NoClassDefFoundError in Java

The NoClassDefFoundError is a runtime error. It occurs when the Java Virtual Machine (JVM) or a ClassLoader instance tries to load a class, but cannot find it at the runtime, even though it was available during the compile time.

In this blog, we will learn why you are getting a NoClassDefFoundError in Java in more detail, also we will see how we can resolve the error NoClassDefFoundError in Java.

Table of Contents:

• Introduction to NoClassDefFoundError in Java
• Common Causes of NoClassDefFoundError in Java
• How to Resolve the NoClassDefFoundError in Java?
• NoClassDefFoundError vs ClassNotFoundException in Java
  • NoClassDefFoundError in Java
  • ClassNotFoundException in Java
• Conclusion

Introduction to NoClassDefFoundError in Java

The NoClassDefFoundError is an error, which occurs when a class is present during compile-time but is not found during the runtime. This also happens even if the class is compiled successfully, but the JVM is not able to find it while executing the program.

Common Causes of NoClassDefFoundError in Java

Here are the following causes of the error:

1. Missing or Incorrect Classpath: The class was available when you compiled the code, but when you try to run it, it is not found because it was not included in the runtime classpath.
2. Static Initialization Failure: If a class has a static block, and an exception is thrown then the class is not loaded.
3. Class File Deletion or Relocation: If your class file is deleted, moved, or renamed after your program is compiled, the JVM will not be able to locate it during the runtime.
4. Mismatch Between Compile-Time and Runtime Class: This can happen if you have changed a class after compiling it (e.g., renaming a class) and do not compile its dependent classes.
5. Compatible Library Versions: If you are using external libraries or JARs files, and the versions do not match between compile-time and runtime, then the JVM will fail to find certain classes that were available during the compilation process.

Example:

Output:

Explanation: The above code will show a NoClassDefFoundError. This error occurs when the class is not available or not in the right location (classpath). Then the program tries to create an object of MyClass, but it is not available. This error only occurs when the class is missing at runtime, even if it was available during the compilation.

How to Resolve the NoClassDefFoundError in Java?

There are many ways to resolve the NoClassDefFoundError in Java, some of them are:

• You should make sure that all the classes or the JAR files are included in the correct classpath when the program is running.
  • Example: Make sure that the MyClass class is at the right place when the program runs, by using java -cp. Main.

• Look for the static initialization of the class for any errors and if occur you can fix them so that the static block runs in a correct way.
  • Example: If MyClass has a static block with an error, you can fix the error or handle it to avoid class loading problems.

• You should ensure that the class file is present in the correct directory or folder or the JAR file. If you are working on a large project, then make sure to check if there are the required classes present.
  • Example: always verify that MyClass.class is in the same directory as the Main.class is included in the correct JAR file.

• Compile the entire project so that all the dependencies get up-to-date.
  • Example: Compile the entire project with javac *.java to ensure no previously used classes are causing the NoClassDefFoundError.

• Always ensure that the version of the library used during the compilation process stays the same as the one used during the runtime process.
  • Example: Always ensure that the same version of MyClass or any of its dependencies, used at compile-time, is used at the runtime to avoid any of the mismatches.

Let’s understand the differences in detail:

NoClassDefFoundError vs ClassNotFoundException in Java

When working with Java, you might encounter NoClassDefFoundError or ClassNotFoundException, both related to missing classes, but they occur in different situations. Let’s now look at both and understand the difference between them:

1. NoClassDefFoundError in Java

• This error generally occurs when a class is available at the compile-time, but is missing at the runtime. Your program may have compiled successfully, but at the runtime, the class cannot be found in the correct path.
• This usually occurs when the class was part of the program during compilation but it is not available or visible in the classpath. The class may have been moved, deleted, or renamed after the compilation process.

Example:

Output:

Explanation: The above code throws a NoClassDefFoundError as the SomeClass is missing at runtime. This error mainly occurs when the JVM is not able to locate the class, which was available during the compile time but cannot be found during the execution.

2. ClassNotFoundException in Java

• This exception arises when you try to load a class at runtime (for example, using Class.forName()), but the class is not found in the correct path.
• The JVM cannot find the class because it is not present when you attempt to load it at runtime.

Example:

Output:

Explanation: The above error occurs when “SomeClass tries to load a class, but it is not found at runtime. This exception is caught in the catch block, and the error is printed. It specifically happens during the dynamic class loading.

Conclusion

NoClassDefFoundError occurs when a class is available at compile-time but is missing at the runtime. To resolve this, ensure that all the required classes or JAR files are included in the classpath, also check for static initialization errors, and confirm the class file’s location. Additionally, you should also ensure library versions. Then properly solving these factors can help prevent the runtime error.

If you want to learn more about Java, you can refer to our Java Course.

FAQs