java.lang.NoClassDefFoundError is a runtime error in Java that occurs when the Java Virtual Machine (JVM) or a ClassLoader instance attempts to load the definition of a class that could not be found. The class definition exists at compile-time but is not available at runtime. This error typically occurs when the classpath is not set correctly or when there is a problem with the JAR file containing the class definition.
Suppose you have two Java classes:
// MainClass.java:
public class MainClass {
public static void main(String[] args) {
HelperClass helper = new HelperClass();
helper.displayMessage();
}
}
// HelperClass.java:
public class HelperClass {
public void displayMessage() {
System.out.println("Hello from HelperClass!");
}
}
Now, let’s say you successfully compile both classes using javac:
javac MainClass.java
javac HelperClass.java
This will create two .class files: MainClass.class and HelperClass.class.
Now, you run the MainClass
java MainClass
At this point, everything should work fine, and you’ll see the output: Hello from HelperClass!
Now, let’s simulate a NoClassDefFoundError. Delete the HelperClass.class file:
rm HelperClass.class
Now, when you try to run MainClass again: java MainClass
You will likely encounter a NoClassDefFoundError similar to the following:
Exception in thread "main" java.lang.NoClassDefFoundError: HelperClass
at MainClass.main(MainClass.java:3)
Caused by: java.lang.ClassNotFoundException: HelperClass
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:582)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:190)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:499)
... 1 more
The following steps should be followed to resolve a NoClassDefFoundError in Java:
Now, to resolve issue in the above example:
1. Check Classpath: Ensure that both MainClass.class and HelperClass.class are in the classpath. If you are using external libraries or JAR files, make sure they are included in the classpath.
2. Recompile: Recompile both classes to regenerate the .class files.
javac MainClass.java
javac HelperClass.java
3. Run Again: Run the MainClass again:
java MainClass
This should resolve the NoClassDefFoundError if the issue was related to a missing class file or a misconfigured classpath.
Remember that this is a simple example, and real-world scenarios may involve more complex class hierarchies and dependencies. Always check your classpath and ensure that all necessary classes and dependencies are available during runtime.