Click here to close now.

Welcome!

Java IoT Authors: Liz McMillan, Elizabeth White, Roger Strukhoff, Pat Romanski, Plutora Blog

Related Topics: Java IoT

Java IoT: Article

EJB 3 Transactions

Understanding and using transactions with EJB 3

Much of the work surrounding the design and development of enterprise applications involves decisions about how to coordinate the flow of persistent data. This includes when and where to cache data, when to apply it to a persistent store (typically the database), how to resolve simultaneous attempts to access the same data and how to resolve errors that might occur while data in the database is in an inconsistent state. A reliable database is capable of handling these issues at a low level in the database tier, but these same issues can exist in the middle (application server) and client tiers as well, and typically require special application logic. One of the principal benefits of using EJB 3 is its support for enterprise-wide services like transaction management and security control. In this article, we will explore how EJB 3 offers transaction services and how you can leverage them to meet your specific requirements.

Understanding Transactions
A transaction is a group of operations that must be performed as a unit. These operations can be synchronous or asynchronous, and can involve persisting data objects, sending mail, validating credit cards, etc. A classic example is a banking transfer, in which one operation debits funds from one account (i.e., updates a record in a database table) and another operation credits those same funds to another account (updates another row in that same, or a different database table). From the perspective of an external application querying both accounts, there must never be a time when these funds can be seen in both accounts. Nor can a moment exist when the funds can be seen in neither account. Only when both operations in this transaction have been successfully performed can the changes be visible from another application context. A group of operations that must be performed together in this way as a unit is known as a transaction.

When the operations in a transaction are performed across databases or other resources that reside on separate computers or processes, this is known as a distributed transaction. Such enterprise-wide transactions require special coordination between the resources involved and can be extremely difficult to program reliably. This is where Java Transaction API (JTA) comes in, providing the interface that resources can implement and to which they can bind, in order to participate in a distributed transaction. The EJB container is a transaction manager that supports JTA and so can participate in distributed transactions involving other EJB containers, as well as third-party JTA resources like many database management systems (DBMS).

The ACID Properties of a Transaction
Transactions come in all shapes and sizes and can involve synchronous and asynchronous operations, but they all have some core features in common, known as their ACID components. ACID refers to the four characteristics that define a robust and reliable transaction: atomicity, consistency, isolation, and durability. Table 1 describes these four components.

EJB 3 addresses these requirements by providing a robust JTA transaction manager and a declarative metadata API that can be specified on interoperable, portable business components. Virtually all Java EE applications require transaction services and EJB brings them to the application developer in a very slick package. From its inception, the EJB framework has provided a convenient way to manage transactions and access control by letting the developer define the behavior declaratively on a method-by-method basis. Beyond these container-provided services, EJB 3 allows developers to turn control over to the application to define transaction event boundaries and other custom behavior.

EJB 3 Transaction Services
The EJB 3 transaction model is built on this JTA model, in which session beans or other application clients provide the transactional context in which enterprise services are performed as a logical unit of work. Enterprise services in the Java EE environment include the creation, retrieval, updating and deletion of entities; the sending of JMS messages to the queue; the execution of MDBs; the firing of mail requests; the invocation of web services; and JDBC operations.

EJB 3 provides a built-in JTA transaction manager, but the real power lies in the declarative services EJB offers to bean providers. Using metadata tags instead of programmatic logic, bean providers can seamlessly participate in JTA transactions and declaratively control the transactional behavior of each business method on an enterprise bean. EJB 3 extends this programming model by providing explicit support for both JTA transactions and non-JTA (resource-local) transactions. Resource-local transactions are restricted to a single resource manager, such as a database connection, but may result in a performance optimization by avoiding the overhead of a distributed transaction monitor. In addition, application builders may leverage the container-provided (JTA-based) services for automatically managing transactions, or they may choose to take control of the transaction boundaries and handle the transaction begin, commit and rollback events explicitly. Within a single application, both approaches may be used alone or in combination if desired. Whereas the choice of whether to have the container or the application itself demarcate transactions is defined on the enterprise bean, the decision of which type of transaction model to use - JTA or resource-local - is determined when a given EntityManager is obtained from within an application. The persistent objects in the game - the entities - are entirely, and happily, unaware of their governing transaction framework. The transactional context in which an entity operates is not part of its definition, so the same entity class may be used in whatever transactional context the application chooses, provided an appropriate EntityManager is created to service the entity's life cycle events.

The EJB 3 container offers declarative demarcation of transaction events, along with the option to demarcate transaction events explicitly in the bean or in the application client code. Let's consider these two approaches separately, beginning with the default option: leveraging container-managed transaction (CMT) demarcation using declarative markup.

Container-Managed Transaction (CMT) Demarcation
EJB 3 provides built-in transaction management services that are available by default to session beans and MDBs. The container demarcates transaction boundaries and automatically begins and commits transactions based on declarative metadata provided by the bean developer.

When an EJB declares its transactional behavior in metadata, the container interposes on calls to the enterprise bean's methods and applies transactional behavior at the session bean's method boundaries. One of a fixed set of options may be specified for each method. The default behavior provided by the container is to check, immediately before invoking the method, whether a transaction context is associated with the current thread. If no transaction context is available, the container begins a new transaction before calling the method. If a transaction is available, the container allows that transaction to be propagated to the method call and made available to the method code. Then, upon returning from the method invocation, the container checks again. If the container was responsible for creating a new transaction context, it automatically commits that transaction after the method is exited (or, if an exception is thrown by that method, it rolls back the transaction it began). If it did not create the transaction, then it allows the transaction to continue unaffected. By interposing on the bean's method calls, the EJB container is able to apply transactional behavior at run time that was specified declaratively at development time.

The default behavior described above is specified by the REQUIRED transaction attribute. You can attribute any one of the six demarcation options shown in Table 2 to any method on a session bean.

All six attributes are typically available for session bean methods, though certain attributes are not available on a session timeout callback method, or when the session bean implements javax.ejb.SessionSynchronization. MDBs support only the REQUIRED and NOT_SUPPORTED attributes. Here is an example of how you would specify the transaction behavior on a session bean method to override the transaction behavior specified (or defaulted) at the bean level:

@TransactionAttribute(TransactionAttributeType.SUPPORTS)
public CustomerOrder createCustomerOrderUsingSupports(Customer customer)
throws Exception { ... }

Table 3 illustrates an EJB's transactional behavior, dependent on its transaction attribute and the presence or absence of a transactional context at the time the session method is called.

Bean-Managed Transaction (BMT) Demarcation
For some enterprise beans, the declarative CMT services may not provide the demarcation granularity they require. For instance, a client may wish to call multiple methods on a session bean without having each method commit its work upon completion. In this case, the client has two options: it can either instantiate its own JTA (or resource-local) transaction, or it can ask the session bean to expose transaction demarcation methods that the client can call to control the transaction boundaries itself.

To address this latter requirement, EJB offers enterprise beans a convenient way to handle their demarcation of transaction events. To turn off the automatic CMT demarcation services, enterprise beans simply specify the @TransactionManagement(TransactionManagementType.BEAN) annotation or assign the equivalent metadata to the session bean in the ejb-jar.xml file. With BMT demarcation, the EJB container still provides the transaction support to the bean. The primary difference is that the bean makes explicit calls to begin, commit and roll back transactions instead of using CMT attributes to declaratively assign transactional behavior to its methods. Also, the container does not propagate transactions begun by a client to beans that elect to demarcate their own transactions. While any given enterprise bean must choose one plan or the other (CMT vs. BMT demarcation) for its methods, both types of beans may interact with each other within a single transaction context.

In the last part of this article, we discuss JPA entity transaction behavior.

How Entities Become Associated with a Transaction Context
From the preceding discussion about how the EJB server acts as a transaction coordinator in associating resources with a transaction context, you may have realized that a JPA entity's persistence context is the resource that gets associated with a transaction. In this way, a persistence context is propagated through method calls so entities in a persistence unit can see each other's intermediate state, through their common persistence context, if they are associated with the same transaction context. Also, the restriction that only one persistence context for any given persistence unit must be associated with a given transaction context ensures that for any entity of type T with identity I, its state will be represented by only one persistence context within any transaction context. Within an application thread, only one transaction context is available at any moment, but the EJB server is free to dissociate one persistence context from that thread and associate a new persistence context for the same persistence unit to satisfy transaction isolation boundaries. When the EJB server does this, the newly instantiated persistence context is not able to see the intermediate changes made to any entities associated with the suspended persistence context.

Container-Managed vs. Application-Managed Persistence Context
The persistence services in EJB 3 let you opt out of container-managed entity persistence altogether and manage the transaction life cycles of your entities explicitly within your application code. When an EntityManager is injected (or looked up through JNDI), it comes in as a container-managed persistence context. The container automatically associates container-managed persistence contexts with any transaction that happens to be in context at the time that the EntityManager is injected. Should an application wish to control how or whether its persistence contexts are associated with transactions, it may obtain an EntityManagerFactory (again, through container injection or JNDI lookup) and explicitly create the EntityManager instances that represent their persistence contexts. An application-managed persistence context is used when the EntityManager is obtained through an EntityManagerFactory-a requirement when running outside the Java EE container.

Transaction-Scoped Persistence Context vs. Extended Persistence Context
When an EntityManager is created, you may specify whether the persistence context that it manages should be bound to the life of a transaction, or whether it should span the life of the EntityManager itself. A persistence context that is created when a transaction is created, and destroyed when the transaction ends, is known as a transaction-scoped persistence context. A persistence context that is created at the time it is injected into the bean (or bound through a JNDI lookup), and is not destroyed until the EntityManager instance is itself destroyed, is called an extended persistence context. Only stateful session beans may use extended persistence contexts. At the time an EntityManager instance is created, its persistence context type is defined, and it may not be changed during the EntityManager's lifetime. The default type is transaction-scoped; to inject an EntityManager by specifying an extended persistence context, you may specify the injection directive with the following:

@PersistenceContext(type = PersistenceContextType.EXTENDED)
private EnterpriseManager em;

or you may define a persistence-context-ref element in the XML descriptor.

Summary
In this article, we began with a discussion of the concepts essential to all transaction behavior, and we then explored both the built-in, declarative features offered by the EJB container, as well as options to bypass this support and coordinate transactions in application code. We concluded by describing the ways that JPA entities can interact with EJBs in a transactional environment.

Now that you are familiar with how to set up and use EJB 3 transactions, you may wish to explore the many related areas also introduced in the EJB 3 and JPA. For an examination of these features, with code samples, check out Beginning EJB 3 Application Development: From Novice to Professional (Apress, 2006).

More Stories By Raghu R. Kodali

Raghu R. Kodali is consulting product manager and SOA evangelist for Oracle Application Server. He leads next-generation SOA initiatives and J2EE feature sets for Oracle Application Server, with particular expertise in EJB, J2EE deployment, Web services, and BPEL. He holds a Masters degree in Computer Science and is a frequent speaker at technology conferences. Raghu is also a technical committee member for the OASIS SOA Blueprints specification, and a board member of Web Services SIG in OAUG. He maintains an active blog at Loosely Coupled Corner (www.jroller.com/page/raghukodali).

More Stories By Jonathan Wetherbee

Jon Wetherbee is a consulting engineer and tech lead for EJB development tools on Oracle's JDeveloper IDE. He has over 12 years of experience in development at Oracle, having built a variety of O/R mapping tools and holding responsibility for Oracle's core EJB toolset since EJB 1.1. In 1999, he received a patent for his work on integrating relational databases in an object-oriented environment.

Jon is co-author of 'Beginning EJB 3 Application Development: From Novice to Professional' (Apress, 2006), and enjoys speaking at user groups on EJB and related topics. Jon holds a BS in cognitive science from Brown University.

Comments (1) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
ejm 10/15/08 10:29:07 AM EDT

Well versed article and nice explanation. Easy to understand especially for us who gives merchant services.

@ThingsExpo Stories
"BSQUARE is in the business of selling software solutions for smart connected devices. It's obvious that IoT has moved from being a technology to being a fundamental part of business, and in the last 18 months people have said let's figure out how to do it and let's put some focus on it, " explained Dave Wagstaff, VP & Chief Architect, at BSQUARE Corporation, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
The WebRTC Meetup, where WebRTC enthusiasts exchange ideas, is being held on Wednesday, June 10, from 7 pm – 9 pm at the 4th WebRTC Summit, June 9-11, 2015, at the Javits Center in New York City, NY. The WebRTC Meetup is being hosted by the New York WebRTC Developer Group. WebRTC is the future of browser-to-browser communications, and continues to make inroads into the traditional, difficult, plug-in web communications world. The 4th WebRTC Summit continues our tradition of delivering the latest and greatest presentations within the world of WebRTC. Topics include voice calling, video chat, ...
Explosive growth in connected devices. Enormous amounts of data for collection and analysis. Critical use of data for split-second decision making and actionable information. All three are factors in making the Internet of Things a reality. Yet, any one factor would have an IT organization pondering its infrastructure strategy. How should your organization enhance its IT framework to enable an Internet of Things implementation? In this session, James Kirkland, Red Hat's Chief Architect for the Internet of Things and Intelligent Systems, will describe how to revolutionize your architecture and...
Aria Systems, which helps enterprises grow recurring revenue, today announced that its co-founder and inventor of cloud billing Brendan O'Brien, will be a featured speaker at the Cloud Expo, June 9-11 at The Javits Center in New York. Aria also will be demonstrating its Active Monetization platform in Booth #424 on the Expo Floor. O'Brien will lead the following sessions: June 9 - 11:00 am - 11:35 am, Room 1A16 with participants from Constant Contact, MGI Research and ATG Enabling Complex Recurring Revenue Strategies – With IoT exploding, massive data will transform businesses with oppor...
Growth hacking is common for startups to make unheard-of progress in building their business. Career Hacks can help Geek Girls and those who support them (yes, that's you too, Dad!) to excel in this typically male-dominated world. Get ready to learn the facts: Is there a bias against women in the tech / developer communities? Why are women 50% of the workforce, but hold only 24% of the STEM or IT positions? Some beginnings of what to do about it!
The enterprise market will drive IoT device adoption over the next five years. In his session at @ThingsExpo, John Greenough, an analyst at BI Intelligence, division of Business Insider, will analyze how companies will adopt IoT products and the associated cost of adopting those products. John Greenough is the lead analyst covering the Internet of Things for BI Intelligence- Business Insider’s paid research service. Numerous IoT companies have cited his analysis of the IoT. Prior to joining BI Intelligence, he worked analyzing bank technology for Corporate Insight and The Clearing House Pay...
The basic integration architecture, as defined by ESBs, hasn’t changed for more than a decade. Most cloud integration providers still rely on an ESB architecture and their proprietary connectors. As a result, enterprise integration projects suffer from constraints of availability and reliability of these connectors that are not re-usable across other integration vendors. However, the rapid adoption of APIs and almost ubiquitous availability of APIs amongst most SaaS and Cloud applications are rapidly redefining traditional integration approaches and their reliance on proprietary connectors. ...
SYS-CON Events announced today that O'Reilly Media has been named “Media Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9–11, 2015, at the Javits Center in New York City, NY. O'Reilly Media spreads the knowledge of innovators through its books, online services, magazines, and conferences. Since 1978, O'Reilly Media has been a chronicler and catalyst of cutting-edge development, homing in on the technology trends that really matter and spurring their adoption by amplifying "faint signals" from the alpha geeks who are creating the future. An active participa...
The 17th International Cloud Expo has announced that its Call for Papers is open. 17th International Cloud Expo, to be held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, brings together Cloud Computing, APM, APIs, Microservices, Security, Big Data, Internet of Things, DevOps and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportunity. Submit your speaking proposal today!
The 4th International Internet of @ThingsExpo, co-located with the 17th International Cloud Expo - to be held November 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA - announces that its Call for Papers is open. The Internet of Things (IoT) is the biggest idea since the creation of the Worldwide Web more than 20 years ago.
Imagine a world where targeting, attribution, and analytics are just as intrinsic to the physical world as they currently are to display advertising. Advances in technologies and changes in consumer behavior have opened the door to a whole new category of personalized marketing experience based on direct interactions with products. The products themselves now have a voice. What will they say? Who will control it? And what does it take for brands to win in this new world? In his session at @ThingsExpo, Zack Bennett, Vice President of Customer Success at EVRYTHNG, will answer these questions a...
SYS-CON Events announced today that BMC will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. BMC delivers software solutions that help IT transform digital enterprises for the ultimate competitive business advantage. BMC has worked with thousands of leading companies to create and deliver powerful IT management services. From mainframe to cloud to mobile, BMC pairs high-speed digital innovation with robust IT industrialization – allowing customers to provide amazing user experiences with optimized IT per...
17th Cloud Expo, taking place Nov 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy. Meanwhile, 94% of enterprises are using some form of XaaS – software, platform, and infrastructure as a service.
With major technology companies and startups seriously embracing IoT strategies, now is the perfect time to attend @ThingsExpo in Silicon Valley. Learn what is going on, contribute to the discussions, and ensure that your enterprise is as "IoT-Ready" as it can be! Internet of @ThingsExpo, taking place Nov 3-5, 2015, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with 17th Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. The Internet of Things (IoT) is the most profound change in personal an...
SYS-CON Events announced today that MetraTech, now part of Ericsson, has been named “Silver Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9–11, 2015, at the Javits Center in New York, NY. Ericsson is the driving force behind the Networked Society- a world leader in communications infrastructure, software and services. Some 40% of the world’s mobile traffic runs through networks Ericsson has supplied, serving more than 2.5 billion subscribers.
2015 predictions circa 1970: houses anticipate our needs and adapt, city infrastructure is citizen and situation aware, office buildings identify and preprocess you. Today smart buildings have no such collective conscience, no shared set of fundamental services to identify, predict and synchronize around us. LiveSpace and M2Mi are changing that. LiveSpace Smart Environment devices deliver over the M2Mi IoT Platform real time presence, awareness and intent analytics as a service to local connected devices. In her session at @ThingsExpo, Sarah Cooper, VP Business of Development at M2Mi, will d...
"For over 25 years we have been working with a lot of enterprise customers and we have seen how companies create applications. And now that we have moved to cloud computing, mobile, social and the Internet of Things, we see that the market needs a new way of creating applications," stated Jesse Shiah, CEO, President and Co-Founder of AgilePoint Inc., in this SYS-CON.tv interview at 15th Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
The Internet of Things is not only adding billions of sensors and billions of terabytes to the Internet. It is also forcing a fundamental change in the way we envision Information Technology. For the first time, more data is being created by devices at the edge of the Internet rather than from centralized systems. What does this mean for today's IT professional? In this Power Panel at @ThingsExpo, moderated by Conference Chair Roger Strukhoff, panelists will addresses this very serious issue of profound change in the industry.
Health care systems across the globe are under enormous strain, as facilities reach capacity and costs continue to rise. M2M and the Internet of Things have the potential to transform the industry through connected health solutions that can make care more efficient while reducing costs. In fact, Vodafone's annual M2M Barometer Report forecasts M2M applications rising to 57 percent in health care and life sciences by 2016. Lively is one of Vodafone's health care partners, whose solutions enable older adults to live independent lives while staying connected to loved ones. M2M will continue to gr...
The world is at a tipping point where the technology, the device and global adoption are converging to such a point that we will see an explosion of a world where smartphone devices not only allow us to talk to each other, but allow for communication between everything – serving as a central hub from which we control our world – MediaTek is at the heart of both driving this and allowing the markets to drive this reality forward themselves. The next wave of consumer gadgets is here – smart, connected, and small. If your ambitions are big, so are ours. In his session at @ThingsExpo, Jack Hu, D...