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It is important to understand that the term transaction has many different yet related meanings in regards to persistence and Object/Relational Mapping. In most use-cases these definitions align, but that is not always the case.
Might refer to the physical transaction with the database.
Might refer to the logical notion of a transaction as related to a persistence context.
Might refer to the application notion of a Unit-of-Work, as defined by the archetypal pattern.
This documentation largely treats the physical and logic notions of transaction as one-in-the-same.
Hibernate uses the JDBC API for persistence. In the world of Java there are 2 well defined mechanism for dealing with transactions in JDBC: JDBC itself and JTA. Hibernate supports both mechanisms for integrating with transactions and allowing applications to manage physical transactions.
Transaction handling per Session is handled by the
org.hibernate.resource.transaction.TransactionCoordinator contract, which
are built by the org.hibernate.resource.transaction.TransactionCoordinatorBuilder
service. TransactionCoordinatorBuilder represents a strategy for dealing with transactions whereas
TransactionCoordinator represents one instance of that strategy related to a Session. Which
TransactionCoordinatorBuilder implementation to use is defined by the
hibernate.transaction.coordinator_class setting.
Hibernate-provided TransactionCoordinatorBuilder implementations
jdbc (the default) - Manages transactions via calls to java.sql.Connection
jta - Manages transactions via JTA. See ???
For details on implementing a custom TransactionCoordinatorBuilder, or simply better understanding how it works, see the Integrations Guide
Hibernate uses JDBC connections and JTA resources directly, without adding any additional locking behavior. Hibernate does not lock objects in memory. The behavior defined by the isolation level of your database transactions does not change when you use Hibernate. The Hibernate Session acts as a transaction-scoped cache providing repeatable reads for lookup by identifier and queries that result in loading entities.
To reduce lock contention in the database, the physical database transaction needs to be as short as
possible. Long database transactions prevent your application from scaling to a highly-concurrent load.
Do not hold a database transaction open during end-user-level work, but open it after the end-user-level
work is finished. This is concept is referred to as transactional write-behind.
Interaction with a JTA system is consolidated behind a single contract named
org.hibernate.engine.transaction.jta.platform.spi.JtaPlatform which
exposes access to the javax.transaction.TransactionManager and
javax.transaction.UserTransaction for that system as well as exposing
the ability to register javax.transaction.Synchronization instances,
check transaction status, etc.
Generally JtaPlatform will need access to JNDI to resolve the JTA TransactionManager, UserTransaction, etc. See Chapter 8, JNDI for details on configuring access to JNDI
Hibernate tries to discover the JtaPlatform it should use through the use of another service named
org.hibernate.engine.transaction.jta.platform.spi.JtaPlatformResolver.
If that resolution does not work, or if you wish to provide a custom implementation you will need to
specify the hibernate.transaction.jta.platform setting. Hibernate provides many
implementations of the JtaPlatform contract, all with short-names:
Built-in JtaPlatform implementations by short name
Borland - JtaPlatform for the Borland Enterprise Server.
Bitronix - JtaPlatform for Bitronix.
JBossAS - JtaPlatform for Arjuna/JBossTransactions/Narnya when used within the JBoss/WildFly Application Server.
JBossTS - JtaPlatform for Arjuna/JBossTransactions/Narnya when used standalone.
JOnAS - JtaPlatform for JOTM when used within JOnAS.
JOTM - JtaPlatform for JOTM when used standalone.
JRun4 - JtaPlatform for the JRun 4 Application Server.
OC4J - JtaPlatform for Oracle's OC4J container.
Orion - JtaPlatform for the Orion Application Server.
Resin - JtaPlatform for the Resin Application Server.
SunOne - JtaPlatform for the SunOne Application Server.
Weblogic - JtaPlatform for the Weblogic Application Server.
WebSphere - JtaPlatform for older versions of the WebSphere Application Server.
WebSphereExtended - JtaPlatform for newer versions of the WebSphere Application Server.
Hibernate provides an API for helping to isolate applications from the differences in the underlying physical transaction system in use. Based on the configured TransactionCoordinatorBuilder, Hibernate will simply do the right thing when this transaction API is used by the application. This allows your applications and components to be more portable move around into different environments.
To use this API, you would obtain the org.hibernate.Transaction
from the Session.
Transaction allows for all the normal operations you'd expect. begin,
commit and rollback. And these calls noop if they
should.
It even exposes some cool methods like:
markRollbackOnly that works in both JTA and JDBC!
getTimeout and setTimeout that again work
in both JTA and JDBC!
registerSynchronization that allows you to register JTA Synchronizations
even in non-JTA environments. In fact in both JTA and JDBC environments, these Synchronizations
are kept locally by Hibernate. In JTA environments Hibernate will only ever register one single
Synchronization with the TransactionManager to avoid ordering problems.
Additionally it exposes a getStatus method that returns an
org.hibernate.resource.transaction.spi.TransactionStatus enum. This method
checks with the underling transaction system if needed, so care should be taken to minimize its use;
it can have a big performance impact in certain JTA set ups.
Lets take a look at using the Transaction API in the various environments.
Example 7.1. Using Transaction API in JDBC
public void doSomeWork() {
StandardServiceRegistry ssr = new StandardServiceRegistryBuilder()
// "jdbc" is the default, but for explicitness
.applySetting( AvailableSettings.TRANSACTION_COORDINATOR_STRATEGY, "jdbc" )
...;
SessionFactory = ...;
Session session = sessionFactory.openSession();
try {
// calls Connection#setAutoCommit(false) to
// signal start of transaction
session.getTransaction().begin();
doTheWork();
// calls Connection#commit(), if an error
// happens we attempt a rollback
session.getTransaction().commit();
}
catch (Exception e) {
// we may need to rollback depending on
// where the exception happened
if ( session.getTransaction().getStatus() == ACTIVE
|| session.getTransaction().getStatus() == MARKED_ROLLBACK ) {
session.getTransaction().rollback();
}
// handle the underlying error
}
finally {
session.close();
}
}
Example 7.2. Using Transaction API in JTA (CMT)
public void doSomeWork() {
StandardServiceRegistry ssr = new StandardServiceRegistryBuilder()
.applySetting( AvailableSettings.TRANSACTION_COORDINATOR_STRATEGY, "jta" )
...;
// Note: depending on the JtaPlatform used and some optional settings,
// the underlying transactions here will be controlled through either
// the JTA TransactionManager or UserTransaction
SessionFactory = ...;
Session session = sessionFactory.openSession();
try {
// Since we are in CMT, a JTA transaction would
// already have been started. This call essentially
// no-ops
session.getTransaction().begin();
doTheWork();
// Since we did not start the transaction (CMT),
// we also will not end it. This call essentially
// no-ops in terms of transaction handling.
session.getTransaction().commit();
}
catch (Exception e) {
// again, the rollback call here would no-op (aside from
// marking the underlying CMT transaction for rollback only).
if ( session.getTransaction().getStatus() == ACTIVE
|| session.getTransaction().getStatus() == MARKED_ROLLBACK ) {
session.getTransaction().rollback();
}
// handle the underlying error
}
finally {
session.close();
}
}
Example 7.3. Using Transaction API in JTA (BMT)
public void doSomeWork() {
StandardServiceRegistry ssr = new StandardServiceRegistryBuilder()
.applySetting( AvailableSettings.TRANSACTION_COORDINATOR_STRATEGY, "jta" )
...;
// Note: depending on the JtaPlatform used and some optional settings,
// the underlying transactions here will be controlled through either
// the JTA TransactionManager or UserTransaction
SessionFactory = ...;
Session session = sessionFactory.openSession();
try {
// Assuming a JTA transaction is not already active,
// this call the TM/UT begin method. If a JTA
// transaction is already active, we remember that
// the Transaction associated with the Session did
// not "initiate" the JTA transaction and will later
// nop-op the commit and rollback calls...
session.getTransaction().begin();
doTheWork();
// calls TM/UT commit method, assuming we are initiator.
session.getTransaction().commit();
}
catch (Exception e) {
// we may need to rollback depending on
// where the exception happened
if ( session.getTransaction().getStatus() == ACTIVE
|| session.getTransaction().getStatus() == MARKED_ROLLBACK ) {
// calls TM/UT commit method, assuming we are initiator;
// otherwise marks the JTA trsnaction for rollback only
session.getTransaction().rollback();
}
// handle the underlying error
}
finally {
session.close();
}
}
In the CMT case we really could have omitted all of the Transaction calls. But the point of the examples was to show that the Transaction API really does insulate your code from the underlying transaction mechanism. In fact if you strip away the comments and the single configruation setting supplied at bootstrap, the code is exactly the same in all 3 examples. In other words, we could develop that code and drop it, as-is, in any of the 3 transaction environments.
The Transaction API tries hard to make the experience consistent across all environments. To that end, it generally defers to the JTA specification when there are differences (for example automatically trying rollback on a failed commit).
This is an anti-pattern of opening and closing a Session for each database call
in a single thread. It is also an anti-pattern in terms of database transactions. Group your database
calls into a planned sequence. In the same way, do not auto-commit after every SQL statement in your
application. Hibernate disables, or expects the application server to disable, auto-commit mode
immediately. Database transactions are never optional. All communication with a database must
be encapsulated by a transaction. Avoid auto-commit behavior for reading data, because many small
transactions are unlikely to perform better than one clearly-defined unit of work, and are more
difficult to maintain and extend.
Using auto-commit does not circumvent database transactions. Instead, when in auto-commit mode, JDBC drivers simply perform each call in an implicit transaction call. It is as if your application called commit after each and every JDBC call.
This is the most common transaction pattern. The term request here relates to the concept of a system
that reacts to a series of requests from a client/user. Web applications are a prime example of this
type of system, though certainly not the only one. At the beginning of handling such a request, the
application opens a Hibernate Session, starts a transaction, performs
all data related work, ends the transaction and closes the Session.
The crux of the pattern is the one-to-one relationship between the transaction and the
Session.
Within this pattern there is a common technique of defining a current session to
simplify the need of passing this Session around to all the application
components that may need access to it. Hibernate provides support for this technique through the
getCurrentSession method of the SessionFactory.
The concept of a "current" session has to have a scope that defines the bounds in which the notion
of "current" is valid. This is purpose of the
org.hibernate.context.spi.CurrentSessionContext contract. There are 2
reliable defining scopes:
First is a JTA transaction because it allows a callback hook to know when it is ending which
gives Hibernate a chance to close the Session and clean up.
This is represented by the
org.hibernate.context.internal.JTASessionContext implementation of
the org.hibernate.context.spi.CurrentSessionContext contract.
Using this implementation, a Session will be opened the first
time getCurrentSession is called within that transaction.
Secondly is this application request cycle itself. This is best represented with the
org.hibernate.context.internal.ManagedSessionContext implementation of
the org.hibernate.context.spi.CurrentSessionContext contract.
Here an external component is responsible for managing the lifecycle and scoping of a "current"
session. At the start of such a scope, ManagedSessionContext's
bind method is called passing in the
Session. At the end, its unbind
method is called.
Some common examples of such "external components" include:
javax.servlet.Filter implementation
AOP interceptor with a pointcut on the service methods
A proxy/interception container
The getCurrentSession() method has one downside in a JTA environment. If
you use it, after_statement connection release mode is also used by default. Due to a limitation of
the JTA specification, Hibernate cannot automatically clean up any unclosed
ScrollableResults or Iterator
instances returned by scroll() or iterate().
Release the underlying database cursor by calling ScrollableResults.close()
or Hibernate.close(Iterator) explicitly from a
finally block.
The session-per-request pattern is not the only valid way of designing units of work. Many business processes require a whole series of interactions with the user that are interleaved with database accesses. In web and enterprise applications, it is not acceptable for a database transaction to span a user interaction. Consider the following example:
Procedure 7.1. An example of a long-running conversation
The first screen of a dialog opens. The data seen by the user is loaded in a particular
Session and database transaction. The user is free to modify the objects.
The user uses a UI element to save their work after five minutes of editing. The modifications are made persistent. The user also expects to have exclusive access to the data during the edit session.
Even though we have multiple databases access here, from the point of view of the user, this series of steps represents a single unit of work. There are many ways to implement this in your application.
A first naive implementation might keep the Session and database transaction open
while the user is editing, using database-level locks to prevent other users from modifying the same
data and to guarantee isolation and atomicity. This is an anti-pattern, because lock contention is a
bottleneck which will prevent scalability in the future.
Several database transactions are used to implement the conversation. In this case, maintaining isolation of business processes becomes the partial responsibility of the application tier. A single conversation usually spans several database transactions. These multiple database accesses can only be atomic as a whole if only one of these database transactions (typically the last one) stores the updated data. All others only read data. A common way to receive this data is through a wizard-style dialog spanning several request/response cycles. Hibernate includes some features which make this easy to implement.
|
Automatic Versioning |
Hibernate can perform automatic optimistic concurrency control for you. It can automatically detect if a concurrent modification occurred during user think time. Check for this at the end of the conversation. |
|
Detached Objects |
If you decide to use the session-per-request pattern, all loaded instances will be in the detached state during user think time. Hibernate allows you to reattach the objects and persist the modifications. The pattern is called session-per-request-with-detached-objects. Automatic versioning is used to isolate concurrent modifications. |
|
Extended Session |
The Hibernate |
Session-per-request-with-detached-objects and session-per-conversation each have advantages and disadvantages.
A Session is not thread-safe. Things that work concurrently, like HTTP requests, session beans,
or Swing workers, will cause race conditions if a Session instance is shared. If you keep your
Hibernate Session in your javax.servlet.http.HttpSession you should
consider synchronizing access to your HttpSession; otherwise, a user that clicks reload fast enough
can use the same Session in two concurrently running threads.
An exception thrown by Hibernate means you have to rollback your database transaction and close the Session immediately. If your Session is bound to the application, you have to stop the application. Rolling back the database transaction does not put your business objects back into the state they were at the start of the transaction. This means that the database state and the business objects will be out of sync. Usually this is not a problem, because exceptions are not recoverable and you will have to start over after rollback anyway.
The Session caches every object that is in a persistent state (watched and checked for changes
by Hibernate). If you keep it open for a long time or simply load too much data, it will grow
endlessly until you get an OutOfMemoryException. One solution is to call
clear() and evict() to manage the
Session cache, but you should consider an alternate means of dealing with large amounts of data
such as a Stored Procedure. Java is simply not the right tool for these kind of operations.
Some solutions are shown in Chapter 11, Batching. Keeping a Session open for the duration of
a user session also means a higher probability of stale data.