JarURL.openStream() throws *IOException: no entry name specified*

Question

<pre>
I have some code (supplied below) which tries to read contents of a resource
referred by a URL.
public class URLTest {
public static void main(String... args) throws Exception {
URL u = new URL(args[0]);
InputStream is = u.openStream();
// more code goes here:
If you create a jar file in /tmp/b.jar and run this program as shown below:
(If your shell treats ! as a special char, then you need to use escape char \ before it)
java pkg.URLTest <b>jar:file:/tmp/b.jar!/</b>
it produces the following exception:
Exception in thread "main" java.io.IOException: no entry name specified
at sun.net.www.protocol.jar.JarURLConnection.getInputStream(JarURLConnection.java:131)
at java.net.URL.openStream(URL.java:1007)
at pkg.URLTest.main(URLTest.java:20)
But, if you pass the argument as jar:file:/tmp/b.jar!/A.class then it opens a stream
that points to A.class which is embedded inside b.jar.
My question is, what is the motivation behind not allowing user to open a stream to
jar:file:/tmp/b.jar!/ which according to the javadocs of JarURLConnection
http://java.sun.com/j2se/1.5.0/docs/api/java/net/JarURLConnection.html
represents a JAR File?
Thanks,
Sahoo
</pre>

Answer 1

I just also came across this. I don't know the motivation behind it. But it is certainly deliberate.
This is the source code for JarURLConnection.getInputStream.
public InputStream getInputStream() throws IOException {
     connect();
     InputStream result = null;
     if (entryName == null) {
          throw new IOException("no entry name specified");
     } else {
          if (jarEntry == null) {
               throw new FileNotFoundException("JAR entry " + entryName +
                    " not found in " +
                    jarFile.getName());
          result = new JarURLInputStream (jarFile.getInputStream(jarEntry));
     return result;
}So it will only let you get an InputStream to a file inside the Jar file, not the Jar file itself.
If you want to do that, then rather than using InputStreams you want to use the JarURLConnection class. My example below shows you how to get at the JarFile and its contents, JarEntry objects,
URL u = new URL("/tmp/b.jar!/");
JarURLConnection juc = (JarURLConnection)u.openConnection();
JarFile jf = juc.getJarFile();
System.out.println("JarFile: " + jf.getName());
Enumeration<JarEntry> entries = jf.entries();
for(JarEntry je = entries.nextElement(); entries.hasMoreElements(); je = entries.nextElement())
     System.out.println("\tJarEntry: " + je.getName());
}