokes the enterprise bean's method, the method executes within the client's transaction. If the client is not associated with a transaction, the container throws the TransactionRequiredException.

Use the Mandatory attribute if the enterprise bean's method must use the transaction of the client.

NotSupported

If the client is running within a transaction and invokes the enterprise bean's method, the container suspends the client's transaction before invoking the method. After the method has completed, the container resumes the client's transaction.

If the client is not associated with a transaction, the container does not start a new transaction before running the method.

Use the NotSupported attribute for methods that don't need transactions. Because transactions involve overhead, this attribute may improve performance.

Supports

If the client is running within a transaction and invokes the enterprise bean's method, the method executes within the client's transaction. If the client is not associated with a transaction, the container does not start a new transaction before running the method.

Because the transactional behavior of the method may vary, you should use the Supports attribute with caution.

Never

If the client is running within a transaction and invokes the enterprise bean's method, the container throws a RemoteException. If the client is not associated with a transaction, the container does not start a new transaction before running the method.

Summary of Transaction Attributes

Table 30-1 summarizes the effects of the transaction attributes. Both the T1 and the T2 transactions are controlled by the container. A T1 transaction is associated with the client that calls a method in the enterprise bean. In most cases, the client is another enterprise bean. A T2 transaction is started by the container just before the method executes.

In the last column of Table 30-1, the word None means that the business method does not execute within a transaction controlled by the container. However, the database calls in such a business method might be controlled by the transaction manager of the DBMS.

Setting Transaction Attributes

Because transaction attributes are stored in the deployment descriptor, they can be changed during several phases of J2EE application development: enterprise bean creation, application assembly, and deployment. However, it is the responsibility of an enterprise bean developer to specify the attributes when creating the bean. The attributes should be modified only by an application developer who is assembling components into larger applications. Do not expect the person deploying the J2EE application to specify the transaction attributes.

Table 30-1 Transaction Attributes and Scope

Transaction Attribute	Client's Transaction	Business Method's Transaction
Required	None	T2
	T1	T1
RequiresNew	None	T2
	T1	T2
Mandatory	None	error
	T1	T1
NotSupported	None	None
	T1	None
Supports	None	None
	T1	T1
Never	None	None
	T1	Error

You can specify the transaction attributes for the entire enterprise bean or for individual methods. If you've specified one attribute for a method and another for the bean, the attribute for the method takes precedence. When you specify attributes for individual methods, the requirements differ with the type of bean. Session beans need the attributes defined for business methods but do not allow them for the create methods. Entity beans require transaction attributes for the business, create, remove, and finder methods. Message-driven beans require transaction attributes (either Required or NotSupported) for the onMessage method.

Rolling Back a Container-Managed Transaction

There are two ways to roll back a container-managed transaction. First, if a system exception is thrown, the container will automatically roll back the transaction. Second, by invoking the setRollbackOnly method of the EJBContext interface, the bean method instructs the container to roll back the transaction. If the bean throws an application exception, the rollback is not automatic but can be initiated by a call to setRollbackOnly. For a description of system and application exceptions, see deploytool Tips for Entity Beans with Bean-Managed Persistence.

The source code for the following example is in the <INSTALL>/j2eetutorial14/examples/ejb/bank directory.

The transferToSaving method of the BankBean example illustrates the setRollbackOnly method. If a negative checking balance occurs, transferToSaving invokes setRollBackOnly and throws an application exception (InsufficientBalanceException). The updateChecking and updateSaving methods update database tables. If the updates fail, these methods throw an SQLException and the transferToSaving method throws an EJBException. Because the EJBException is a system exception, it causes the container to automatically roll back the transaction. Here is the code for the transferToSaving method:

public void transferToSaving(double amount) throws
 InsufficientBalanceException {
 checkingBalance -= amount;
 savingBalance += amount;
 try {
 updateChecking(checkingBalance);
 if (checkingBalance < 0.00) {
 context.setRollbackOnly();
 throw new InsufficientBalanceException();
 }
 updateSaving(savingBalance);
 } catch (SQLException ex) {
 throw new EJBException 
 ("Transaction failed due to SQLException: " 
 + ex.getMessage());
 }
}

When the container rolls back a transaction, it always undoes the changes to data made by SQL calls within the transaction. However, only in entity beans will the container undo changes made to instance variables. (It does so by automatically invoking the entity bean's ejbLoad method, which loads the instance variables from the database.) When a rollback occurs, a session bean must explicitly reset any instance variables changed within the transaction. The easiest way to reset a session bean's instance variables is by implementing the SessionSynchronization interface.

Synchronizing a Session Bean's Instance Variables

The SessionSynchronization interface, which is optional, allows you to synchronize the instance variables with their corresponding values in the database. The container invokes the SessionSynchronization methods--afterBegin, beforeCompletion, and afterCompletion--at each of the main stages of a transaction.

The afterBegin method informs the instance that a new transaction has begun. The container invokes afterBegin immediately before it invokes the business method. The afterBegin method is a good place to load the instance variables from the database. The BankBean class, for example, loads the checkingBalance and savingBalance variables in the afterBegin method:

public void afterBegin() {
 System.out.println("afterBegin()");
 try {
 checkingBalance = selectChecking();
 savingBalance = selectSaving();
 } catch (SQLException ex) {
 throw new EJBException("afterBegin Exception: " +
 ex.getMessage());
 }
}

The container invokes the beforeCompletion method after the business method has finished, but just before the transaction commits. The beforeCompletion method is the last opportunity for the session bean to roll back the transaction (by calling setRollbackOnly). If it hasn't already updated the database with the values of the instance variables, the session bean can do so in the beforeCompletion method.

The afterCompletion method indicates that the transaction has completed. It has a single boolean parameter whose value is true if the transaction was committed and false if it was rolled back. If a rollback occurred, the session bean can refresh its instance variables from the database in the afterCompletion method:

public void afterCompletion(boolean committed) {
 System.out.println("afterCompletion: " + committed);
 if (committed == false) {
 try {
 checkingBalance = selectChecking();
 savingBalance = selectSaving();
 } catch (SQLException ex) {
 throw new EJBException("afterCompletion SQLException:
   " + ex.getMessage());
 }
 }
}

Compiling the BankBean Example

To compile the classes and interfaces in the BankBean example, follow these steps:

In a terminal window, go to this directory:

<INSTALL>/j2eetutorial14/examples/ejb/bank/

Create the database tables and data by typing

asant create-db_common

---

Note: Application Server 8.2 includes a copy of the open source Derby database server. Application Server 8.0/8.1 includes the PointBase database server. If you are using Application Server 8.0/8.1, either follow the instructions in the J2EE Tutorial at http://java.sun.com/j2ee/1.4/docs/tutorial-update6/doc/index.html that works with Application Server 8.0/8.1 or upgrade to Application Server 8.2 (see http://java.sun.com/j2ee/1.4/download.html#appserv to download).

---

Type the following command to build the enterprise bean's classes and interfaces:

asant build

Packaging the BankBean Example

The BankBean session bean uses container-managed transactions. These steps assume that you are familiar with the steps needed to create and deploy an enterprise application using deploytool, as described in Chapter 24.

Creating the J2EE Application

Create a new application named BankApp in:

<INSTALL>/j2eetutorial14/examples/ejb/bank/

Packaging the Enterprise Bean

Create a new enterprise bean in BankApp by selecting FileRight ArrowNewRight ArrowEnterprise Bean.
In the EJB JAR screen:
Select Create New JAR Module in Application.
Enter BankJAR under JAR Name.
Click Edit.
Navigate to <INSTALL>/j2eetutorial14/examples/ejb/bank/build/.
Select Bank.class, BankBean.class, BankHome.class, and InsufficientBalanceException.class.
Click Add.
Click OK.
Click Next.
In the General screen:
Select BankBean under Enterprise Bean Class.
Enter BankBean under Enterprise Bean Name.
Select Stateful Session under Enterprise Bean Type.
Select BankHome under Remote Home Interface.
Select Bank under Remote Interface.
Select Next.
Click Finish.
Select BankBean in deploytool's tree.
In the Resource Ref's tab:
Click Add.
Set the Coded Name to jdbc/BankDB.
Set the JNDI Name to jdbc/ejbTutorialDB.
In the Transactions tab:
Select Container-Managed under Transaction Management.
Verify that getCheckingBalance(), getSavingBalance(), and transferToSaving() have the Required transaction attribute.

Packaging the Application Client

Create a new application client in BankApp by selecting FileRight ArrowNewRight ArrowApplication Client.
In the JAR File Contents screen:
Select BankApp under Create New AppClient Module in Application.
Enter BankClient under AppClient Name.
Click Edit.
Navigate to <INSTALL>/j2eetutorial14/examples/ejb/bank/build/.
Select BankClient.class.
Click Add.
Click OK.
Click Next.
In the General screen:
Select BankClient under Main Class.
Select (Use container-managed authentication) under Callback Handler Class.
Click Next.
Click Finish.

Specifying the Application Client's Enterprise Bean Reference

When it invokes the lookup method, BankClient refers to the home of an enterprise bean:

Object objref = 
initial.lookup("java:comp/env/ejb/SimpleBank");

You specify this reference as follows:

In the tree, select BankClient.
Select the EJB Refs tab.
Click Add.
In the Coded Name field, enter ejb/SimpleBank.
In the EJB Type field, select Session.
In the Interfaces field, select Remote.
In the Home Interface field, enter BankHome.
In the Local/Remote Interface field, enter Bank.
Click OK.
Select the line you just added.
Under Sun-specific Settings for ejb/SimpleBank, select JNDI Name.
In the JNDI Name field select, BankBean.
Select FileRight ArrowSave.

Deploying the J2EE Application

Select BankApp in deploytool.
Select ToolsRight ArrowDeploy.
Under Connection Settings, enter the user name and password for the Application Server.
Tell deploytool to create a JAR file that contains the client stubs:
Check the Return Client JAR box.
In the field below the checkbox, enter <INSTALL>/j2eetutorial14/examples/ejb/bank/.
Click OK.
In the Distribute Module dialog box, click Close when the deployment completes successfully.

Running the Application Client

In a terminal window, go to the <INSTALL>/j2eetutorial14/
examples/ejb/bank/ directory.
Type the following command:

appclient -client BankAppClient.jar

In the terminal window, the client displays these lines:

checking: 60.0
saving: 540.0

Methods Not Allowed in Container-Managed Transactions

You should not invoke any method that might interfere with the transaction boundaries set by the container. The list of prohibited methods follows:

The commit, setAutoCommit, and rollback methods of java.sql.Connection
The getUserTransaction method of javax.ejb.EJBContext
Any method of javax.transaction.UserTransaction

You can, however, use these methods to set boundaries in bean-managed transactions.