|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.
Fifty billion connected devices and still no winning protocols standards. HTTP, WebSockets, MQTT, and CoAP seem to be leading in the IoT protocol race at the moment but many more protocols are getting introduced on a regular basis. Each protocol has its pros and cons depending on the nature of the communications. Does there really need to be only one protocol to rule them all? Of course not. In his session at @ThingsExpo, Chris Matthieu, co-founder and CTO of Octoblu, walk you through how Oct...
Oct. 21, 2016 04:30 AM EDT Reads: 3,062
Major trends and emerging technologies – from virtual reality and IoT, to Big Data and algorithms – are helping organizations innovate in the digital era. However, to create real business value, IT must think beyond the ‘what’ of digital transformation to the ‘how’ to harness emerging trends, innovation and disruption. Architecture is the key that underpins and ties all these efforts together. In the digital age, it’s important to invest in architecture, extend the enterprise footprint to the cl...
Oct. 21, 2016 04:15 AM EDT Reads: 1,696
Almost everyone sees the potential of Internet of Things but how can businesses truly unlock that potential. The key will be in the ability to discover business insight in the midst of an ocean of Big Data generated from billions of embedded devices via Systems of Discover. Businesses will also need to ensure that they can sustain that insight by leveraging the cloud for global reach, scale and elasticity.
Oct. 21, 2016 04:00 AM EDT Reads: 10,921
A critical component of any IoT project is what to do with all the data being generated. This data needs to be captured, processed, structured, and stored in a way to facilitate different kinds of queries. Traditional data warehouse and analytical systems are mature technologies that can be used to handle certain kinds of queries, but they are not always well suited to many problems, particularly when there is a need for real-time insights.
Oct. 21, 2016 03:15 AM EDT Reads: 3,832
One of biggest questions about Big Data is “How do we harness all that information for business use quickly and effectively?” Geographic Information Systems (GIS) or spatial technology is about more than making maps, but adding critical context and meaning to data of all types, coming from all different channels – even sensors. In his session at @ThingsExpo, William (Bill) Meehan, director of utility solutions for Esri, will take a closer look at the current state of spatial technology and ar...
Oct. 21, 2016 03:15 AM EDT Reads: 1,641
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 Arch...
Oct. 21, 2016 02:00 AM EDT Reads: 5,895
The IoT industry is now at a crossroads, between the fast-paced innovation of technologies and the pending mass adoption by global enterprises. The complexity of combining rapidly evolving technologies and the need to establish practices for market acceleration pose a strong challenge to global enterprises as well as IoT vendors. In his session at @ThingsExpo, Clark Smith, senior product manager for Numerex, will discuss how Numerex, as an experienced, established IoT provider, has embraced a ...
Oct. 21, 2016 01:30 AM EDT Reads: 862
Everyone knows that truly innovative companies learn as they go along, pushing boundaries in response to market changes and demands. What's more of a mystery is how to balance innovation on a fresh platform built from scratch with the legacy tech stack, product suite and customers that continue to serve as the business' foundation. In his General Session at 19th Cloud Expo, Michael Chambliss, Head of Engineering at ReadyTalk, will discuss why and how ReadyTalk diverted from healthy revenue an...
Oct. 21, 2016 01:15 AM EDT Reads: 2,909
SYS-CON Events announced today that Hitrons Solutions will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Hitrons Solutions Inc. is distributor in the North American market for unique products and services of small and medium-size businesses, including cloud services and solutions, SEO marketing platforms, and mobile applications.
Oct. 21, 2016 12:30 AM EDT Reads: 1,920
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
Oct. 21, 2016 12:30 AM EDT Reads: 9,580
SYS-CON Events announced today that Super Micro Computer, Inc., a global leader in Embedded and IoT solutions, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 7-9, 2017, at the Javits Center in New York City, NY. Supermicro (NASDAQ: SMCI), the leading innovator in high-performance, high-efficiency server technology, is a premier provider of advanced server Building Block Solutions® for Data Center, Cloud Computing, Enterprise IT, Hadoop/Big Data, HPC and ...
Oct. 21, 2016 12:15 AM EDT Reads: 3,451
SYS-CON Events announced today that Streamlyzer will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. Streamlyzer is a powerful analytics for video streaming service that enables video streaming providers to monitor and analyze QoE (Quality-of-Experience) from end-user devices in real time.
Oct. 21, 2016 12:00 AM EDT Reads: 849
You have great SaaS business app ideas. You want to turn your idea quickly into a functional and engaging proof of concept. You need to be able to modify it to meet customers' needs, and you need to deliver a complete and secure SaaS application. How could you achieve all the above and yet avoid unforeseen IT requirements that add unnecessary cost and complexity? You also want your app to be responsive in any device at any time. In his session at 19th Cloud Expo, Mark Allen, General Manager of...
Oct. 21, 2016 12:00 AM EDT Reads: 755
SYS-CON Events announced today that LeaseWeb USA, a cloud Infrastructure-as-a-Service (IaaS) provider, will exhibit at the 19th International Cloud Expo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. LeaseWeb is one of the world's largest hosting brands. The company helps customers define, develop and deploy IT infrastructure tailored to their exact business needs, by combining various kinds cloud solutions.
Oct. 20, 2016 11:45 PM EDT Reads: 3,772
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 sessi...
Oct. 20, 2016 11:15 PM EDT Reads: 5,024
@ThingsExpo has been named the Top 5 Most Influential M2M Brand by Onalytica in the ‘Machine to Machine: Top 100 Influencers and Brands.' Onalytica analyzed the online debate on M2M by looking at over 85,000 tweets to provide the most influential individuals and brands that drive the discussion. According to Onalytica the "analysis showed a very engaged community with a lot of interactive tweets. The M2M discussion seems to be more fragmented and driven by some of the major brands present in the...
Oct. 20, 2016 10:45 PM EDT Reads: 11,066
The Open Connectivity Foundation (OCF), sponsor of the IoTivity open source project, and AllSeen Alliance, which provides the AllJoyn® open source IoT framework, today announced that the two organizations’ boards have approved a merger under the OCF name and bylaws. This merger will advance interoperability between connected devices from both groups, enabling the full operating potential of IoT and representing a significant step towards a connected ecosystem.
Oct. 20, 2016 10:45 PM EDT Reads: 1,093
SYS-CON Events announced today that Roundee / LinearHub will exhibit at the WebRTC Summit at @ThingsExpo, which will take place on November 1–3, 2016, at the Santa Clara Convention Center in Santa Clara, CA. LinearHub provides Roundee Service, a smart platform for enterprise video conferencing with enhanced features such as automatic recording and transcription service. Slack users can integrate Roundee to their team via Slack’s App Directory, and '/roundee' command lets your video conference ...
Oct. 20, 2016 10:30 PM EDT Reads: 1,958
As cloud adoption continues to transform business, today’s global enterprises are challenged with managing a growing amount of information living outside of the data center. The rapid adoption of IoT and increasingly mobile workforce are exacerbating the problem. Ensuring secure data sharing and efficient backup poses capacity and bandwidth considerations as well as policy and regulatory compliance issues.
Oct. 20, 2016 10:15 PM EDT Reads: 2,166
In the next forty months – just over three years – businesses will undergo extraordinary changes. The exponential growth of digitization and machine learning will see a step function change in how businesses create value, satisfy customers, and outperform their competition. In the next forty months companies will take the actions that will see them get to the next level of the game called Capitalism. Or they won’t – game over. The winners of today and tomorrow think differently, follow different...
Oct. 20, 2016 09:45 PM EDT Reads: 746