unfortunately, websphere has a lot of commonly used libraries in its own installation. They often conflict with the versions you want to use in your application.
The options are to remove the library jars from the application and use what websphere provides or change the class loader settings to 'parent last' to make sure the libs bundled with the app are found first. (this will not eliminate the slf4j warning, but will make sure your implementation is used).
Sorry, I can't give more details where the settings are found (its a few years that I last worked with websphere).
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