Looking at ZipFileSystem's close() implementation, the ExistingChannelCloser facility seems to be kind of unneeded. ExistingChannelCloser is a way to keep existing, open InputStreams against entries in the Zip filesystem alive, even when the sync() method is called and writes a new zip archive. However, the sync() method is only used by the close() implementation and there it is a requirement that all existing InputStreams be closed before calling sync(). So this facility is obsolete and shall be removed.

Some prior discussion with additional information can be found here:
https://mail.openjdk.java.net/pipermail/core-libs-dev/2019-April/059839.html