Welcome!

Java Authors: Pat Romanski, Carmen Gonzalez, Victoria Livschitz, Elizabeth White, Liz McMillan

Related Topics: Java

Java: Article

Bean-Managed Persistence Using a Proxy List

Bean-Managed Persistence Using a Proxy List

Say you're writing an Enterprise JavaBean that represents a persistent object, such as a customer or a product. You have two choices for getting data (such as customer name and product number) from the bean to the database and back:

  • You can let the bean's runtime environment ­ its container, in EJB speak ­ do the heavy lifting for you....
  • ....or you can provide the logic yourself along with your bean.
It seems like an easy choice. Why write code when you don't need to? Frequently, in fact, container-managed persistence will be a good match for a project. However, if you want your bean to be portable across multiple EJB servers, or if you find that the container-managed persistence provided by your chosen EJB server is inadequate, you'll need to turn to bean-managed persistence.

According to the current version of the Enterprise JavaBeans Specification (v1.1), a compliant EJB container isn't required to provide any support for mapping the container-managed fields to a database schema. For instance, it could use Java serialization to save the beans to a file. Most if not all commercial and open-source containers will map fields to table columns in a database. But some will do it better than others. You'll have trouble with some products, for example, if you want to map your bean to multiple rows in multiple tables.

In fact, you probably do want to map your bean to multiple rows in multiple tables. Your beans should be coarse-grained, managing their own dependent objects. Your order bean should have line items implemented as a helper class, rather than as references to line-item EJBs. Your customer bean should have addresses rather than references to address beans. The overhead of bean-to-bean calls between an order and its line items, or a customer and its addresses, would be prohibitive. Your EJB server can provide you with many services, such as declarative transactions, security, and even load balancing and failover. But there's no free lunch, and the price you pay is indirection. Every call you make goes through a layer whose purpose is to provide these services (see Figure 1). Managing dependent objects reduces the frequency of trips through this indirection layer (see Figure 2). And a bean with multiple dependent objects needs to be stored in multiple tables, in multiple rows, if you want to maintain a normalized schema.

Technically, bean-managed persistence doesn't mean you have to write your own database access code. It just means the bean, rather than the container, provides the persistence logic. Several good object-relational mapping tools are on the market, and they can be portable from server to server along with your bean. But you may find it impossible to use these tools because of cost (some have runtime fees and/or a hefty per-developer price tag), distribution practicalities or reasons of your own. This article will tell you what you need to do to write your own persistence for a coarse-grained entity bean.

Requirements for Dependent Objects
An efficient implementation of bean-managed persistence for dependent objects will have two features:

  • Load-on-demand
  • Partitioned storage logic

Load-on-demand means that the dependent objects aren't loaded until they're actually needed. The EJB framework will call a function in your entity bean to indicate that persistent data should be made available to the bean's business logic. The bean could load the dependent data at this point. But if the business logic doesn't make use of certain dependent data during the current transaction, that database access was wasted. For instance, changing a customer's credit rating may not require access to any address, so the addresses shouldn't be loaded. If the dependent data is accessed, it can be loaded at that time. (This is also known as lazy loading.) Partitioned storage logic is necessary so that the bean updates the database the way a relational database expects: new data is inserted, changed data is updated, discarded data is deleted and unchanged data is left alone (see Figure 3). The alternative ­ wiping out the records and reinserting them ­ is too horrible even to contemplate.

A Good Idiom
To implement load-on-demand, you could scatter calls throughout your business logic to functions with names like "ensureAddressListLoaded" and "ensureLineItemListLoaded" ­ that is, if you want to be the poster child for ugly code. And to store your dependent objects to the database, you could have each object keep track of its status: NEW, UPDATED, DELETED or UNMODIFIED. As you totaled the line items in a purchase order, you'd need to check each object to see if it had been deleted (see Figure 4). Don't forget, or you're going to have some unhappy customers.

A better idiom is to group the logic related to persistence with a collection class. Your business logic for an order probably works with a list of line items. To delete an object, the most natural thing to do is probably to remove it from the list. To add an object, the most natural thing is to append it to the list. And simply calling a method on the list should be the signal to your persistence logic that the items in the list need to be loaded from the database. If you use a smart list that knows how to do these things, nothing else needs to be done from the perspective of the business logic programmer. In the example of totaling line items in a purchase order, you'd simply iterate through the objects in the list (see Figure 5).

Behind the scenes, a smart list implementation is keeping track of an "isLoaded" variable. When the list is accessed, it checks this variable first to see if the data needs to be loaded from the database. If so, it loads it. It keeps a set of references to all the objects it loads to distinguish them from new objects added to the list. If an object is removed from the list, it's added to an internal list of deleted objects. This deleted objects list is used by the persistence logic, but not, typically, by the business logic programmer.

The Proxy List
The smart list needs a layer of indirection between the list interface and the actual list storage. At this layer a method call on the list to remove objects will first add those objects to the internal deleted objects list. Also, a call to any function will check to see if the list data has been loaded. One possible way to gain this indirection is simply to implement the java.util.List interface and delegate the calls to a private internal "backing" list. A more elegant way is to use the java.lang.reflect.Proxy class newly available in JDK 1.3. (Note: Using the Proxy class limits your bean's portability to servers that support JDK 1.3. All the techniques and code discussed in this article are easily adapted to the "delegation list" compatible with earlier JDKs.)

The Proxy class dynamically creates an implementation of an interface that will automatically forward all its calls to a middle layer called InvocationHandler (also in package java.lang.reflect). In a subclass of InvocationHandler you can forward the method call (or not), take action before or after forwarding it, alter its parameters and change the returned object (see Figure 6). As you can see, this is more than enough functionality to implement our smart list. The uses for this Proxy are many: it can be used to handle user interface events and to provide a "poor man's multiple inheritance," and has even been used to implement an open-source EJB server, EJBoss. (For more information on the Proxy and InvocationHandler classes, see the article at http://java.sun.com/products/jfc/tsc/articles/generic-listener2/index.html on Sun's Web site.)

Take a look at ListInvocationHandler (see Listing 1). It's the smart list implementation that keeps track of deleted objects, the set of original objects and whether the data has been loaded from the database. It also takes as a constructor parameter its "backing" list so that any class implementing the List interface (LinkedList or ArrayList) can be used, depending on how the data is typically accessed. The main functionality is in the invoke method. Here I check to make sure the data has been loaded from the database. I also check for any List function that removes an object so I can make a copy to use for calling "delete" later on the database. An important point: several List functions will return a reference to the backing list unless these too are interposed on. Any method that returns an Iterator (which points to the backing list) must instead be made to return an Iterator pointing to the interposed list. I did this using ­ you guessed it ­ another Proxy (see Listing 2). Any method that returns a Collection must either be interposed on or made unmodifiable.

Persistence Details
My smart list implementation uses two persistence-specific interfaces (see Figure 7). The first, PersistentOperations (see Listing 3), is implemented by the smart list itself (in ListInvocationHandler). It's for list operations needed by the bean's persistence plumbing, rather than the business logic. You can get a list of deleted objects to actually delete them. You can get the set of original objects to decide between insert and update operations. You can add an object to the list so that it's tagged as an original object rather than a new one. You can tell the list that the bean has been asked to save itself to persistent storage, and to take whatever actions are necessary. For symmetry more than need, you can also ask the list to load itself from persistent storage. Typically, you'll let the list decide this for itself.

The second persistence-specific interface used by the list is DataStore (see Listing 4). Although your business logic can treat the smart list as a regular list, you need to put the SQL somewhere, and this interface is the gateway to that "somewhere." Your bean will pass an implementation of this interface to the smart list factory (DemandListFactory; see Listing 5). When the list needs to save or restore its data, it will call methods in this interface, passing a reference to its PersistentOperations interface.

Implementation Example
A simplified example of using a proxy list with an EJB with bean-managed persistence is available on the Java Developer's Journal Web site. The bean implements a customer-has-line-items model. A customer has an ID and a name, and an unlimited number of line items of products he or she has ordered. In order to give the smart list a workout, I've written functions so the business logic programmer can add, delete or change a line item, and use the entire set of line items at once. The list is initialized when the bean is created (a new customer record is inserted) or loaded (an existing customer record is read from the database). An anonymous DataStore implementation is passed to the DemandListFactory, which will call bean methods whenever DataStore methods are called by the list.

One bean method will do a simple select on the database table where line items are stored. As it iterates through the result set, it calls the PersistentOperations's addFromStore method, which will indicate to the list that the object already exists in the database and needs to be updated, not inserted, when the list is stored. Another bean method that stores the list is only slightly more complicated. It must use the information available from the PersistentOperations interface to partition the objects into three sets: insertions, updates and deletes. You'll notice that I'm using an isModified function to further partition updates from unmodified instances. It's possible to do this in the smart list as well by keeping a copy of each original object and then comparing it to the object's state just before the database update. There are disadvantages to this technique, however, depending on the memory required to keep copies of those objects and the processing time required to compare object states. In any case, implementing this is beyond the scope of this article.

To test my Customer implementation, I've included a stateless session EJB that will give it a good workout. Fronting an entity EJB with a "business process" session, EJB is a common design pattern. Here it allows us to include multiple adds, deletes, updates and totals within one transaction. Since the typical EJB server will load and store persistent data on transaction boundaries (load when the transaction begins and store when it ends), this is important for our testing. Obviously, the stateless session EJB isn't a good example of an actual business process.

Conclusion
It isn't difficult to write an efficient, easy-to-use Enterprise JavaBean using bean-managed persistence. Well, what I should really say is that it's not much harder than writing the SQL code that your bean will use. If you need to roll your own persistence for EJBs, there's no better place for the persistence logic than a collection class. In this case we used a list, but the same technique would work for a Set or a Map, or even a tree document, depending on your needs. And if your target market allows you to write to the JDK 1.3, the new Proxy class can increase the expressiveness and effectiveness of your code.

More Stories By Daniel O'Connor

Daniel O'Connor is an independent software developer writing enterprise management software for the
not-for-profit field. He has a BA from Williams College and an MA from SUNY University at Albany. 

Comments (0)

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.


@ThingsExpo Stories
Cultural, regulatory, environmental, political and economic (CREPE) conditions over the past decade are creating cross-industry solution spaces that require processes and technologies from both the Internet of Things (IoT), and Data Management and Analytics (DMA). These solution spaces are evolving into Sensor Analytics Ecosystems (SAE) that represent significant new opportunities for organizations of all types. Public Utilities throughout the world, providing electricity, natural gas and water, are pursuing SmartGrid initiatives that represent one of the more mature examples of SAE. We have s...
The security devil is always in the details of the attack: the ones you've endured, the ones you prepare yourself to fend off, and the ones that, you fear, will catch you completely unaware and defenseless. The Internet of Things (IoT) is nothing if not an endless proliferation of details. It's the vision of a world in which continuous Internet connectivity and addressability is embedded into a growing range of human artifacts, into the natural world, and even into our smartphones, appliances, and physical persons. In the IoT vision, every new "thing" - sensor, actuator, data source, data con...
The Internet of Things is tied together with a thin strand that is known as time. Coincidentally, at the core of nearly all data analytics is a timestamp. When working with time series data there are a few core principles that everyone should consider, especially across datasets where time is the common boundary. In his session at Internet of @ThingsExpo, Jim Scott, Director of Enterprise Strategy & Architecture at MapR Technologies, discussed single-value, geo-spatial, and log time series data. By focusing on enterprise applications and the data center, he will use OpenTSDB as an example t...
How do APIs and IoT relate? The answer is not as simple as merely adding an API on top of a dumb device, but rather about understanding the architectural patterns for implementing an IoT fabric. There are typically two or three trends: Exposing the device to a management framework Exposing that management framework to a business centric logic Exposing that business layer and data to end users. This last trend is the IoT stack, which involves a new shift in the separation of what stuff happens, where data lives and where the interface lies. For instance, it's a mix of architectural styles ...
The 3rd International Internet of @ThingsExpo, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that its Call for Papers is now open. The Internet of Things (IoT) is the biggest idea since the creation of the Worldwide Web more than 20 years ago.
An entirely new security model is needed for the Internet of Things, or is it? Can we save some old and tested controls for this new and different environment? In his session at @ThingsExpo, New York's at the Javits Center, Davi Ottenheimer, EMC Senior Director of Trust, reviewed hands-on lessons with IoT devices and reveal a new risk balance you might not expect. Davi Ottenheimer, EMC Senior Director of Trust, has more than nineteen years' experience managing global security operations and assessments, including a decade of leading incident response and digital forensics. He is co-author of t...
The Internet of Things will greatly expand the opportunities for data collection and new business models driven off of that data. In her session at @ThingsExpo, Esmeralda Swartz, CMO of MetraTech, discussed how for this to be effective you not only need to have infrastructure and operational models capable of utilizing this new phenomenon, but increasingly service providers will need to convince a skeptical public to participate. Get ready to show them the money!
The Internet of Things will put IT to its ultimate test by creating infinite new opportunities to digitize products and services, generate and analyze new data to improve customer satisfaction, and discover new ways to gain a competitive advantage across nearly every industry. In order to help corporate business units to capitalize on the rapidly evolving IoT opportunities, IT must stand up to a new set of challenges. In his session at @ThingsExpo, Jeff Kaplan, Managing Director of THINKstrategies, will examine why IT must finally fulfill its role in support of its SBUs or face a new round of...
One of the biggest challenges when developing connected devices is identifying user value and delivering it through successful user experiences. In his session at Internet of @ThingsExpo, Mike Kuniavsky, Principal Scientist, Innovation Services at PARC, described an IoT-specific approach to user experience design that combines approaches from interaction design, industrial design and service design to create experiences that go beyond simple connected gadgets to create lasting, multi-device experiences grounded in people's real needs and desires.
Enthusiasm for the Internet of Things has reached an all-time high. In 2013 alone, venture capitalists spent more than $1 billion dollars investing in the IoT space. With "smart" appliances and devices, IoT covers wearable smart devices, cloud services to hardware companies. Nest, a Google company, detects temperatures inside homes and automatically adjusts it by tracking its user's habit. These technologies are quickly developing and with it come challenges such as bridging infrastructure gaps, abiding by privacy concerns and making the concept a reality. These challenges can't be addressed w...
The Domain Name Service (DNS) is one of the most important components in networking infrastructure, enabling users and services to access applications by translating URLs (names) into IP addresses (numbers). Because every icon and URL and all embedded content on a website requires a DNS lookup loading complex sites necessitates hundreds of DNS queries. In addition, as more internet-enabled ‘Things' get connected, people will rely on DNS to name and find their fridges, toasters and toilets. According to a recent IDG Research Services Survey this rate of traffic will only grow. What's driving t...
Scott Jenson leads a project called The Physical Web within the Chrome team at Google. Project members are working to take the scalability and openness of the web and use it to talk to the exponentially exploding range of smart devices. Nearly every company today working on the IoT comes up with the same basic solution: use my server and you'll be fine. But if we really believe there will be trillions of these devices, that just can't scale. We need a system that is open a scalable and by using the URL as a basic building block, we open this up and get the same resilience that the web enjoys.
Connected devices and the Internet of Things are getting significant momentum in 2014. In his session at Internet of @ThingsExpo, Jim Hunter, Chief Scientist & Technology Evangelist at Greenwave Systems, examined three key elements that together will drive mass adoption of the IoT before the end of 2015. The first element is the recent advent of robust open source protocols (like AllJoyn and WebRTC) that facilitate M2M communication. The second is broad availability of flexible, cost-effective storage designed to handle the massive surge in back-end data in a world where timely analytics is e...
We are reaching the end of the beginning with WebRTC, and real systems using this technology have begun to appear. One challenge that faces every WebRTC deployment (in some form or another) is identity management. For example, if you have an existing service – possibly built on a variety of different PaaS/SaaS offerings – and you want to add real-time communications you are faced with a challenge relating to user management, authentication, authorization, and validation. Service providers will want to use their existing identities, but these will have credentials already that are (hopefully) i...
"Matrix is an ambitious open standard and implementation that's set up to break down the fragmentation problems that exist in IP messaging and VoIP communication," explained John Woolf, Technical Evangelist at Matrix, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
P2P RTC will impact the landscape of communications, shifting from traditional telephony style communications models to OTT (Over-The-Top) cloud assisted & PaaS (Platform as a Service) communication services. The P2P shift will impact many areas of our lives, from mobile communication, human interactive web services, RTC and telephony infrastructure, user federation, security and privacy implications, business costs, and scalability. In his session at @ThingsExpo, Robin Raymond, Chief Architect at Hookflash, will walk through the shifting landscape of traditional telephone and voice services ...
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 his session at Internet of @ThingsExpo, James Kirkland, Chief Architect for the Internet of Things and Intelligent Systems at Red Hat, described how to revolutioniz...
Bit6 today issued a challenge to the technology community implementing Web Real Time Communication (WebRTC). To leap beyond WebRTC’s significant limitations and fully leverage its underlying value to accelerate innovation, application developers need to consider the entire communications ecosystem.
The definition of IoT is not new, in fact it’s been around for over a decade. What has changed is the public's awareness that the technology we use on a daily basis has caught up on the vision of an always on, always connected world. If you look into the details of what comprises the IoT, you’ll see that it includes everything from cloud computing, Big Data analytics, “Things,” Web communication, applications, network, storage, etc. It is essentially including everything connected online from hardware to software, or as we like to say, it’s an Internet of many different things. The difference ...
Cloud Expo 2014 TV commercials will feature @ThingsExpo, which was launched in June, 2014 at New York City's Javits Center as the largest 'Internet of Things' event in the world.