|By Lakshmi Shankar, Simon Burns||
|March 22, 2006 02:00 PM EST||
The class loading component is fundamental to the Java virtual machine. Though developers generally have a good grasp of the basics of class loading, when a problem occurs, they may have a hard time diagnosing and determining a solution. In this four-part article series, we discuss the various class loading problems that you may encounter in your Java development and illustrate why they occur and how to resolve them. These insights should help you understand and resolve common Java exceptions, such as NoClassDefFoundError and ClassNotFoundException, in addition to more challenging problems, such as class loader constraint violations and deadlocks. In this first article, we describe in detail how Java class loading works and discuss the tools available in the JVM to help you diagnose class loading problems.
Class loaders are responsible for loading classes into the Java Virtual Machine (JVM). Simple applications can use the Java platform's built-in class loading facility to load their classes; more complex applications tend to define their own custom class loaders. No matter what kind of class loader you're using, however, there are many problems that can occur in the class loading process. If you want to avoid such problems, you need to understand the fundamental mechanics of class loading. When problems do occur, an appreciation of the available diagnostic features and debugging techniques should help you resolve them.
In this series of articles, we'll provide an in-depth look at class loading problems and use comprehensive examples to illustrate them. The first section of this introductory article describes the fundamentals of class loading; the second introduces some JVM debugging features. The next three articles in this series will focus on resolving class loading exceptions and illustrate some of the trickier class loading problems that you may encounter.
Fundamentals of class loading
This section describes the core concepts of class loading to provide a knowledge base for the rest of this series.
Class loader delegation
The class loader delegation model is the graph of class loaders that pass loading requests to each other. The bootstrap class loader is at the root of this graph. Class loaders are created with a single delegation parent and looks for a class in the following places:
Because the parent class loader is always given the opportunity to load a class first, the class is loaded by the class loader nearest the root. This means that all core bootstrap classes are loaded by the bootstrap loader, which makes sure that the correct versions of classes such as java.lang.Object are loaded. This also has the effect of only allowing a class loader to see classes loaded by itself or its parent or ancestors; it cannot see classes loaded by its children.
Figure 1 shows the three standard class loaders.
Unlike all other class loaders , the bootstrap class loader (also known as the primordial class loader) cannot be instantiated by Java code. (Often, this is because it is implemented natively as part of the VM itself.) This class loader loads the core system classes from the boot classpath, which is normally the JAR files located in the jre/lib directory. However, you can modify this classpath using the -Xbootclasspath command-line options (which we describe later).
The extension class loader (also known as the standard extensions class loader) is a child of the bootstrap class loader. Its primary responsibility is to load classes from the extension directories, normally located the jre/lib/ext directory. This provides the ability to simply drop in new extensions, such as various security extensions, without requiring modification to the user's classpath.
The system class loader (also known as the application class loader) is the class loader responsible for loading code from the path specified by the CLASSPATH environment variable. By default, this class loader is the parent of any class loader created by the user. This is also the class loader returned by the ClassLoader.getSystemClassLoader() method.
Table 1 summarizes the command-line options for setting the classpaths of the three standard class loaders:
The phases of class loading
The loading of a class can essentially be broken down into three phases: loading, linking, and initializing.
Most, if not all, problems relating to class loading can be tracked down to a problem occurring in one of these phases. Therefore, a thorough understanding of each phase helps in the diagnosing of class loading problems. The phases are illustrated in Figure 2:
The loading phase consists of locating the required class file (by searching though the respective classpaths) and loading in the bytecode. Within the JVM, the loading process gives a very basic memory structure to the class object. Methods, fields, and other referenced classes are not dealt with at this stage. As a result, the class is not usable.
Linking is the most complicated of the three phases. It can be broken down into three main stages:
- Bytecode verification. The class loader does a number of checks on the bytecodes of the class to ensure that it is well formed and well behaved.
- Class preparation. This stage prepares the necessary data structures that represent fields, methods, and implemented interfaces that are defined within each class.
- Resolving. In this stage, the class loader loads all the other classes referenced by a particular class. The classes can be referenced in a number of ways:
- Method signatures
- Local variables used in methods
At the end of these three phases, a class is fully loaded and is ready for use. Note that class loading can be performed in a lazy manner and therefore some parts of the class loading process may be done on first use of the class rather than at load time.
Explicit vs. implicit loading
There are two ways in which classes can be loaded -- explicitly or implicitly -- with subtle variations between the two. Explicit class loading occurs when a class is loaded using one of the following method calls:
- cl.loadClass() (where cl is an instance of java.lang.ClassLoader)
- Class.forName() (the starting class loader is the defining class loader of the current class)
Implicit class loading occurs when a class is loaded as result of a reference, instantiation, or inheritance (not via an explicit method call). In each of these cases, the loading is initiated under the covers and the JVM resolves the necessary references and loads the class. As with explicit class loading, if the class is already loaded, then a reference is simply returned; otherwise, the loader goes through the delegation model to load the class.
As more intelligent IoT applications shift into gear, they’re merging into the ever-increasing traffic flow of the Internet. It won’t be long before we experience bottlenecks, as IoT traffic peaks during rush hours. Organizations that are unprepared will find themselves by the side of the road unable to cross back into the fast lane. As billions of new devices begin to communicate and exchange data – will your infrastructure be scalable enough to handle this new interconnected world?
Sep. 1, 2015 08:00 AM EDT Reads: 165
While many app developers are comfortable building apps for the smartphone, there is a whole new world out there. In his session at @ThingsExpo, Narayan Sainaney, Co-founder and CTO of Mojio, will discuss how the business case for connected car apps is growing and, with open platform companies having already done the heavy lifting, there really is no barrier to entry.
Sep. 1, 2015 08:00 AM EDT Reads: 144
With the Apple Watch making its way onto wrists all over the world, it’s only a matter of time before it becomes a staple in the workplace. In fact, Forrester reported that 68 percent of technology and business decision-makers characterize wearables as a top priority for 2015. Recognizing their business value early on, FinancialForce.com was the first to bring ERP to wearables, helping streamline communication across front and back office functions. In his session at @ThingsExpo, Kevin Roberts, GM of Platform at FinancialForce.com, will discuss the value of business applications on wearable ...
Sep. 1, 2015 08:00 AM EDT
WebRTC has had a real tough three or four years, and so have those working with it. Only a few short years ago, the development world were excited about WebRTC and proclaiming how awesome it was. You might have played with the technology a couple of years ago, only to find the extra infrastructure requirements were painful to implement and poorly documented. This probably left a bitter taste in your mouth, especially when things went wrong.
Sep. 1, 2015 03:00 AM EDT Reads: 462
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.
Aug. 31, 2015 09:00 PM EDT Reads: 372
The Internet of Things is in the early stages of mainstream deployment but it promises to unlock value and rapidly transform how organizations manage, operationalize, and monetize their assets. IoT is a complex structure of hardware, sensors, applications, analytics and devices that need to be able to communicate geographically and across all functions. Once the data is collected from numerous endpoints, the challenge then becomes converting it into actionable insight.
Aug. 31, 2015 07:00 PM EDT
Consumer IoT applications provide data about the user that just doesn’t exist in traditional PC or mobile web applications. This rich data, or “context,” enables the highly personalized consumer experiences that characterize many consumer IoT apps. This same data is also providing brands with unprecedented insight into how their connected products are being used, while, at the same time, powering highly targeted engagement and marketing opportunities. In his session at @ThingsExpo, Nathan Treloar, President and COO of Bebaio, will explore examples of brands transforming their businesses by t...
Aug. 31, 2015 06:00 PM EDT Reads: 243
With the proliferation of connected devices underpinning new Internet of Things systems, Brandon Schulz, Director of Luxoft IoT – Retail, will be looking at the transformation of the retail customer experience in brick and mortar stores in his session at @ThingsExpo. Questions he will address include: Will beacons drop to the wayside like QR codes, or be a proximity-based profit driver? How will the customer experience change in stores of all types when everything can be instrumented and analyzed? As an area of investment, how might a retail company move towards an innovation methodolo...
Aug. 31, 2015 02:30 PM EDT Reads: 464
The Internet of Things (IoT) is about the digitization of physical assets including sensors, devices, machines, gateways, and the network. It creates possibilities for significant value creation and new revenue generating business models via data democratization and ubiquitous analytics across IoT networks. The explosion of data in all forms in IoT requires a more robust and broader lens in order to enable smarter timely actions and better outcomes. Business operations become the key driver of IoT applications and projects. Business operations, IT, and data scientists need advanced analytics t...
Aug. 31, 2015 02:30 PM EDT Reads: 422
Contrary to mainstream media attention, the multiple possibilities of how consumer IoT will transform our everyday lives aren’t the only angle of this headline-gaining trend. There’s a huge opportunity for “industrial IoT” and “Smart Cities” to impact the world in the same capacity – especially during critical situations. For example, a community water dam that needs to release water can leverage embedded critical communications logic to alert the appropriate individuals, on the right device, as soon as they are needed to take action.
Aug. 31, 2015 12:00 PM EDT
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. 31, 2015 11:30 AM EDT Reads: 899
SYS-CON Events announced today that Micron Technology, Inc., a global leader in advanced semiconductor systems, 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. Micron’s broad portfolio of high-performance memory technologies – including DRAM, NAND and NOR Flash – is the basis for solid state drives, modules, multichip packages and other system solutions. Backed by more than 35 years of technology leadership, Micron's memory solutions enable the world's most innovative computing, consumer,...
Aug. 31, 2015 11:15 AM EDT Reads: 232
Through WebRTC, audio and video communications are being embedded more easily than ever into applications, helping carriers, enterprises and independent software vendors deliver greater functionality to their end users. With today’s business world increasingly focused on outcomes, users’ growing calls for ease of use, and businesses craving smarter, tighter integration, what’s the next step in delivering a richer, more immersive experience? That richer, more fully integrated experience comes about through a Communications Platform as a Service which allows for messaging, screen sharing, video...
Aug. 31, 2015 10:30 AM EDT Reads: 667
SYS-CON Events announced today that Pythian, a global IT services company specializing in helping companies leverage disruptive technologies to optimize revenue-generating systems, has been named “Bronze Sponsor” of SYS-CON's 17th Cloud Expo, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Founded in 1997, Pythian is a global IT services company that helps companies compete by adopting disruptive technologies such as cloud, Big Data, advanced analytics, and DevOps to advance innovation and increase agility. Specializing in designing, imple...
Aug. 31, 2015 10:15 AM EDT Reads: 309
In his session at @ThingsExpo, Lee Williams, a producer of the first smartphones and tablets, will talk about how he is now applying his experience in mobile technology to the design and development of the next generation of Environmental and Sustainability Services at ETwater. He will explain how M2M controllers work through wirelessly connected remote controls; and specifically delve into a retrofit option that reverse-engineers control codes of existing conventional controller systems so they don't have to be replaced and are instantly converted to become smart, connected devices.
Aug. 31, 2015 09:30 AM EDT Reads: 125
SYS-CON Events announced today that IceWarp 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. IceWarp, the leader of cloud and on-premise messaging, delivers secured email, chat, documents, conferencing and collaboration to today's mobile workforce, all in one unified interface
Aug. 31, 2015 04:00 AM EDT Reads: 429
As more and more data is generated from a variety of connected devices, the need to get insights from this data and predict future behavior and trends is increasingly essential for businesses. Real-time stream processing is needed in a variety of different industries such as Manufacturing, Oil and Gas, Automobile, Finance, Online Retail, Smart Grids, and Healthcare. Azure Stream Analytics is a fully managed distributed stream computation service that provides low latency, scalable processing of streaming data in the cloud with an enterprise grade SLA. It features built-in integration with Azur...
Aug. 28, 2015 07:45 PM EDT Reads: 221
Akana has announced the availability of the new Akana Healthcare Solution. The API-driven solution helps healthcare organizations accelerate their transition to being secure, digitally interoperable businesses. It leverages the Health Level Seven International Fast Healthcare Interoperability Resources (HL7 FHIR) standard to enable broader business use of medical data. Akana developed the Healthcare Solution in response to healthcare businesses that want to increase electronic, multi-device access to health records while reducing operating costs and complying with government regulations.
Aug. 26, 2015 07:00 AM EDT Reads: 195
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. 2, 2015 11:15 AM EDT Reads: 558
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: 490