如何从嵌套存档中读取ZipEntry CRC（zip中的zip）

Question

See comments of selected answer, the issue was with my test data and not the code.

I'm trying to read the CRCs of a Zip archive using the ZipEntry object. This works for zips directly on the file system.

zStream = new ZipInputStream(new FileInputStream(zipPath))
zEntry = zStream.getNextEntry()
zEntry.getCrc()

I want to get the CRC from a nested zipEntry, but I always returns -1

zStream = new ZipInputStream(new ZipFile(zipPath).getInputStream(zEntry))
zStream.getNextEntry().getCrc()
// always equal -1

Is there a way to read the CRC without extracting the nested zip to the FS? Why doesn't it work as is?

Answer 1

zStream = new ZipInputStream(new ZipFile(zipPath).getInputStream(zEntry))

That won't work. Try this:

ZipInputStream innerZip = new ZipInputStream(zStream);

after you've got to the (outer) ZEntry concerned, then iterate innerZip 's own ZEntries and get their CRCs etc, whatever you want.

Answer 2

If reading one entry after another, the implementation of java.util.zip.ZipInputStream does not deliver the CRC of the current entry. It is available (for the previous entry) only after calling getNextEntry() again!

AFAIK this is not documented but urgently needs to.

Answer 3

This really appears to be a bug/limitation of the ZipEntry interface. There should be an API to calculate the CRC.

Here is some Groovy code to generate a missing CRC:

long calculateCRC( zStream) {
    def crc32 = new CRC32()
    crc32.update( IOUtils.toByteArray(zStream))
    return crc32.value
}