|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.
SYS-CON Events announced today that Intelligent Systems Services will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Established in 1994, Intelligent Systems Services Inc. is located near Washington, DC, with representatives and partners nationwide. ISS’s well-established track record is based on the continuous pursuit of excellence in designing, implementing and supporting nationwide clients’ mission-critical systems. ISS has completed many successful projects in Healthcare, Commercial, Manufacturing, ...
Apr. 25, 2015 02:15 AM EDT Reads: 2,758
Sonus Networks introduced the Sonus WebRTC Services Solution, a virtualized Web Real-Time Communications (WebRTC) offer, purpose-built for the Cloud. The WebRTC Services Solution provides signaling from WebRTC-to-WebRTC applications and interworking from WebRTC-to-Session Initiation Protocol (SIP), delivering advanced real-time communications capabilities on mobile applications and on websites, which are accessible via a browser.
Apr. 25, 2015 02:00 AM EDT Reads: 1,952
SYS-CON Events announced today that B2Cloud, a provider of enterprise resource planning software, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. B2cloud develops the software you need. They have the ideal tools to help you work with your clients. B2Cloud’s main solutions include AGIS – ERP, CLOHC, AGIS – Invoice, and IZUM
Apr. 25, 2015 02:00 AM EDT Reads: 3,553
SYS-CON Events announced today that Tufin, the market-leading provider of Security Policy Orchestration Solutions, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. As the market leader of Security Policy Orchestration, Tufin automates and accelerates network configuration changes while maintaining security and compliance. Tufin's award-winning Orchestration Suite™ gives IT organizations the power and agility to enforce security policy across complex, multi-vendor enterprise networks. With more than 1...
Apr. 25, 2015 01:45 AM EDT Reads: 3,567
SYS-CON Events announced today that Cloudian, Inc., the leading provider of hybrid cloud storage solutions, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Cloudian, Inc., is a Foster City, California - based software company specializing in cloud storage software. The main product is Cloudian, an Amazon S3-compliant cloud object storage platform, the bedrock of cloud computing systems, that enables cloud service providers and enterprises to build reliable, affordable and scalable cloud storage solu...
Apr. 25, 2015 01:00 AM EDT Reads: 2,723
“With easy-to-use SDKs for Atmel’s platforms, IoT developers can now reap the benefits of realtime communication, and bypass the security pitfalls and configuration complexities that put IoT deployments at risk,” said Todd Greene, founder & CEO of PubNub. PubNub will team with Atmel at CES 2015 to launch full SDK support for Atmel’s MCU, MPU, and Wireless SoC platforms. Atmel developers now have access to PubNub’s secure Publish/Subscribe messaging with guaranteed ¼ second latencies across PubNub’s 14 global points-of-presence. PubNub delivers secure communication through firewalls, proxy ser...
Apr. 25, 2015 01:00 AM EDT Reads: 3,838
SYS-CON Events announced today that Gridstore™, the leader in hyper-converged infrastructure purpose-built to optimize Microsoft workloads, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Gridstore™ is the leader in hyper-converged infrastructure purpose-built for Microsoft workloads and designed to accelerate applications in virtualized environments. Gridstore’s hyper-converged infrastructure is the industry’s first all flash version of HyperConverged Appliances that include both compute and storag...
Apr. 25, 2015 12:45 AM EDT Reads: 4,633
SYS-CON Events announced today that IDenticard will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. IDenticard™ is the security division of Brady Corp (NYSE: BRC), a $1.5 billion manufacturer of identification products. We have small-company values with the strength and stability of a major corporation. IDenticard offers local sales, support and service to our customers across the United States and Canada. Our partner network encompasses some 300 of the world's leading systems integrators and security s...
Apr. 25, 2015 12:00 AM EDT Reads: 5,237
Containers and microservices have become topics of intense interest throughout the cloud developer and enterprise IT communities. Accordingly, attendees at the upcoming 16th Cloud Expo at the Javits Center in New York June 9-11 will find fresh new content in a new track called PaaS | Containers & Microservices Containers are not being considered for the first time by the cloud community, but a current era of re-consideration has pushed them to the top of the cloud agenda. With the launch of Docker's initial release in March of 2013, interest was revved up several notches. Then late last...
Apr. 24, 2015 09:00 PM EDT Reads: 2,783
So I guess we’ve officially entered a new era of lean and mean. I say this with the announcement of Ubuntu Snappy Core, “designed for lightweight cloud container hosts running Docker and for smart devices,” according to Canonical. “Snappy Ubuntu Core is the smallest Ubuntu available, designed for security and efficiency in devices or on the cloud.” This first version of Snappy Ubuntu Core features secure app containment and Docker 1.6 (1.5 in main release), is available on public clouds, and for ARM and x86 devices on several IoT boards. It’s a Trend! This announcement comes just as...
Apr. 24, 2015 08:00 PM EDT Reads: 1,324
SYS-CON Events announced today the IoT Bootcamp – Jumpstart Your IoT Strategy, being held June 9–10, 2015, in conjunction with 16th Cloud Expo and Internet of @ThingsExpo at the Javits Center in New York City. This is your chance to jumpstart your IoT strategy. Combined with real-world scenarios and use cases, the IoT Bootcamp is not just based on presentations but includes hands-on demos and walkthroughs. We will introduce you to a variety of Do-It-Yourself IoT platforms including Arduino, Raspberry Pi, BeagleBone, Spark and Intel Edison. You will also get an overview of cloud technologies s...
Apr. 24, 2015 05:30 PM EDT Reads: 2,976
“In the past year we've seen a lot of stabilization of WebRTC. You can now use it in production with a far greater degree of certainty. A lot of the real developments in the past year have been in things like the data channel, which will enable a whole new type of application," explained Peter Dunkley, Technical Director at Acision, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
Apr. 24, 2015 03:00 PM EDT Reads: 4,403
Health care systems across the globe are under enormous strain, as facilities reach capacity and costs continue to rise. M2M and the Internet of Things have the potential to transform the industry through connected health solutions that can make care more efficient while reducing costs. In fact, Vodafone's annual M2M Barometer Report forecasts M2M applications rising to 57 percent in health care and life sciences by 2016. Lively is one of Vodafone's health care partners, whose solutions enable older adults to live independent lives while staying connected to loved ones. M2M will continue to gr...
Apr. 24, 2015 03:00 PM EDT Reads: 1,470
While not quite mainstream yet, WebRTC is starting to gain ground with Carriers, Enterprises and Independent Software Vendors (ISV’s) alike. WebRTC makes it easy for developers to add audio and video communications into their applications by using Web browsers as their platform. But like any market, every customer engagement has unique requirements, as well as constraints. And of course, one size does not fit all. In her session at WebRTC Summit, Dr. Natasha Tamaskar, Vice President, Head of Cloud and Mobile Strategy at GENBAND, will explore what is needed to take a real time communications ...
Apr. 24, 2015 03:00 PM EDT Reads: 1,759
The best mobile applications are augmented by dedicated servers, the Internet and Cloud services. Mobile developers should focus on one thing: writing the next socially disruptive viral app. Thanks to the cloud, they can focus on the overall solution, not the underlying plumbing. From iOS to Android and Windows, developers can leverage cloud services to create a common cross-platform backend to persist user settings, app data, broadcast notifications, run jobs, etc. This session provides a high level technical overview of many cloud services available to mobile app developers, includi...
Apr. 24, 2015 03:00 PM EDT Reads: 1,331
SYS-CON Events announced today that Vicom Computer Services, Inc., a provider of technology and service solutions, will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. They are located at booth #427. Vicom Computer Services, Inc. is a progressive leader in the technology industry for over 30 years. Headquartered in the NY Metropolitan area. Vicom provides products and services based on today’s requirements around Unified Networks, Cloud Computing strategies, Virtualization around Software defined Data Ce...
Apr. 24, 2015 02:30 PM EDT Reads: 1,709
Dave will share his insights on how Internet of Things for Enterprises are transforming and making more productive and efficient operations and maintenance (O&M) procedures in the cleantech industry and beyond. Speaker Bio: Dave Landa is chief operating officer of Cybozu Corp (kintone US). Based in the San Francisco Bay Area, Dave has been on the forefront of the Cloud revolution driving strategic business development on the executive teams of multiple leading Software as a Services (SaaS) application providers dating back to 2004. Cybozu's kintone.com is a leading global BYOA (Build Your O...
Apr. 24, 2015 02:00 PM EDT Reads: 1,484
SYS-CON Events announced today that Ciqada will exhibit at SYS-CON's @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Ciqada™ makes it easy to connect your products to the Internet. By integrating key components - hardware, servers, dashboards, and mobile apps - into an easy-to-use, configurable system, your products can quickly and securely join the internet of things. With remote monitoring, control, and alert messaging capability, you will meet your customers' needs of tomorrow - today! Ciqada. Let your products take flight. For more inform...
Apr. 24, 2015 01:00 PM EDT Reads: 1,876
SYS-CON Media announced today that @WebRTCSummit Blog, the largest WebRTC resource in the world, has been launched. @WebRTCSummit Blog offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. @WebRTCSummit Blog can be bookmarked ▸ Here @WebRTCSummit conference site can be bookmarked ▸ Here
Apr. 24, 2015 01:00 PM EDT Reads: 2,470
What exactly is a cognitive application? In her session at 16th Cloud Expo, Ashley Hathaway, Product Manager at IBM Watson, will look at the services being offered by the IBM Watson Developer Cloud and what that means for developers and Big Data. She'll explore how IBM Watson and its partnerships will continue to grow and help define what it means to be a cognitive service, as well as take a look at the offerings on Bluemix. She will also check out how Watson and the Alchemy API team up to offer disruptive APIs to developers.
Apr. 24, 2015 12:00 PM EDT Reads: 1,798