Welcome!

Java IoT Authors: Elizabeth White, Yeshim Deniz, Liz McMillan, Carmen Gonzalez, Yakov Fain

Related Topics: Java IoT

Java IoT: Article

Improved Observer/ Observable

Improved Observer/ Observable

The Observer Design Pattern is among the most useful for object-oriented software design. The JDK itself makes heavy use of a variant of this pattern in the 1.1 AWT event delegation model. The JDK also provides a reusable embodiment of the pattern in the form of the java.util.Observer interface and the java.util.Observable class. If you've done much serious Java programming you've more than likely had occasion to use these classes.

The idea of the pattern is to model a one-to-many dependency without tightly coupling the observed object with its many observers. When the observed object changes in some interesting way it can automatically notify all of its observers without knowing them directly. For more on this pattern, see Qusay Mahmoud's article, "Implementing Design Patterns in Java" (JDJ, Vol. 2, Iss. 5), or the Design Patterns book.

Unfortunately, a number of weaknesses have been identified in the JDK's Observer/Observable classes. Peter Coad and Mark Mayfield discuss some of these weaknesses in their excellent book, Java Design. These weaknesses significantly limit the reusability and power of the classes, which is a shame since powerful reusability is a big part of what object-oriented design is all about.

Most of the weaknesses are due to the fact that java.util.Observable is a class rather than an interface; or rather, that it is a class without a corresponding interface. This implies that the only way to reuse Observable is to subclass it. You can't take an existing class and tack on the role of Observable by having it implement an Observable interface because there is no Observable interface.

What if you have a class that is already in a class hierarchy and also needs to play the role of an Observable? Since Java doesn't support multiple inheritance, you're out of luck. That class cannot extend from Observable because it is already extending from some other class.

It also means that you are stuck with the one and only implementation of Observable in java.util.Observable. For a variety of reasons, you may want to use an alternate implementation - e.g., to do the notification in a separate thread or in a particular order. You may even want to vary the implementation of Observable at runtime. There is no Observable interface for your alternate implementations to implement. You cannot reuse Observable by composition so you cannot vary the composed Observable implementation at runtime.

The designers of the Observable class broke two general principles of object-oriented design with Java. The first principle is to design with interfaces rather than classes. Whenever possible, avoid committing yourself to a particular implementation of an interface. The second principle is to favor reuse by composition over reuse by inheritance unless a class hierarchy is clearly indicated. By omitting an Observable interface and making some of its methods protected, the designers made it impossible to reuse Observable by composition.

A minor weakness in Observable is the necessity to call setChanged() before notifySubscribers(). The intention there seems to be to eliminate needless notifications in cases where there is no interesting change to the Observable. There may be situations in which this two-stage notification is appropriate, but it isn't the simplest case and programmers shouldn't be forced to use this implementation in all situations. Also, setChanged() is protected, further reinforcing the necessity to reuse the class only by inheritance.

Now let's look at the Observer interface. The weakness here is its tight coupling with the Observable class. The first parameter to the update() method is unnecessarily typed as an Observable. If it were typed more generally as a simple Object, the Observer interface would be more reusable. It then could be used with any Observable implementation or even in any situation, completely unrelated to Observer/Observable, which called for a void method with two Object parameters.

Despite all these weaknesses, I was initially reluctant to ignore the JDK classes in favor of the homegrown replacements suggested by Coad and Mayfield. After all, the JDK classes are already locally available to every VM and they do serve their purpose nicely for the majority of cases. On the other hand, designing for maximal reuse is crucial to the success of any object-oriented design. A little forethought early in the game can lead to great savings down the road. Luckily, when I started a new project some months ago, I decided to take the plunge and start using the improved classes.

More recently I began designing a number of distributed three-tier Java apps using Remote Method Invocation (RMI). It turns out that the Observer pattern has great application to remote applications. A remote object that lives on the server often needs to be observed by multiple objects living on multiple clients. When the remote object changes, all clients need to find out in order to, for example, update the user's view.

The JDK's Observer/Observable classes are of no use here. They do not extend from java.rmi.Remote and their method signatures do not allow for the possible throwing of RemoteException. Both of these are required of any Remote interface for RMI. So the JDK's Observable/Observer classes can never be implemented as an RMI remote interface of a RemoteObject.

The solution that I've seen in articles and books is to write a separate set of classes for Remote Observable/Observer. This always seemed wrong to me. Why should I have to write, support and use two disjoint sets of classes to do basically the same thing albeit in different situations? What if I have a class that needs to notify both local and remote Observers? You mean it has to implement both versions of Observable?!

Having already severed my dependence on the JDK's Observable classes, I was able to painlessly enhance my classes to support Remote Method Invocation. The same classes can now be used for local Observers as well as remote Observers or even for a mixture of remote and local Observers.

The interfaces are shown in Listing 1. In order to avoid confusion with the JDK classes, I use the synonymous terminology of Publisher/Subscriber rather than Observable/Observer.

Notice that the Publisher and Subscriber interfaces extend from java.rmi.Remote and all their methods may throw RemoteException. This is so that these interfaces may be implemented by remote classes for use with RMI. These additions do not preclude these classes from being used in a strictly local situation without RMI. In fact, the same Publisher could be used to publish to a mixture of local and remote Subscribers.

A simple basic implementation of Publisher is presented in Listing 2. This Publisher can be used for both local and remote Subscribers.

The notifySubscribers() method deserves some elaboration. The call to the Subscribers' update() method is in a try/catch clause to deal with possibly thrown RemoteExceptions. Two particular RemoteExceptions, ConnectException and NoSuchObjectException, are considered serious enough to consider the Subscriber to be dead. Subscribers that are considered dead are removed from the list of Subscribers for this Publisher. Other RemoteExceptions may be indications of transient failures that could correct themselves. This is the type of fuzzy logic you need to apply in networked situations where errors are unpredictable and non-deterministic.

The dead Subscribers are not removed from the list of Subscribers until after the list is enumerated. At first glance you might ask, why not remove the dead Observers right away, within the catch clauses? This is because of a quirk in java.util.Vector that makes it unsafe to manipulate a Vector while it is being enumerated. This quirk (bug?) is a discussion for another time, but a few words about the workaround I chose are warranted here.

As I discover Subscribers that are to be considered dead, I accumulate them in a Vector. Only after enumeration of all Subscribers do I go through the Vector of dead Subscribers and actually remove them. Another workaround that I could have used is to clone the Vector of Subscribers before enumerating it. Then I could operate on the original Vector while enumerating the clone. I chose the workaround I did because it has little overhead for the more usual case when no RemoteException is thrown. Even the deadSubs Vector is not allocated until and unless it is needed. This technique is known as lazy instantiation.

The preferred way to reuse BasicPublisher is by composition. Any class that needs to play the role of a Publisher should implement the Publisher interface and include a reference to an instance of BasicPublisher to do the work of a Publisher. Listing 3 is a rough outline of this.

Class XX is free to extend from some other class if it needs to and it can publish changes to subscribers, whether local or remote, by calling pub.notifySubscribers(). You can also reuse BasicPublisher by subclassing it, but this is not the preferred way. It eliminates the need to delegate the Publisher interface but it reduces the extendibility of the class because the class can no longer be a subclass of any other class.

To use these classes with RMI, the classes that implement Publisher and Subscriber must be remote objects. Without going into too much detail on RMI, let me just point out that any class can be made remote in two simple steps. First, add the following to its constructor:

UnicastRemoteObject.exportObject(this);

and recompile. Second, run the rmic post compiler on the class. That's it. Your class is now remote. Remote publishers can now publish changes to local subscribers in the same VM or to remote subscribers in other VMs.

In summary, these two interfaces and class constitute a far more powerful and reusable embodiment of the Observer design pattern than the JDK's Observer/Observable. They can even be used for remote Observers with RMI. This is just the sort of weapon in your object-oriented arsenal that makes Java programming such a joy.

References
Gamma, E., Johnson, R. and Vlissides, J., "Design Patterns: Elements of Object-Oriented Architecture", Addison-Wesley, Reading, MA, 1995.
Coad , P. and Mayfield, M., "Java Design: Building Better Apps and Applets", Yourdon Press, Upper Saddle River, NJ, 1997.

More Stories By Steven Schwell

Steven Schwell is a Senior Developer and Java Guru in the New York office of Micromuse, Inc., a leading provider of Service Level Management software. Steve is currently developing a number of large distributed Java apps. He holds a M.S. in Computer Science from Columbia University.

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
SYS-CON Events announced today that ContentMX, the marketing technology and services company with a singular mission to increase engagement and drive more conversations for enterprise, channel and SMB technology marketers, has been named “Sponsor & Exhibitor Lounge Sponsor” of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2016, at the Javits Center in New York City, New York. “CloudExpo is a great opportunity to start a conversation with new prospects, but what happens after the...
SYS-CON Events announced today the How to Create Angular 2 Clients for the Cloud Workshop, being held June 7, 2016, in conjunction with 18th Cloud Expo | @ThingsExpo, at the Javits Center in New York, NY. Angular 2 is a complete re-write of the popular framework AngularJS. Programming in Angular 2 is greatly simplified. Now it’s a component-based well-performing framework. The immersive one-day workshop led by Yakov Fain, a Java Champion and a co-founder of the IT consultancy Farata Systems and...
Customer experience has become a competitive differentiator for companies, and it’s imperative that brands seamlessly connect the customer journey across all platforms. With the continued explosion of IoT, join us for a look at how to build a winning digital foundation in the connected era – today and in the future. In his session at @ThingsExpo, Chris Nguyen, Group Product Marketing Manager at Adobe, will discuss how to successfully leverage mobile, rapidly deploy content, capture real-time d...
SYS-CON Events announced today that BMC Software has been named "Siver Sponsor" of SYS-CON's 18th Cloud Expo, which will take place on June 7-9, 2015 at the Javits Center in New York, New York. BMC is a global leader in innovative software solutions that help businesses transform into digital enterprises for the ultimate competitive advantage. BMC Digital Enterprise Management is a set of innovative IT solutions designed to make digital business fast, seamless, and optimized from mainframe to mo...
What a difference a year makes. Organizations aren’t just talking about IoT possibilities, it is now baked into their core business strategy. With IoT, billions of devices generating data from different companies on different networks around the globe need to interact. From efficiency to better customer insights to completely new business models, IoT will turn traditional business models upside down. In the new customer-centric age, the key to success is delivering critical services and apps wit...
Join us at Cloud Expo | @ThingsExpo 2016 – June 7-9 at the Javits Center in New York City and November 1-3 at the Santa Clara Convention Center in Santa Clara, CA – and deliver your unique message in a way that is striking and unforgettable by taking advantage of SYS-CON's unmatched high-impact, result-driven event / media packages.
In his keynote at 18th Cloud Expo, Andrew Keys, Co-Founder of ConsenSys Enterprise, will provide an overview of the evolution of the Internet and the Database and the future of their combination – the Blockchain. Andrew Keys is Co-Founder of ConsenSys Enterprise. He comes to ConsenSys Enterprise with capital markets, technology and entrepreneurial experience. Previously, he worked for UBS investment bank in equities analysis. Later, he was responsible for the creation and distribution of life ...
SYS-CON Events announced today that MobiDev will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. MobiDev is a software company that develops and delivers turn-key mobile apps, websites, web services, and complex software systems for startups and enterprises. Since 2009 it has grown from a small group of passionate engineers and business managers to a full-scale mobile software company with over 200 develope...
SoftLayer operates a global cloud infrastructure platform built for Internet scale. With a global footprint of data centers and network points of presence, SoftLayer provides infrastructure as a service to leading-edge customers ranging from Web startups to global enterprises. SoftLayer's modular architecture, full-featured API, and sophisticated automation provide unparalleled performance and control. Its flexible unified platform seamlessly spans physical and virtual devices linked via a world...
Companies can harness IoT and predictive analytics to sustain business continuity; predict and manage site performance during emergencies; minimize expensive reactive maintenance; and forecast equipment and maintenance budgets and expenditures. Providing cost-effective, uninterrupted service is challenging, particularly for organizations with geographically dispersed operations.
SYS-CON Events announced today TechTarget has been named “Media Sponsor” of SYS-CON's 18th International Cloud Expo, which will take place on June 7–9, 2016, at the Javits Center in New York City, NY, and the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. TechTarget is the Web’s leading destination for serious technology buyers researching and making enterprise technology decisions. Its extensive global networ...
SYS-CON Events announced today that Commvault, a global leader in enterprise data protection and information management, has been named “Bronze Sponsor” of SYS-CON's 18th International Cloud Expo, which will take place on June 7–9, 2016, at the Javits Center in New York City, NY, and the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Commvault is a leading provider of data protection and information management...
As cloud and storage projections continue to rise, the number of organizations moving to the cloud is escalating and it is clear cloud storage is here to stay. However, is it secure? Data is the lifeblood for government entities, countries, cloud service providers and enterprises alike and losing or exposing that data can have disastrous results. There are new concepts for data storage on the horizon that will deliver secure solutions for storing and moving sensitive data around the world. ...
SYS-CON Events announced today Object Management Group® has been named “Media Sponsor” of SYS-CON's 18th International Cloud Expo, which will take place on June 7–9, 2016, at the Javits Center in New York City, NY, and the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that MangoApps will exhibit at SYS-CON's 18th International Cloud Expo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, NY. MangoApps provides modern company intranets and team collaboration software, allowing workers to stay connected and productive from anywhere in the world and from any device. For more information, please visit https://www.mangoapps.com/.
The essence of data analysis involves setting up data pipelines that consist of several operations that are chained together – starting from data collection, data quality checks, data integration, data analysis and data visualization (including the setting up of interaction paths in that visualization). In our opinion, the challenges stem from the technology diversity at each stage of the data pipeline as well as the lack of process around the analysis.
The IoTs will challenge the status quo of how IT and development organizations operate. Or will it? Certainly the fog layer of IoT requires special insights about data ontology, security and transactional integrity. But the developmental challenges are the same: People, Process and Platform. In his session at @ThingsExpo, Craig Sproule, CEO of Metavine, will demonstrate how to move beyond today's coding paradigm and share the must-have mindsets for removing complexity from the development proc...
Designing IoT applications is complex, but deploying them in a scalable fashion is even more complex. A scalable, API first IaaS cloud is a good start, but in order to understand the various components specific to deploying IoT applications, one needs to understand the architecture of these applications and figure out how to scale these components independently. In his session at @ThingsExpo, Nara Rajagopalan is CEO of Accelerite, will discuss the fundamental architecture of IoT applications, ...
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, wh...
SYS-CON Events announced today that Tintri Inc., a leading producer of VM-aware storage (VAS) for virtualization and cloud environments, will exhibit at the 18th International CloudExpo®, which will take place on June 7-9, 2016, at the Javits Center in New York City, New York, and the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.