How to Fix the `java.lang.NoClassDefFoundError: org/apache/log4j/LogManager` Exception in Java?

What does the `java.lang.NoClassDefFoundError: org/apache/log4j/LogManager` error mean and how can it be resolved in Java?

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The `java.lang.NoClassDefFoundError` is thrown when a particular class definition could not be found during runtime. In this case, `org/apache/log4j/LogManager` indicates that the Log4j logging framework class is not accessible to the Java application. This usually happens due to missing libraries in the classpath.

<dependency>
    <groupId>log4j</groupId>
    <artifactId>log4j</artifactId>
    <version>1.2.17</version>
</dependency>

Causes

• The Log4j library is not included in the project's build dependencies.
• Incorrect version of Log4j is used, incompatible with the application.
• The classpath is not set properly to include Log4j library.

Solutions

• Ensure that Log4j is included in your project's dependencies (Maven, Gradle, or IDE setup).
• Add the correct version of the Log4j library that matches your application requirements.
• Check and configure the classpath correctly to include all necessary libraries.

Mistake: Forgetting to add Log4j to the project dependencies.

Solution: Verify your build configuration (Maven, Gradle) to make sure Log4j is included.

Mistake: Using an outdated version of Log4j that does not contain `LogManager`.

Solution: Update the Log4j version to the latest compatible version.

Mistake: Not refreshing the project after adding new dependencies.

Solution: Perform a project clean and rebuild to ensure all libraries are properly linked.