Constructing JarInputStream from byte[]

Question

i want to use a 2 step process for accessing the contents of jar files.
first, i create the jar file from the shell:
$ jar cfv /tmp/x.jar *class
i inspected the exact size of the file from the shell:
*$ ls -l /tmp/x.jar*
_(step #1):_
i read a jar file from the file system into a byte[] and store it as an instance variable.
_(step #2):_
at a later time, sometimes i read classes from this byte[] .
so, this is my code:
if(jarFileImage.length == RealFileLength) { System.out.println("exact same size as in file system."); } // ok
JarInputInputStream jin = new JarInputStream(new ByteArrayInputStream((byte[]) jarFileImage)); // ok
if(jin.available() == 1) { System.out.println("so far so good"); } // ok
JarEntry je = jin.getNextJarEntry(); // <--- error "null pointer exception"
// this also fails :
Manifest mani = jin.getManifest();   // <--- error "null pointer exception"Streams are so basic and straight forward, i can't see what to inspect further.
Edited by: kappa9h on Feb 19, 2008 7:10 AM

Answer 1

i have made good progress and am way past the null-pointer exception.
What is this? An extra Input in there?i think that the root cause is really a networking issue. specifically,
i am serializing objects into the payload of DatagramPackets and multicasting them with udp .
to guarantee remote de-serialization, i include the relevant jar file in the same datagram.
JarInputInputStream jin = new JarInputStream(new ByteArrayInputStream((byte[]) jarFileImage));here, *((byte[]) jarFileImage)* is the jar file. the other part of the payload is the serialized object.
the two problems are:
(1) my understanding is that you can't read/write primitives like bytes/ints/doubles/etc. on the same Stream that you serialize objects.
for example:
objOutStream.writeObject(myObj);
objOutStream.writeObject((List<Byte>) jarImageAsObj); // okverse:
objOutStream.writeObject(myObj);
objOutStream.write((byte[]) jarImage);  // error(2) there is a maximum size of a DatagramPacket .
notice also that treating byte[] as a collection of Byte objects confounds the max. packet size. issue.
i did solve issue #1. i could not solve issue #2.
i think:
max DatagramPacket size is 64-k? more than anything i hope to expand this.
but there is also an issue of DatagramSocket buffer sizes. there are so many
moving parts such that i don't really understand.
for small jar files, everything works ok.
but when i use relatively large jar files, i get this:
java.io.EOFException
at java.io.ObjectInputStream$PeekInputStream.readFully(ObjectInputStream.java:2279)
at java.io.ObjectInputStream$BlockDataInputStream.readInt(ObjectInputStream.java:2272)
which hints to me that, indeed, the limited size of the DatagramPacket payload truncates the jar file.
i don't have a clear view of whats really going on here. i am mostly beginner.
if anyone has an ideas, i will do more testing.
otherwise, i give-up and use my tcp based class loader.
but this breaks the encapsulation that i want.
thanks for reading so much.
Edited by: kappa9h on Feb 20, 2008 3:32 AM