Locking and Concurrency in Java Persistence 2.0
By edort on Sep 11, 2009
The Java Persistence API (informally referred to as JPA) provides a plain old Java object (POJO)-based persistence model for Java EE and Java SE applications. It handles the details of how relational data is mapped to Java objects, and it standardizes Object/Relational (O/R) mapping. The latest update to JPA, Java Persistence 2.0, adds a number of new features such as additional O/R mapping functionality and new query language capabilities. Another area that has been enhanced in JPA 2.0 is locking and concurrency.
This Tech Tip highlights the new locking and concurrency features in JPA 2.0 and provides an application that demonstrates these capabilities.
Locking and Concurrency
Locking is a technique for handling database transaction concurrency. When two or more database transactions concurrently access the same data, locking is used to ensure that only one transaction at a time can change the data.
There are generally two locking approaches: optimistic and pessimistic. Optimistic locking assumes that there will be infrequent conflicts between concurrent transactions, that is, they won't often try to read and change the same data at the same time. In optimistic locking, the objective is to give concurrent transactions a lot of freedom to process simultaneously, but to detect and prevent collisions. Two transactions can access the same data simultaneously. However, to prevent collisions, a check is made to detect any changes made to the data since the data was last read.
Pessimistic locking assumes that transactions will frequently collide. In pessimistic locking, a transaction that reads the data locks it. Another transaction cannot change the data until the first transaction commits the read.
Optimistic locking works best for applications where concurrent transactions do not conflict. Pessimistic locking works best where concurrent transactions do conflict.
With JPA it is possible to lock an entity. This allows you to control when, where, and which kind of locking to use for an entity. Recall that in JPA, an entity is a lightweight persistence domain object. Typically, an entity represents a table in a relational database, with each entity instance corresponding to a row in that table.
Locking Support in JPA 1.0
JPA 1.0 only supports optimistic read or optimistic write locking. In this support, any transaction can read
and update an entity. However, when a transaction commits, JPA checks the
version attribute of the entity
to determine if it was updated since the entity was last read. If the
version attribute was updated since
the entity was last read, JPA throws an exception. The advantage of this approach is that no database locks are held.
This can result in better scalability than for pessimistic locking. The disadvantage of this approach is that the user
or application must refresh and retry failed updates.
A versioned entity is marked with the
@Version annotation, as illustrated in the following code
and its corresponding database schema has a version column, such as that created by the following SQL statement:
version attribute can be an int, short, long, or timestamp. It is incremented when a transaction
successfully commits. This results in an SQL operation such as the following:
Figure 1 illustrates optimistic locking.
|Figure 1. Optimistic Locking|
Here, two concurrent transactions attempt to update Part
p. Transaction 1 commits first. In response,
JPA increments the
version attribute for the
p entity. When Transaction 2 commits, JPA
OptimisticLockException because the
version attribute for the
is higher than it was when Transaction 2 last read the
p entity. As a result, Transaction 2 is rolled back.
You can further control the way JPA manages locking on a versioned entity by specifying a lock mode. You do this through
lock() method of the
EntityManager class. Here is the method signature:
The first method parameter is the entity instance that needs to be locked in the transaction. The second method parameter is the lock mode.
In JPA 1.0, the lock mode value could only be one of the following:
READ. In this case, the JPA entity manager performs the optimistic locking operations as previously described. It locks the entity and before a transaction commits, checks the entity's version attribute to determine if it has been updated since the entity was last read. If the version attribute has been updated, the entity manager throws an
OptimisticLockExceptionand rolls back the transaction.
WRITE. In this case, the entity manager performs the same optimistic locking operations as for the READ lock mode. However, it also updates the entity's version column.
Additional Locking Support in JPA 2.0
JPA 2.0 adds five new lock modes. Two of these are used for optimistic locking. JPA 2.0 also adds support for pessimistic locking and provides three lock modes for pessimistic locking. The two new optimistic lock modes are:
OPTIMISTIC. This is the same as the
READlock mode. The
READlock mode is still supported in JPA 2.0, but specifying
OPTIMISTICis recommended for new applications.
OPTIMISTIC_FORCE_INCREMENT. This is the same as the
WRITElock mode. The
WRITElock mode is still supported in JPA 2.0, but specifying
OPTIMISTIC_FORCE_INCREMENTis recommended for new applications.
The three new pessimistic lock modes are:
PESSIMISTIC_READ. The entity manager locks the entity as soon as a transaction reads it. The lock is held until the transaction completes. This lock mode is used when you want to query data using repeatable-read semantics. In other words, you want to ensure that the data is not updated between successive reads. This lock mode does not block other transactions from reading the data.
PESSIMISTIC_WRITE. The entity manager locks the entity as soon as a transaction updates it. This lock mode forces serialization among transactions attempting to update the entity data. This lock mode is often used when there is a high likelihood of update failure among concurrent updating transactions.
PESSIMISTIC_FORCE_INCREMENT. The entity manager locks the entity when a transaction reads it. It also increments the entity's
versionattribute when the transaction ends, even if the entity is not modified.
JPA 2.0 also provides multiple ways to specify the lock mode for an entity. You can specify the lock mode
find() methods of the
EntityManager. In addition,
if you call the
EntityManager.refresh() method, it refreshes the state of the entity instance
from the database and locks it based on the entity's lock mode.
You can also set the lock mode for a query through the
setLockMode() method of the
interface. And you can specify a lock mode for the results returned by a named query through the
setLockMode element of the
Let's look at some examples of the new locking support in JPA 2.0.
OPTIMISTIC Lock Mode
The typical use case for
OPTIMISTIC lock mode is where an entity has an intrinsic dependency on one or
more entities to ensure consistency, for example, when there is a relationship between two entities. In the example
shown in Figure 2, Transaction 1 on the left updates the price for part
version attribute. Transaction 2 on the right submits a bid for a user,
u1. If the part price is lower than the user's current bid, Transaction 2 increases the bid.
Figure 2. Using
In this scenario, you don't want Transaction 2 to commit if Transaction T1 changes the price for the part
after Transaction T2 reads the price. So
OPTIMISTIC lock mode is a good choice:
Before committing Transaction 2, the entity manager checks the
version attribute for the
version attribute is higher than when p1 was last read, so the entity manager throws an
OptimisticLockException and rolls back Transaction2. Note that checking
attribute for an update would not throw an exception. That's because Transaction 1 updates
version attribute — it does not increment
OPTIMISTIC_FORCE_INCREMENT Lock Mode
OPTIMISTIC_FORCE_INCREMENT lock mode causes an optimistic lock failure if another transaction tries to
modify the locked entity. The common use for this lock is to guarantee consistency among entities in a relationship.
Figure 3 shows an example of
OPTIMISTIC_FORCE_INCREMENT lock mode.
Figure 3. Using
Transaction 2 on the right wants to ensure that the price for a part
p1 does not change during the transaction,
so it locks the
p1 entity as follows:
Transaction 2 then calls
em.flush()— this increments
in the database. Any parallel attempt to update
p1 will throw an
roll back. As you can see, Transaction 1 attempts to update
p1's price after Transaction 2 calls
em.flush(). When Transaction T1 attempts to commit, the entity manager checks the
version attribute. Because the attribute has been updated since the last read, the entity manager
OptimisticLockException and rolls back Transaction T1.
PESSIMISTIC Lock Modes
The pessimistic lock modes lock a database row when data is read. This is the equivalent to the action
taken in response to the SQL statement
SELECT . . . FOR UPDATE [NOWAIT]. Pessimistic locking ensures
that transactions do not update the same entity at the same time. This can simplify application code, but it limits
concurrent access to the data, something that can cause poor scalability and may cause deadlocks. Pessimistic locking
is better for applications with a higher risk of contention among concurrent transactions.
The following figures show various examples of
PESSIMISTIC lock modes:
- Figure 4 shows an example of reading an entity and in a later step
setting it in
- Figure 5 shows an example of reading an entity and at the same time
setting it in
- Figure 6 shows an example of reading an entity and in a later step
setting it in
Figure 4. Setting
Figure 5. Setting
Figure 6. Setting
The right locking approach to use depends on your application. Some questions you might want to ask to help make the decision are:
- What is the risk of contention among concurrent transactions?
- What are the requirements for scalability?
- What are the requirements for user retrying after a failure?
Accompanying this tip is a sample application that demonstrates some of the locking support
in JPA 2.0. The application is also available in the Java EE 6
SDK Preview release — look for "The Java Persistence API Locking Sample Application" in the
samples directory of the Java EE 6 SDK Preview release download package.
The application consists of a client, a servlet, entity classes for part and user data, and stateless session beans that
provide the logic for accessing and updating the data. The client calls the servlet to initialize the data. The client then
makes multiple requests to the servlet that simulate parallel read and update operations. These operations are performed
by the beans. Some of the operations are performed using optimistic locking, some using pessimistic locking. For example,
the following method,
updateWithOptimisticReadLock() demonstrates parallel operations performed using
updateWithOptimisticReadLock() method calls the
updatePrice() method in the
bean to find a user and then update the price of a part. The
updateWithOptimisticReadLock() method then waits to
allow parallel method calls to find other users before calling the
updateBid() method in the
updateBid() method sets an optimistic lock for the part and then submits a user bid that is based on the
part price, as shown below:
updateWithOptimisticReadLock() method then calls
em.flush(). At that point, the entity
manager performs a version check on the part entity. If any transaction submitted by any of the other users updates the part
while it is locked, the entity manager increments the part's version attribute. If the version attribute for the part
is higher than it was when the part was last read by the set bid transaction, the
method throws an
OptimisticLockException and rolls back that transaction.
You can find the source code for the application in the
To run the sample application, do the following:
- If you haven't already done so, download the Java EE 6 SDK Preview release. Also be sure to have an installed version of the Java Platform Standard Edition (Java SE) 6 SDK.
- Download the sample application.
- Set up your build environment and configure the application server by following the common build instructions.
- Change to the samples_install_dir
/javaee6/jpa/lockingdirectory, where samples_install_dir is where you installed the sample application.
- Build, deploy, and run the sample application by entering the following command on the command line:
You can replace the
ant allcommand with the following set of commands:
ant default— compiles and packages the application.
ant dir— deploys the application to the application server.
ant run— runs the test Java client.
In response, you should see output similar to the following:
The operations, which are identified in the
tc parameter values in the URL calls, are as follows:
updateWOL. Finds a part. Simulates think time to allow parallel threads to find users in parallel. Updates the part using optimistic locking.
updateWOR. Finds a part and a user. Simulates think time to allow parallel threads to find users in parallel. Locks the part using optimistic locking. Updates the user.
updateWOW. Finds a part and a user. Simulates think time to allow parallel threads to find users in parallel. Locks the part using an OPTIMISTIC_FORCE_INCREMENT lock. Updates the user.
updateWRP. Finds a part. Simulates think time to allow parallel threads to find users in parallel. Locks the part using a PESSIMISTIC_READ lock. Updates the part.
updateWRR. Finds a part. Simulates think time to allow parallel threads to find users in parallel. Refreshes using a PESSIMISTIC_WRITE lock. Updates the part.
updateWPL. Finds a part using a PESSIMISTIC_WRITE lock. Simulates think time to allow parallel threads to find users in parallel. Updates the part.
Notice that some update operations that use optimistic locking, such as
updateWOL, fail, while all update
operations that use pessimistic locking, such as
updateWPL, are successful. However, the time it takes to update
using pessimistic locking is much higher than that taken using optimistic locking.
Use the command
ant clean to undeploy the sample application and to remove temporary directories.
For more information, see the following resources:
- Preventing Non-Repeatable Reads in JPA Using EclipseLink
- Java Persistence API 2.0: What's New ?
- What's New and Exciting in JPA 2.0
- Beginning Java EE 6 Platform with GlassFish 3: From Novice to Professional
- Java Persistence API: Best Practices and Tips
About the Author
Carol McDonald is a Java Technology Evangelist at Sun Microsystems. As a software developer since 1986, Carol's experience has been in the technology areas of distributed network applications and protocols, including Java EE technology, XML, Internet/Intranet applications, LDAP, Distributed Network Management (CMIP,SNMP) and Email (X.400,X.500). Besides Java, Carol is also fluent in French and German. Read Carol McDonald's blog.