This error indicates a defect in the plugin's main JAR. There are two known causes for this problem:
- The plugin JAR got corrupted during download to your machine.
- The plugin JAR was improperly assembled or deployed to the plugin repository.
To check whether the plugin JAR is intact, inspect your local repository which is usually located at ${user.home}/.m2/repository. For a plugin with the coordinates com.company:custom-maven-plugin:1.0, the file to check is com/company/custom-maven-plugin/1.0/custom-maven-plugin-1.0.jar. If this file doesn't match the copy in the plugin repository, e.g. as noticeable by checksum mismatches, simply delete the local file and have Maven re-download it. If you have verified that your local copy of the JAR and the JAR in the plugin repository are bytewise identical but Maven still complains, please report this to the maintainer of the plugin.