ClassNotFoundException and NoClassDefFoundError are both related to class loading errors in Java, but they occur in different circumstances.
ClassNotFoundException occurs when the JVM tries to load a class at runtime, but it cannot find the definition of the class. This can happen when the class is not on the classpath, or when the class is in a different package or module that is not visible to the classloader. For example, if you try to load a class using Class.forName(“com.example.MyClass”), but the com.example package is not on the classpath, you will get a ClassNotFoundException.
NoClassDefFoundError, on the other hand, occurs when the JVM tries to load a class that was available at compile time, but is not available at runtime. This can happen when a class that is a dependency of another class is present at compile time, but is missing at runtime. For example, if you have a class that uses the Apache Commons Lang library, but the Commons Lang JAR file is not on the classpath at runtime, you will get a NoClassDefFoundError.
In summary, ClassNotFoundException occurs when a class is not found at runtime, while NoClassDefFoundError occurs when a class was present at compile time but is missing at runtime. To resolve these errors, you need to ensure that all necessary classes are available on the classpath at runtime. This may involve adding JAR files to the classpath or ensuring that all required modules are available.