|By Bill Burke||
|December 4, 2003 12:00 AM EST||
Aspect-oriented programming (AOP) is a promising new paradigm that came out of Xerox PARC a few years ago and is just now becoming mature and mainstream. A natural complement to object-oriented programming, it has the promise of easing the management of complex systems and making their organization much more intuitive, extendable, and flexible. AOP makes OOP multidimensional.
What is an Aspect? An Aspect is a common functionality that's scattered across methods, classes, object hierarchies, or object models. Functionality that your class or object model shouldn't be concerned about, functionality that doesn't belong as it's not what the object is all about. The AOP-ites like to call this type of functionality crosscutting concerns, as the behavior is cutting across multiple points in your object models, and yet is distinctly different from the classes it's crosscutting. AOP allows you to abstract and seamlessly componentize these concerns and apply them to your applications in a unique way that regular object-oriented programs cannot achieve very easily.
A simple example of a crosscutting concern is timing and metrics. Let's say you wanted to add code similar to Listing 1 to your application that would measure the amount of time it would take to invoke a particular method.
There are a few problems with this approach:
1. You have to manually add this code to multiple different files, methods, and classes, which is a pain; if you want to change which methods are profiled you have to manually edit those files. In other words, this is hard to turn off and on and difficult to maintain.
2. The profiling code really doesn't belong sprinkled throughout your application code. It makes your code bloated and harder to read as you have to enclose the timings within a try-finally block.
3. If you want to add other metrics like a method count or a failure count, you would have to modify all the files where you manually inserted the profiling code. It's very difficult to maintain, expand, and extend your metrics functionality as it's dispersed throughout your entire code base.
This is a tiny example of how you can have common code that is sprinkled across many unrelated modules of your application; code that intrudes on the overall purpose of the Java class you're implementing. Aspect-oriented programming provides a way to pull together these common behaviors into a manageable unit and apply them to your code base. Let's look at how AOP would implement and solve this problem.
Defining an Aspect
The first thing that should be done to aspectize the metrics functionality would be to create an Aspect. The try-finally block that we originally had within the BankAccount.withdraw method should be extracted and encapsulated into its own object. Having this code within its own object enables us to easily expand and maintain any additional metrics we may want to calculate later on in the development cycle. For this object to work, it must be able to wrap around and obtain contextual information about the particular method you want to add profiling to so that metrics can be displayed. There are a few AOP frameworks out there, so rather than picking one framework to give an example in, let's look at some pseudo code that could be easily translated into a real framework later on (see Listing 2).
The MetricsAspect class pulls together the metrics functionality into one maintainable, extendable unit. The invoke method at line 3 should be called in place of the actual method you want to provide metrics for. All AOP frameworks should provide some form of abstraction for wrapping/intercepting a method call. Line 8 wraps and delegates to the actual method. Line 13 assumes that you can obtain contextual information about the method call from the AOP framework you are using.
Applying an Aspect
Now that we have extracted out the metrics functionality into a componentized Aspect, how can we apply it? This is where a pointcut comes in. A pointcut defines an entry point within your code base. It describes an event. An entry point could be a field access, a method call, or a constructor call. An event could be an exception being thrown. A pointcut is a way for you to define where you want your aspects applied. Let's look at some pseudo XML configuration for a pointcut that any AOP framework should be able to do in some form or another.
1. <method-pointcut expr="com.mc.BankAccount.withdraw(double amount)">
4. <method-pointcut expr="com.mc.billing.*">
Lines 1-3 define a pointcut that applies the metrics aspect to the specific method BankAccount.withdraw. Lines 4-6 define a general pointcut to apply the metrics aspect on all methods in all classes under the com.mc. billing package name. Most AOP frameworks have a rich set of pointcut expressions that you can use to apply your aspects. You can attach your aspects on an individual one-on-one basis to each Java class in your application, or you can use a more complex pointcut to specify a wide range of classes with one expression.
What this example shows is that with AOP, you're able to pull together crosscutting behavior into one object and sprinkle it easily and simply throughout your code base without making code unreadable or polluted with functionality that doesn't belong with the business logic you are implementing. Common crosscutting functionality can be maintained and extended in one place.
Another thing to notice is that the code within the BankAccount class has no idea that it's being profiled. The application developer was allowed to focus on writing business logic rather than being distracted with writing the code candy and syntactic sugar of profiling. Needed orthogonal behavior could be snapped on after the fact quite easily without even touching this existing code base. This is a very subtle significant part of AOP as this complete obliviousness allows aspects to be layered on top of or below the functionality they are crosscutting. A layered design allows you, as a system designer, to more easily snap on or remove functionality or behavior that you need. For instance, maybe you only snap on the metrics functionality when you're doing some benchmarks but want to remove this within production. Or, if the AOP framework allows for it, maybe you want to turn on metrics in production to determine where bottlenecks are.
In the early days of object-oriented programming, it was user-interface applications that helped to scope and discover object-oriented patterns and techniques. If you look at the Gang of Four's Design Patterns book (the bible of object-oriented programming), you'll see that GUIs are used in many of the coding examples that describe the patterns in the book. As GUIs helped formulate the early patterns of OO, middleware is shaping up to be the killer app for aspect-oriented programming.
Middleware, by nature, is crosscutting. It has functionality that's common across object hierarchies that really should not be mingled with business logic. The evolution of middleware has always been to abstract out how it is applied to regular simple objects. AOP completes this evolution as middleware functionality can be applied after the fact without changing the code or design of the existing business model. Packaging up middleware into a set of aspects frees developers to focus on writing the plain Java objects that make up their application's specific behavior rather than forcing them to work under an API dictated to them by their system architecture.
Take J2EE, for instance. It can be sliced and diced and served à la carte to your object model rather than going through the sometimes cumbersome and unnecessary process of implementing an EJB. For instance, let's say you were using EJB solely for the purpose of defining transactions. Transaction demarcation lines could be drawn within any class at any point using AOP. Instead of extending SessionBean and writing home, remote, and local interfaces; deciding on a JNDI binding; and defining all your <ejb-ref>s in XML; all you would have to do is define a pointcut for the method of the class you want a transaction started from and attach the transactional aspect to trigger the desired behavior.
1. <method-pointcut expr="com.mc.BankAccount.withdraw(double amount)">
2. <attach-aspect class="org.vendor.transaction.RequiredAspect"/>
You can apply these same techniques to a multitude of middleware technology like remoteness, ACID, replicated caching, oneway, simple asynchronous invocations, role-based security, and persistence. AOP prevents system programming from intruding into your object model. It has the potential to completely separate the concern of middleware from your application logic. This can make your code easier to maintain and read, and more flexible as you can make system architecture decisions later on in the development process. It's a pure layered approach to applying middleware.
AOP is a new paradigm for expanding code reuse and easing the maintainability of your code base. It provides mechanisms to easily componentize code that is scattered throughout your object model and really needs to be organized centrally into one set of objects. When combined with something like middleware, it has the ability to isolate your business logic from the confines of system architecture, thus making your applications even more resistant to change as the landscape of APIs and public specifications changes over time. As framework developers focus on providing their functionality through aspects, the term pointcut will be morphed into pointclick as aspects are applied to an object model through the point-and-click interfaces of an IDE.
|Mike Jozwiak 12/29/03 08:37:57 AM EST|
I have not looked into this, other than skimming this article.
Maybe I am missing something.
Why can''t/shouldn''t this be done with simmple objects?
|Magesh Narayanan 12/22/03 04:35:04 PM EST|
|Irene 12/14/03 08:39:36 PM EST|
Very clear explanation of Aspect Programming!
|V 12/10/03 11:36:52 AM EST|
Look for the link at the bottom of the articles that says: Source Code.
|mARK 12/09/03 04:36:04 PM EST|
where does one find the "listing 1" mentioned in the article?
SYS-CON Events announced today that HPM Networks will exhibit at the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. For 20 years, HPM Networks has been integrating technology solutions that solve complex business challenges. HPM Networks has designed solutions for both SMB and enterprise customers throughout the San Francisco Bay Area.
Aug. 1, 2015 04:45 PM EDT Reads: 481
For IoT to grow as quickly as analyst firms’ project, a lot is going to fall on developers to quickly bring applications to market. But the lack of a standard development platform threatens to slow growth and make application development more time consuming and costly, much like we’ve seen in the mobile space. In his session at @ThingsExpo, Mike Weiner, Product Manager of the Omega DevCloud with KORE Telematics Inc., discussed the evolving requirements for developers as IoT matures and conducted a live demonstration of how quickly application development can happen when the need to comply wit...
Aug. 1, 2015 03:15 PM EDT Reads: 326
The Internet of Everything (IoE) brings together people, process, data and things to make networked connections more relevant and valuable than ever before – transforming information into knowledge and knowledge into wisdom. IoE creates new capabilities, richer experiences, and unprecedented opportunities to improve business and government operations, decision making and mission support capabilities.
Aug. 1, 2015 10:00 AM EDT Reads: 294
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 @ThingsExpo, James Kirkland, Red Hat's Chief Architect for the Internet of Things and Intelligent Systems, described how to revolutionize your archit...
Jul. 30, 2015 07:30 PM EDT Reads: 1,413
MuleSoft has announced the findings of its 2015 Connectivity Benchmark Report on the adoption and business impact of APIs. The findings suggest traditional businesses are quickly evolving into "composable enterprises" built out of hundreds of connected software services, applications and devices. Most are embracing the Internet of Things (IoT) and microservices technologies like Docker. A majority are integrating wearables, like smart watches, and more than half plan to generate revenue with APIs within the next year.
Jul. 30, 2015 02:30 PM EDT Reads: 124
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 Opening Keynote at 16th Cloud Expo, Sandy Carter, IBM General Manager Cloud Ecosystem and Developers, and a Social Business Evangelist, d...
Jul. 30, 2015 12:00 PM EDT Reads: 2,068
In his keynote at 16th Cloud Expo, Rodney Rogers, CEO of Virtustream, discussed the evolution of the company from inception to its recent acquisition by EMC – including personal insights, lessons learned (and some WTF moments) along the way. Learn how Virtustream’s unique approach of combining the economics and elasticity of the consumer cloud model with proper performance, application automation and security into a platform became a breakout success with enterprise customers and a natural fit for the EMC Federation.
Jul. 30, 2015 09:00 AM EDT Reads: 2,170
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 addressed this very serious issue of profound change in the industry.
Jul. 29, 2015 03:00 PM EDT Reads: 1,287
Discussions about cloud computing are evolving into discussions about enterprise IT in general. As enterprises increasingly migrate toward their own unique clouds, new issues such as the use of containers and microservices emerge to keep things interesting. In this Power Panel at 16th Cloud Expo, moderated by Conference Chair Roger Strukhoff, panelists addressed the state of cloud computing today, and what enterprise IT professionals need to know about how the latest topics and trends affect their organization.
Jul. 29, 2015 02:00 PM EDT Reads: 1,194
It is one thing to build single industrial IoT applications, but what will it take to build the Smart Cities and truly society-changing applications of the future? The technology won’t be the problem, it will be the number of parties that need to work together and be aligned in their motivation to succeed. In his session at @ThingsExpo, Jason Mondanaro, Director, Product Management at Metanga, discussed how you can plan to cooperate, partner, and form lasting all-star teams to change the world and it starts with business models and monetization strategies.
Jul. 28, 2015 04:30 PM EDT Reads: 1,772
Converging digital disruptions is creating a major sea change - Cisco calls this the Internet of Everything (IoE). IoE is the network connection of People, Process, Data and Things, fueled by Cloud, Mobile, Social, Analytics and Security, and it represents a $19Trillion value-at-stake over the next 10 years. In her keynote at @ThingsExpo, Manjula Talreja, VP of Cisco Consulting Services, discussed IoE and the enormous opportunities it provides to public and private firms alike. She will share what businesses must do to thrive in the IoE economy, citing examples from several industry sectors.
Jul. 28, 2015 11:00 AM EDT Reads: 2,047
There will be 150 billion connected devices by 2020. New digital businesses have already disrupted value chains across every industry. APIs are at the center of the digital business. You need to understand what assets you have that can be exposed digitally, what their digital value chain is, and how to create an effective business model around that value chain to compete in this economy. No enterprise can be complacent and not engage in the digital economy. Learn how to be the disruptor and not the disruptee.
Jul. 27, 2015 10:00 AM EDT Reads: 2,038
Akana has released Envision, an enhanced API analytics platform that helps enterprises mine critical insights across their digital eco-systems, understand their customers and partners and offer value-added personalized services. “In today’s digital economy, data-driven insights are proving to be a key differentiator for businesses. Understanding the data that is being tunneled through their APIs and how it can be used to optimize their business and operations is of paramount importance,” said Alistair Farquharson, CTO of Akana.
Jul. 27, 2015 09:00 AM EDT Reads: 330
Business as usual for IT is evolving into a "Make or Buy" decision on a service-by-service conversation with input from the LOBs. How does your organization move forward with cloud? In his general session at 16th Cloud Expo, Paul Maravei, Regional Sales Manager, Hybrid Cloud and Managed Services at Cisco, discusses how Cisco and its partners offer a market-leading portfolio and ecosystem of cloud infrastructure and application services that allow you to uniquely and securely combine cloud business applications and services across multiple cloud delivery models.
Jul. 27, 2015 08:00 AM EDT Reads: 1,907
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, analyzed 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 Payment...
Jul. 26, 2015 09:00 PM EDT Reads: 1,581
"Optimal Design is a technology integration and product development firm that specializes in connecting devices to the cloud," stated Joe Wascow, Co-Founder & CMO of Optimal Design, in this SYS-CON.tv interview at @ThingsExpo, held June 9-11, 2015, at the Javits Center in New York City.
Jul. 25, 2015 02:00 PM EDT Reads: 399
SYS-CON Events announced today that CommVault has been named “Bronze Sponsor” of SYS-CON's 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. A singular vision – a belief in a better way to address current and future data management needs – guides CommVault in the development of Singular Information Management® solutions for high-performance data protection, universal availability and simplified management of data on complex storage networks. CommVault's exclusive single-platform architecture gives companies unp...
Jul. 25, 2015 01:00 PM EDT Reads: 1,965
Electric Cloud and Arynga have announced a product integration partnership that will bring Continuous Delivery solutions to the automotive Internet-of-Things (IoT) market. The joint solution will help automotive manufacturers, OEMs and system integrators adopt DevOps automation and Continuous Delivery practices that reduce software build and release cycle times within the complex and specific parameters of embedded and IoT software systems.
Jul. 25, 2015 12:15 PM EDT Reads: 481
"ciqada is a combined platform of hardware modules and server products that lets people take their existing devices or new devices and lets them be accessible over the Internet for their users," noted Geoff Engelstein of ciqada, a division of Mars International, in this SYS-CON.tv interview at @ThingsExpo, held June 9-11, 2015, at the Javits Center in New York City.
Jul. 25, 2015 12:00 PM EDT Reads: 1,544
Internet of Things is moving from being a hype to a reality. Experts estimate that internet connected cars will grow to 152 million, while over 100 million internet connected wireless light bulbs and lamps will be operational by 2020. These and many other intriguing statistics highlight the importance of Internet powered devices and how market penetration is going to multiply many times over in the next few years.
Jul. 25, 2015 09:00 AM EDT Reads: 1,494