The class that is not available on the classpath

java.lang.ClassNotFoundException
Javadoc
Thrown when an application tries to load in a class through its string name using:
  • The forName method in class Class.
  • The findSystemClass method in class ClassLoader .
  • The loadClass method in class ClassLoader.

but no definition for the class with the specified name could be found.

As of release 1.4, this exception has been retrofitted to conform to the general purpose exception-chaining mechanism. The "optional exception that was raised while loading the class" that may be provided at construction time and accessed via the {@link #getException()} method is now known as the cause, and may be accessed via the {@link Throwable#getCause()} method, as well as the aforementioned "legacy method." @author unascribed @see java.lang.Class#forName(java.lang.String) @see java.lang.ClassLoader#findSystemClass(java.lang.String) @see java.lang.ClassLoader#loadClass(java.lang.String, boolean) @since JDK1.0

at java.net.URLClassLoader$1.run
at java.net.URLClassLoader$1.run
at java.security.AccessController.doPrivileged
(source)
at java.net.URLClassLoader.findClass
(source)
3 frames hidden

Typical Exception Messages

  1. org.xmlpull.v1.XmlPullParserFactory
  2. com.mysql.jdbc.Driver
  3. org.slf4j.LoggerFactory
  4. cpw.mods.fml.common.launcher.FMLTweaker
  5. org.apache.commons.logging.LogFactory
  6. javafx.application.Application
  7. org.apache.log4j.Logger
  8. com.google.common.base.Function
  9. org.lwjgl.LWJGLException
  10. org.apache.commons.lang3.text.StrLookup

Origin

Specific cases

Solution

poroszd, 3 months ago

java.lang.ClassNotFoundException is one of the most frequent exceptions in Java. It is a runtime error, however, it does not signal a bug in your application logic, but rather an issue with the environment (build system, dependency management, runtime environment, etc.).

According to the javadoc, it "is thrown when an application tries to load in a class through its string name". There are several different situations leading to this error and you have to examine the stack trace thoroughly to be able to decide how to solve this problem.

The first thing to check is whether you get this checked exception directly, or wrapped in a java.class.NoClassDefFoundException. The latter happens when the class not found was resolved during compilation time, but not found during runtime, which is a bit different than the first case and it needs special care. If this is what happened to you, then you should read that article (coming soon).

Next we ask what is the class that was not found. Fortunately, in most cases, you can see it in the message of the exception. If this class was specified by you (either directly in the code, or via a configuration file), double check that you did not misspell it. Some practical advices:

  • Make sure you use the fully qualified name of the class (e.g. org.postgresql.ds.PGSimpleDataSource)
  • When loading a nested class, make sure you use the proper name (joined with a $ sign, e.g. test.MyClass$InnerClass).
  • If you are unsure, you can check the name of the class in the code by using MyClass.class.getName()

If none of the above helped then you have an issue with your classpath, which requires some time and experience to resolve. When the java.lang.ClassNotFoundException is related to a third party library, you might want to ask for help in their support forums. However, if you want to debug it yourself, let me equip you with some theoretical and practical knowledge.

  • To understand what is class loading in Java, start with this StackOverflow question or the Java Language Specification §12.2. Roughly speaking, the classes that your application use are located in jar files on the file system. The classpath is a list of jar files which are searched through when your application loads a class. The java.lang.ClassNotFoundException means that the class you wanted to load is not present in any of the jar files on the classpath.
  • The configuration of your classpath depends on your runtime environment (i.e. do you run the application from console, from IDE, web container, etc), you can find more information on the specific forums. However, if you can use breakpoints to stop at specific points in your application, stop at the first line of java.net.URLClassLoader.findClass and check the value of the field ucp.
  • You should have an idea on which jar file contains the class that could not be loaded, and look for that jar in the classpath. If it is not there, you have to add that jar to the classpath (precise details depend on the runtime environment).
  • If you find the jar you suspect to contain your class, check if it is true by opening the jar file (they are simple zip files actually) and look for the class definition in the directory structure in it (the directory structure must be identical to the package structure of the java code). At this point you should realize that the class is not present, otherwise the problem would have been found previously during debugging.
  • There are two reasons for class not being present in the jar: the first is that you have an incorrect version of the library you wanted to use. To fix this, you have to tweak your build system (maven, ant, sbt, gradle etc) by making sure that you configured the right version of the library as the dependency, and that you don’t have conflicting library versions. To find out how to do this, look for help on the forums of that build system.
  • The other reason of the class not being present is that it’s a bug in that library. If this is a third-party library, you might want to check the support forum and the release notes. If this library was written by you, then try to rebuild this library and check if the class you look for is present in the resulting jar file.

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