How to fix Exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory in Java

This error means your code or any external library you are using in your application is using the SLF4J library, an open source logging library, but it is not able to find the required JAR file e.g. slf4j-api-1.7.2.jar hence it's throwing Exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory. If you look at the error, you will see that it's saying it is not able to find the class org/slf4j/LoggerFactory or org.slf4j.LoggerFactory. The package name indicates that it's part of SLF4j, hence you need SLF4j JAR files e.g. slf4j-api-1.7.2.jar in your application's classpath. So, go ahead and download the JAR file from SLFj website or from Maven Central repository and restart your application.