How to Fix Exception in Thread Main Java.Lang.NoClassDefFoundError

1. Exception in thread "main" java.lang.NoClassDefFoundError in Java
2. Causes of Exception in thread "main" java.lang.NoClassDefFoundError in Java
3. Solution to Exception in thread "main" java.lang.NoClassDefFoundError in Java

In today’s post, we will talk about java.lang.NoClassDefFoundError that occurred in the main thread of Java. But before we go into it, we must understand what the java.lang.NoClassDefFoundError signifies.

Exception in thread "main" java.lang.NoClassDefFoundError in Java

The java.lang.NoClassDefFoundError indicates that your application’s code or any external library you are using is attempting to use the SLF4J library, which is an open source logging library.

Still, it cannot do so because it cannot locate the necessary library. Because of this, an exception named java.lang.NoClassDefFoundError is being thrown in the main thread.

Because the Java program only looks for class files in the directory it is now in, if your class file is not in the directory it is currently in, you will need to add it to the classpath.

The solution to this issue is to put this. A class file can now be found in the classpath.

If your class file is included inside a jar file, then the jar file has to be included in the classpath. Classpath may be an absolute and complete path, or it can be a relative path associated with a directory.

Classpath is a system-wide environment variable that, when set, directs the system to class files located in specific folders.

Causes of Exception in thread "main" java.lang.NoClassDefFoundError in Java

The following are the typical causes that have triggered exceptions in main thread java.lang.NoClassDefFoundError in various Java programs. The issue you’re experiencing might very well be one of these reasons.

1. Invalid entry in the classpath - When executing a Java program using the java-jar command, the ClassPath property of the manifest file does not include the definition of that class. This is one possible consequence of the exception.

2. Problems related to the environment - We’ve encountered unusual causes of java.lang.NoClassDefFoundError. When working in Java, we keep getting this exception.

   In one particular instance, one of the shell scripts launching our Java program altered the value of the CLASSPATH environment variable.

   This was causing our Java application not to start properly. Therefore, we should use the java-classpath command to start the Java application and specify the classpath in the start script by providing a list of JAR files to include in the classpath.

3. No JAR file in the classpath - The most concise description for this issue is that the .jar file was not included in the classpath. The issue will no longer appear as soon as you include that JAR in the classpath.

   You can identify which JAR file a specific class belongs to in Eclipse by using keyboard shortcuts such as search resource Ctrl+R.

   After you have determined which JAR file the class belongs to, you can examine whether or not that JAR is present in the classpath.

4. Overriding of class loaders - There is a request for class loading from the parent class loader, and then the child class loader attempts to load the requested class.

   If the parent class loader loads any class, then the child class loader can see it, but the other way around, the parent class loader doesn’t know what the child class loader has loaded.

Solution to Exception in thread "main" java.lang.NoClassDefFoundError in Java

1. We need to configure a CLASSPATH environment variable so that we can fix the exception that occurs in Windows. The following settings must be configured for us to establish a classpath.

   set CLASSPATH = % CLASSPATH % ;
   .;
   

   %CLASSPATH% signifies the existing classpath to be added, and it refers to the directory in which the program is now running.

2. You’ll need to set the build path for your project. It is possible to customize it by right-clicking on the Project tab and selecting Properties followed by Java Build Path and then selecting choose libraries for jars or classes in the folder set source.

3. The exception manifests in servers when the jar file is missing from the classpath. To resolve this issue, check out the whole stack trace, determine the missing jar file’s name, and make an effort to add it to either the server classpath or the application classpath.

4. To use this class, we need to include either the class itself or the .jar file that provides for it in the java classpath. We must type a . at the beginning of the Java class name whenever we execute it from the command line.