Why does excluding META-INF helps with " could not find or load main method " in gradle

Question

I was trying to create a fat jar for my project in gradle . I was using this code to create the jar .

jar {
    duplicatesStrategy = DuplicatesStrategy.EXCLUDE
    manifest {
        attributes(
                'Main-Class': 'com.kroger.cxp.app.Main'
        )
    }
    from {
        configurations.runtimeClasspath.collect { it.isDirectory() ? it : zipTree(it) }
    }
}

But When I tried to runt the jar it threw "could not find or load main Class" Error. I searched and changed my code to

jar {
    duplicatesStrategy = DuplicatesStrategy.EXCLUDE
    manifest {
        attributes(
                'Main-Class': 'com.kroger.cxp.app.Main'
        )
    }
    from {
        configurations.runtimeClasspath.collect { it.isDirectory() ? it : zipTree(it) }
    }{
        exclude "META-INF/*.SF"
        exclude "META-INF/*.DSA"
        exclude "META-INF/*.RSA"
    }
}

After adding these exclude META-INF lines I was able to run the jar but i still don't understand how this fixed the issue ? What the these META-INF files and how removing them helps here.

Answer 1

As the JAR file specification points out, the META-INF directory in JAR files contains a manifest file, may contain a indexation file to speedup class loaders, and may contain a subdirectory for configuration purposes.

Your question concerns another file type there -- the signature files of SF and DSA resp. RSA type. For each SF file there has to be a DSA or RSA file of same name. Basically, all files which your JAR file contains are listed there with their verification hash signatures. For details take a look on the paragraph about signed JAR files.

So in case the JAR file got e.g. corrupted or manipulated, the JAR can't be run, as the hash over the file which has been changed doesn't equal to the hash value recorded in the signature files. Also missing files will cause an error, as they're declared in META-INF but are not there, where they should be.

So in your first attempt you created a JAR file with META-INF signature files which do not fit to the actual content of your JAR. Therefore you got an error when trying to run the JAR.

In your second attempt you simply kicked out the offending signature files and basically generated an unsigned JAR file. That works as signing is not obligatory for JARs.

You can be either content with that result, or may consider to sign your JAR with jarsigner.

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P.S.: You will find META-INF also in APK and AAR files for Android OS, as those are JAR files with a certain structure. However, it's obligatory to sign APKs and AARs, while it's optional for JARs.