NoClassDefFoundError

Why This Happens

NoClassDefFoundError differs from ClassNotFoundException in that the class was found during compilation but is missing at runtime. This typically indicates a missing dependency JAR, a classpath configuration issue, or a static initializer failure in the class.

The Problem

// Compiled with library.jar but running without it
import com.library.Helper;
Helper.doSomething(); // NoClassDefFoundError at runtime

The Fix

// Ensure library.jar is included in runtime classpath
// Maven: check scope is not 'provided' if needed at runtime
// <dependency>
//   <groupId>com.library</groupId>
//   <artifactId>helper</artifactId>
//   <scope>compile</scope>  <!-- not 'provided' -->
// </dependency>

Step-by-Step Fix

1. 1

   Identify the missing class

   Read the error message for the class name. The slashes (/) indicate the package path.

2. 2

   Check runtime dependencies

   Verify that the JAR containing the class is present in the runtime classpath. Check if Maven/Gradle scope is set to 'provided' when it should be 'compile'.

3. 3

   Check for initialization errors

   If the class exists but its static initializer failed, the root cause is an earlier ExceptionInInitializerError. Check logs for that.