ClassNotFoundException/NoClassDefFoundError in my Java web application

What does this mean?

First, let’s see the meaning of java.lang.ClassNotFoundException:

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.

Usually, this happens when trying to open a connection manually in this form:

String jdbcDriver = "...'; //name of your driver
Class.forName(jdbcDriver);

Or when you refer to a class that belongs to an external library and strangely this class cannot be loaded when the application server tries to deploy the application.

Let’s see the meaning of java.lang.NoClassDefFoundError (emphasis mine):

Thrown if the Java Virtual Machine or a ClassLoader instance tries to load in the definition of a class (as part of a normal method call or as part of creating a new instance using the new expression) and no definition of the class could be found.

The searched-for class definition existed when the currently executing class was compiled, but the definition can no longer be found.

The last part says it all: the class existed at compile time i.e. when I compiled the application through my IDE, but it is not available at runtime i.e. when the application is deployed.

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how can I fix it?

In Java web applications, all third party libraries used by your application must go in WEB-INF/lib folder. Make sure that all the necessary libraries (jars) are placed there. You can check this easily:

- <webapp folder>
  - WEB-INF
    - lib
      + jar1
      + jar2
      + ...
  - META-INF
  - <rest of your folders>

This problem usually arises for JDBC connectivity jars (MySQL, Derby, MSSQL, Oracle, etc.) or web MVC frameworks libraries like JSF or Spring MVC.

Take into account that some third party libraries rely on other third party libraries, so you have to add all of them in WEB-INF/lib in order to make the application work. A good example of this is RichFaces 4 libraries, where you have to download and add the external libraries manually.

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Note for Maven users: you should not experience these problems unless you have set the libraries as provided, test or system. If set to provided, you’re responsible to add the libraries somewhere in the classpath. You can find more info about the dependency scopes here: Introduction to the Dependency Mechanism

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In case the library must be shared among several applications that will be deployed on your application server e.g. MySQL connector for two applications, there’s another alternative. Instead of deploying two war files each with their own MySQL connector library, place this library in the common library folder of the server application, this will enable the library to be in the classpath of all the deployed applications.

This folder vary from application server.

• Tomcat 7/8: <tomcat_home>/lib
• JBoss 7/Wildfly: <jboss_home>/standalone/lib