Java 'java.lang.NoClassDefFoundError' vs 'java.lang.ClassNotFoundException' at runtime: what is the difference and how do I fix a missing class?

Cause: The two are different failures. ClassNotFoundException is a checked exception thrown when code loads a class by its string name (Class.forName, ClassLoader.loadClass) and no definition can be found. NoClassDefFoundError is an Error (a LinkageError) thrown when the JVM or a class loader needs a class's definition to run normally (a method call or a new expression) and cannot find it, even though the class was present when the calling code was compiled. In practice: a jar is missing from the runtime classpath, a dependency was marked provided/test scope or excluded, two versions conflict, the jar is not in the deployed artifact (thin jar instead of fat jar), or a class's static initializer failed earlier (then the first error is the real cause and later ones say 'Could not initialize class'). Fix (best first): 1. Read the whole stack trace and look for an earlier 'Caused by' or an ExceptionInInitializerError. If one exists, fix that initializer failure first. 2. Identify the missing class name, then find which jar should provide it (for Maven: mvn dependency:tree, and look for it with -Dincludes=groupId:artifactId). 3. Make sure the dependency is on the runtime classpath: compile or runtime scope, not provided or test, unless the container supplies it. Remember Maven scopes: provided is expected from the JDK or container at runtime and is not transitive. 4. Check that the packaged application actually contains the jar (executable/fat jar, WAR WEB-INF/lib, or the -cp / --class-path used at launch). Compiling successfully proves nothing about the launch classpath. 5. Resolve version conflicts by declaring the dependency explicitly or using dependencyManagement, then rebuild cleanly. 6. For ClassNotFoundException from reflection or plugin loading, check the string name for typos and that the right class loader is used. Verify: Rebuild from clean, start the application using the real launch command, and confirm the code path that failed now runs without either error. Sources: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/NoClassDefFoundError.html https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/ClassNotFoundException.html https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html