Why does InputStreamReader throw a NPE when reading from a jar?

Question

I'm trying to iterate over the class files in a known directory using streams. The ultimate goal is to obtain the class names for all classes that exist in a particular package, then load the classes at runtime and use reflection to obtain the names & values of all the static constants. This works when I run the program from source on my machine, but when I run it as a jar the BufferedReader throws a NPE from both ready() and readLine(). Here's the code (with error handling & best practices omitted for brevity):

private void printClassNamesInPackage(final String strPackage) throws Exception {
    // The returned implementation of InputStream seems to be at fault
    final InputStream       packageStream = getClass().getClassLoader().getResourceAsStream( strPackage );
    final InputStreamReader streamReader  = new InputStreamReader( packageStream );
    final BufferedReader    reader        = new BufferedReader( streamReader );
    // Throws NPE from inside ready() - SEE STACKTRACE BELOW
    // reader.ready()
    String strLine;
    // Throws NPE from inside readLine() -  SEE STACKTRACE BELOW
    while ( null != (strLine = reader.readLine()) ) {
        System.out.println( strLine );
    }
}

The stacktrace from reader.ready():

java.lang.NullPointerException
    at java.io.FilterInputStream.available(FilterInputStream.java:142)
    at sun.nio.cs.StreamDecoder.inReady(StreamDecoder.java:343)
    at sun.nio.cs.StreamDecoder.implReady(StreamDecoder.java:351)
    at sun.nio.cs.StreamDecoder.ready(StreamDecoder.java:165)
    at java.io.InputStreamReader.ready(InputStreamReader.java:178)
    at java.io.BufferedReader.ready(BufferedReader.java:436)

The stacktrace from reader.readLine():

java.lang.NullPointerException
    at java.io.FilterInputStream.read(FilterInputStream.java:116)
    at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:264)
    at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:306)
    at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158)
    at java.io.InputStreamReader.read(InputStreamReader.java:167)
    at java.io.BufferedReader.fill(BufferedReader.java:136)
    at java.io.BufferedReader.readLine(BufferedReader.java:299)
    at java.io.BufferedReader.readLine(BufferedReader.java:362)

Stepping through the execution reveals the following InputStream implementations:

• From source: java.io.ByteArrayInputStream
• From jar: sun.net.www.protocol.jar.JarURLConnection$JarURLInputStream

Looking further into JarURLInputStream I find that the inherited (from FilterInputStream) field InputStream in is null, which leads to the resulting NPE.

Unfortunately, that's as deep as I was able to get in the debugger.

Any ideas on how to properly do this? What am I missing or doing wrong? Thank you!

Answer 1

Folders don't return an InputStream with some list of all files or something. Use JarInputStream to extract the JAR programmatically. You can find an example here. For reference, here's a slight modified extract of relevance:

public static List<String> getClassNamesInPackage(String jarName, String packageName) throws IOException {
    JarInputStream jarFile = new JarInputStream(new FileInputStream(jarName));
    packageName = packageName.replace(".", "/");
    List<String> classes = new ArrayList<String>();

    try {
        for (JarEntry jarEntry; (jarEntry = jarFile.getNextJarEntry()) != null;) {
            if ((jarEntry.getName().startsWith(packageName)) && (jarEntry.getName().endsWith(".class"))) {
                classes.add(jarEntry.getName().replace("/", "."));
            }
        }
    } finally {
        jarFile.close();
    }

    return classes;
}