Click here to close now.


Java IoT Authors: Anders Wallgren, Betty Zakheim, Pat Romanski, Dana Gardner, Automic Blog

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:


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.

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
Container technology is shaping the future of DevOps and it’s also changing the way organizations think about application development. With the rise of mobile applications in the enterprise, businesses are abandoning year-long development cycles and embracing technologies that enable rapid development and continuous deployment of apps. In his session at DevOps Summit, Kurt Collins, Developer Evangelist at, examined how Docker has evolved into a highly effective tool for application delivery by allowing increasingly popular Mobile Backend-as-a-Service (mBaaS) platforms to quickly crea...
Discussions of cloud computing have evolved in recent years from a focus on specific types of cloud, to a world of hybrid cloud, and to a world dominated by the APIs that make today's multi-cloud environments and hybrid clouds possible. In this Power Panel at 17th Cloud Expo, moderated by Conference Chair Roger Strukhoff, panelists addressed the importance of customers being able to use the specific technologies they need, through environments and ecosystems that expose their APIs to make true change and transformation possible.
With major technology companies and startups seriously embracing IoT strategies, now is the perfect time to attend @ThingsExpo 2016 in New York and 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 cha...
Internet of @ThingsExpo, taking place June 7-9, 2016 at Javits Center, New York City and Nov 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA, is co-located with the 18th International @CloudExpo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world and ThingsExpo New York Call for Papers is now open.
The cloud. Like a comic book superhero, there seems to be no problem it can’t fix or cost it can’t slash. Yet making the transition is not always easy and production environments are still largely on premise. Taking some practical and sensible steps to reduce risk can also help provide a basis for a successful cloud transition. A plethora of surveys from the likes of IDG and Gartner show that more than 70 percent of enterprises have deployed at least one or more cloud application or workload. Yet a closer inspection at the data reveals less than half of these cloud projects involve production...
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! In her Day 2 Keynote at 17th Cloud Expo, Sandy Carter, IBM General Manager Cloud Ecosystem and Developers, and a Social Business Evangelist, wil...
Cloud computing delivers on-demand resources that provide businesses with flexibility and cost-savings. The challenge in moving workloads to the cloud has been the cost and complexity of ensuring the initial and ongoing security and regulatory (PCI, HIPAA, FFIEC) compliance across private and public clouds. Manual security compliance is slow, prone to human error, and represents over 50% of the cost of managing cloud applications. Determining how to automate cloud security compliance is critical to maintaining positive ROI. Raxak Protect is an automated security compliance SaaS platform and ma...
In his keynote at @ThingsExpo, Chris Matthieu, Director of IoT Engineering at Citrix and co-founder and CTO of Octoblu, focused on building an IoT platform and company. He provided a behind-the-scenes look at Octoblu’s platform, business, and pivots along the way (including the Citrix acquisition of Octoblu).
There are over 120 breakout sessions in all, with Keynotes, General Sessions, and Power Panels adding to three days of incredibly rich presentations and content. Join @ThingsExpo conference chair Roger Strukhoff (@IoT2040), June 7-9, 2016 in New York City, for three days of intense 'Internet of Things' discussion and focus, including Big Data's indespensable role in IoT, Smart Grids and Industrial Internet of Things, Wearables and Consumer IoT, as well as (new) IoT's use in Vertical Markets.
Today air travel is a minefield of delays, hassles and customer disappointment. Airlines struggle to revitalize the experience. GE and M2Mi will demonstrate practical examples of how IoT solutions are helping airlines bring back personalization, reduce trip time and improve reliability. In their session at @ThingsExpo, Shyam Varan Nath, Principal Architect with GE, and Dr. Sarah Cooper, M2Mi’s VP Business Development and Engineering, explored the IoT cloud-based platform technologies driving this change including privacy controls, data transparency and integration of real time context with p...
The Internet of Things (IoT) is growing rapidly by extending current technologies, products and networks. By 2020, Cisco estimates there will be 50 billion connected devices. Gartner has forecast revenues of over $300 billion, just to IoT suppliers. Now is the time to figure out how you’ll make money – not just create innovative products. With hundreds of new products and companies jumping into the IoT fray every month, there’s no shortage of innovation. Despite this, McKinsey/VisionMobile data shows "less than 10 percent of IoT developers are making enough to support a reasonably sized team....
We all know that data growth is exploding and storage budgets are shrinking. Instead of showing you charts on about how much data there is, in his General Session at 17th Cloud Expo, Scott Cleland, Senior Director of Product Marketing at HGST, showed how to capture all of your data in one place. After you have your data under control, you can then analyze it in one place, saving time and resources.
Just over a week ago I received a long and loud sustained applause for a presentation I delivered at this year’s Cloud Expo in Santa Clara. I was extremely pleased with the turnout and had some very good conversations with many of the attendees. Over the next few days I had many more meaningful conversations and was not only happy with the results but also learned a few new things. Here is everything I learned in those three days distilled into three short points.
As organizations realize the scope of the Internet of Things, gaining key insights from Big Data, through the use of advanced analytics, becomes crucial. However, IoT also creates the need for petabyte scale storage of data from millions of devices. A new type of Storage is required which seamlessly integrates robust data analytics with massive scale. These storage systems will act as “smart systems” provide in-place analytics that speed discovery and enable businesses to quickly derive meaningful and actionable insights. In his session at @ThingsExpo, Paul Turner, Chief Marketing Officer at...
DevOps is about increasing efficiency, but nothing is more inefficient than building the same application twice. However, this is a routine occurrence with enterprise applications that need both a rich desktop web interface and strong mobile support. With recent technological advances from Isomorphic Software and others, rich desktop and tuned mobile experiences can now be created with a single codebase – without compromising functionality, performance or usability. In his session at DevOps Summit, Charles Kendrick, CTO and Chief Architect at Isomorphic Software, demonstrated examples of com...
In his General Session at 17th Cloud Expo, Bruce Swann, Senior Product Marketing Manager for Adobe Campaign, explored the key ingredients of cross-channel marketing in a digital world. Learn how the Adobe Marketing Cloud can help marketers embrace opportunities for personalized, relevant and real-time customer engagement across offline (direct mail, point of sale, call center) and digital (email, website, SMS, mobile apps, social networks, connected objects).
The Internet of Everything is re-shaping technology trends–moving away from “request/response” architecture to an “always-on” Streaming Web where data is in constant motion and secure, reliable communication is an absolute necessity. As more and more THINGS go online, the challenges that developers will need to address will only increase exponentially. In his session at @ThingsExpo, Todd Greene, Founder & CEO of PubNub, exploreed the current state of IoT connectivity and review key trends and technology requirements that will drive the Internet of Things from hype to reality.
Two weeks ago (November 3-5), I attended the Cloud Expo Silicon Valley as a speaker, where I presented on the security and privacy due diligence requirements for cloud solutions. Cloud security is a topical issue for every CIO, CISO, and technology buyer. Decision-makers are always looking for insights on how to mitigate the security risks of implementing and using cloud solutions. Based on the presentation topics covered at the conference, as well as the general discussions heard between sessions, I wanted to share some of my observations on emerging trends. As cyber security serves as a fou...
Continuous processes around the development and deployment of applications are both impacted by -- and a benefit to -- the Internet of Things trend. To help better understand the relationship between DevOps and a plethora of new end-devices and data please welcome Gary Gruver, consultant, author and a former IT executive who has led many large-scale IT transformation projects, and John Jeremiah, Technology Evangelist at Hewlett Packard Enterprise (HPE), on Twitter at @j_jeremiah. The discussion is moderated by me, Dana Gardner, Principal Analyst at Interarbor Solutions.
Too often with compelling new technologies market participants become overly enamored with that attractiveness of the technology and neglect underlying business drivers. This tendency, what some call the “newest shiny object syndrome” is understandable given that virtually all of us are heavily engaged in technology. But it is also mistaken. Without concrete business cases driving its deployment, IoT, like many other technologies before it, will fade into obscurity.