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

Question

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

Stacktrace element 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) 

Stacktrace element 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) 

The execution execution shows the following InputStream implementations:

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

Looking further at the JarURLInputStream , I found that the inherited field (from FilterInputStream ) of InputStream in is null , which results in an NPE result.

Unfortunately, this is so deep that I was able to get into the debugger.

Any ideas on how to do this correctly? What am I missing or is something wrong? Thanks!

Answer 1

Folders do not return an InputStream with some list of all files or anything like that. Use JarInputStream to retrieve the JAR programmatically. Here you can find an example here . For reference, here is a small 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; }