Reading files from an embedded ZIP archive

Question

I have a ZIP archive that's embedded inside a larger file. I know the archive's starting offset within the larger file and its length.

Are there any Java libraries that would enable me to directly read the files contained within the archive? I am thinking along the lines of ZipFile.getInputStream(). Unfortunately, ZipFile doesn't work for this use case since its constructors require a standalone ZIP file.

For performance reasons, I cannot copy the ZIP achive into a separate file before opening it.

edit: Just to be clear, I do have random access to the file.

Answer 1

I've come up with a quick hack (which needs to get sanitized here and there), but it reads the contents of files from a ZIP archive which is embedded inside a TAR. It uses Java6, FileInputStream, ZipEntry and ZipInputStream. 'Works on my local machine':

final FileInputStream ins = new FileInputStream("archive.tar");
// Zip starts at 0x1f6400, size is not needed
long toSkip = 0x1f6400;
// Safe skipping
while(toSkip > 0)
    toSkip -= ins.skip(toSkip);

final ZipInputStream zipin = new ZipInputStream(ins);
ZipEntry ze;
while((ze = zipin.getNextEntry()) != null)
{
    final byte[] content = new byte[(int)ze.getSize()];
    int offset = 0;
    while(offset < content.length)
    {
        final int read = zipin.read(content, offset, content.length - offset);
        if(read == -1)
            break;
        offset += read;
    }
    // DEBUG: print out ZIP entry name and filesize
    System.out.println(ze + ": " + offset);
}
zipin.close();

Answer 2

1.create FileInputStream fis=new FileInputStream(..);

1. position it at the start of embedded zipfile: fis.skip(offset);

2. open ZipInputStream(fis)

Answer 3

I suggest using TrueZIP, it provides file system access to many kinds of archives. It worked well for me in the past.