Findbugs exclude files

This is likely happening due to the fact that you have other file types under build/classes/main than Java class files. Currently this message is bubbling up to the console as described in this post. You can work around the issue with the following code:

tasks.withType(FindBugs) {
    // file types you want to exclude e.g.
    exclude '**/*.conf'
    exclude '**/*.properties'
    exclude '**/*.properties'
}