VIVEK SHAH

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Difference between ClassNotFoundException and NoClassDefFoundError

Posted by vivek shah
Wednesday, January 12, 2011
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As Java developers we've all seen java.lang.ClassNotFoundException and java.lang.NoClassDefFoundError at some point and spend hours trying to track issues with Classloader, CLASSPATH and missing classes.
A ClassNotFoundException is thrown when the reported class is not found by the ClassLoader. This typically means that the class is missing from the CLASSPATH. It could also mean that the class in question is trying to be loaded from another class which was loaded in a parent classloader and hence the class from the child classloader is not visible. This is sometimes the case when working in more complex environments like an App Server (WebSphere is infamous for such classloader issues).
People often tend to confuse java.lang.NoClassDefFoundError with java.lang.ClassNotFoundException however there's an important distinction. For example an exception (an error really since java.lang.NoClassDefFoundError is a subclass of java.lang.Error) like

java.lang.NoClassDefFoundError:
org/apache/crimson/tree/DOMImplementationImpl


does not mean that the DOMImplementationImpl class is not in the CLASSPATH. Infact its quite the opposite. It means that the class DOMImplementationImpl was found by the ClassLoader however when trying to load the class, it ran into an error reading the class definition. This typically happens when the class in question has static blocks or members which use a Class that's not found by the ClassLoader. So to find the culprit, view the source of the class in question (DOMImplementationImpl in this case) and look for code using static blocks or static members. If you don't have access the the source, then simply decompile it using JAD.
On examining the code, say you find a line of code like below, make sure that the class SomeClass in in your CLASSPATH.

private static SomeClass foo = new SomeClass();


Tip : To find out which jar a class belongs to, you can use the web site jarFinder. This allows you to specify a class name using wildcards and it searches for the class in its database of jars. jarhoo allows you to do the same thing but its no longer free to use.
If you would like to locate the which jar a class belongs to in a local path, you can use a utility like jarscan. You just specify the class you'd like to locate and the root directory path where you'd like it to start searching for the class in jars and zip files.

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Findbugs, PMD, Checkstyle, Macker (For Better Java Development) Part-I

Posted by vivek shah
Monday, January 10, 2011
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Macker is a build-time architectural rule checking utility for Java developers. It's meant to model the architectural ideals programmers always dream up for their projects, and then break -- it helps keep code clean and consistent. You can tailor a rules file to suit a specific project's structure, or write some general "good practice" rules for your code. Macker doesn't try to shove anybody else's rules down your throat; it's flexible, and writing a rules file is part of the development process for each unique project.

PMD scans Java source code and looks for potential problems like:
  • Possible bugs - empty try/catch/finally/switch statements
  • Dead code - unused local variables, parameters and private methods
  • Suboptimal code - wasteful String/StringBuffer usage
  • Overcomplicated expressions - unnecessary if statements, for loops that could be while loops
  • Duplicate code - copied/pasted code means copied/pasted bugs  
Checkstyle is a development tool to help programmers write Java code that adheres to a coding standard. It automates the process of checking Java code to spare humans of this boring (but important) task. This makes it ideal for projects that want to enforce a coding standard.

FindBugs is just one of many available static analysis tools


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