getInpustream on a URL representing the whole jar file leads to java.io.IOException: no entry name specified

[Jaikiran Pai]

Please consider the following code:

import java.net.*;
import java.io.*;
import java.util.jar.*;

public class SimpleTest {
    public static void main(final String[] args) throws Exception {
        final String jarLocation = "/tmp/some-jar.jar";
        URI uri = new URI("jar:file", null, jarLocation + "!/", null);
        // URL will be of the form jar:file:/tmp/some-jar.jar!/
        URL url = uri.toURL();
        System.out.println("URL is " + url);
        try (final InputStream is = url.openStream()) {
            System.out.println("Stream opened " + is.getClass().getName());
            final JarInputStream jarIs = new JarInputStream(is);
            System.out.println("Jar inputstream opened " + jarIs);
        }
    }
}

What it does is construct a jar file URL, uses that URL to open a stream
and then passes that stream along to java.util.jar.JarInputStream
constructor for it to parse jar entries out of it.

From what I can say, the URL that is formed is correct based on what's
explained in the docs of JarURLConnection[1]. That documentation also
explicitly states that the entry is optional and in such a case the URL
represents the entire jar file.

When I run the above code, I end up with the following exception:

Exception in thread "main" java.io.IOException: no entry name specified
    at
java.base/sun.net.www.protocol.jar.JarURLConnection.getInputStream(JarURLConnection.java:160)
    at java.base/java.net.URL.openStream(URL.java:1140)
    at SimpleTest.main(SimpleTest.java:12)

So it fails to open a stream out of the jar URL. I haven't found any
documentation which states what's being done here is not allowed. Is
this an incorrect use of the API in the sample program?

Java version in use is:

openjdk version "11.0.8" 2020-07-14
OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.8+10)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.8+10, mixed mode)

But I could even reproduce this on Java 8.

[1]
https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/net/JarURLConnection.html

-Jaikiran