Java Class Loading Error/ Exception

-
Have you encountered some class loading/ definition issues in Java recently?
To expedite troubleshooting such issues, you need to be aware of the 3 main Java errors/ exceptions and to be able to differentiate amongst them. They are:
  1. java.lang.ClassNotFoundException
  2. java.lang.NoClassDefFoundError
  3. java.lang.UnsupportedClassVersionError

ClassNotFoundException

Happens when a class cannot be loaded at runtime. This offending class is usually not known a priori
This is often caused by the class loader not being able to dynamically load a required class. Examples of such offending class loaders are
  1. Class.forName()
  2. Classloader.findSystemClass()
  3. Classloader.loadClass()
  4. An IoC container (e.g. Spring)
The exception is thrown with the class that is not found. E.g.
 java.lang.ClassNotFoundException: oracle.jdbc.driver.OracleDriver

To resolve this, check the class against the classpath. The situation would be a lot more complicated when multiple class loaders come into play (E.g. when working with the Java EE containers)

NoClassDefFoundError

Happens when a class that existed at compile time could not be properly loaded at runtime.
The offending class was referenced in code (e.g. instantiated, static methods were called) and existed during compilation but now, could not be properly loaded.

There is a unique case to the NoClassDefFoundError where class initialisation fails.
E.g. java.lang.NoClassDefFoundError: Could not initialize class javax.swing.RepaintManager
This happens when the offending class is found but could not be properly initialised (due to exception/ error in the static initialiser or constructor)

Again in general, the fix could be simply making sure that the classpath contains the class definition. However, in the case of class initialisation failure, the solution would be far more complex.

UnsupportedClassVersionError

Happens when the class was compiled in a higher JDK version but running in a lower one.
To fix this, either:
  1. recompile the offending class/ jar to the lower version, or
  2. upgrade the JRE to the higher version

Comments

Post a Comment

Popular posts from this blog

Understanding ITIL Service Management the UML way…

-
Image
I originally had a great deal of difficult reconciling the differences amongst ITIL Change Management, Incident Management, and Problem Management. Having researched on the subjects and understanding them better now, I’ve tried to put the concepts together using a UML class diagram to capture my understanding and to keep it imprinted. Similarly, I wanted to differentiate the various categories of changes (Standard, Emergency, Normal) using an easy to understand & self-explanatory manner – a UML Activity Diagram!
Read more

How to depict (Professional-Looking) Logical Network Diagrams in Astah

-
Image
I have always had to create (professional-looking) UML diagrams using Visio-like icons. You’ve got to admit that showing a bunch of boring UML boxes hardly interests anyone less technical (like anyone whose designation has the words “manager” or “director”). This is actually possible in Astah! I’ve created a template for future projects that allows me to produce logical network diagrams that look like the following: To create this diagram, you need to do the following: Assign various icons to stereotypes. I created the following for my use: Create a new UML Deployment Diagram Every element in the diagram is created as a Node The nodes are subsequently assigned the appropriate <<stereotype>>. E.g. Internet is <<internet>>, WebZone FW is <<firewall>>, Web LB is <<loadBalancer>> Ensure the “Icon Notation” is set to “Customized Icon” for each node Instead of the boring boxes, the corresponding icons automatically gets displayed!
Read more