Why Does My SLF4J Application Throw 'Unresolved Load Class Error' and 'NoClassDefFoundError' on Some Application Servers?

SLF4J: Unresolved Load Class Error and NoClassDefFoundError on Different Application Servers

When deploying an application that utilizes SLF4J as a dependency, developers may encounter load class errors on certain application servers. This article addresses two common errors: the failed load class "org.slf4j.impl.StaticLoggerBinder" and the NoClassDefFoundError for the same class.

The reported error suggests that WebSphere 6.1 may have conflicting SLF4J dependencies, leading to a fallback to the no-operation logger implementation. This issue is not observed on other application servers, such as tcServer.

To resolve this issue:

1. Check Classpaths: Verify that the SLF4J jar file (e.g., slf4j-api.jar) is included and that there are no duplicate SLF4J jars in the classpath.
2. Add slf4j-simple to WebSphere Deployment: For WebSphere 6.1, the NoClassDefFoundError may indicate the presence of an outdated SLF4J version within the application server's bundled jars. Adding the slf4j-simple-1.6.1.jar to the application bundle, alongside the slf4j-api-1.6.1.jar, can resolve this issue.

3. Use Maven Dependencies: If using Maven, include the following dependencies in your pom.xml file:

   <dependency>
       <groupId>org.slf4j</groupId>
       <artifactId>slf4j-api</artifactId>
       <version>${slf4j.version}</version>
   </dependency>
   <dependency>
       <groupId>org.slf4j</groupId>
       <artifactId>slf4j-simple</artifactId>
       <version>${slf4j.version}</version>
   </dependency>

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   Replace ${slf4j.version} with the latest version of SLF4J.

By following these steps, you can resolve the load class errors and successfully deploy your SLF4J-dependent application on WebSphere 6.1 and other application servers.

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