Well, theoretically you can simply decompress JAR files using Java, and then repack the content to a ZIP archive. It is possible, however the reasons for this are questionable.
Convert Compressed archive file package for Java classes and data to WinZIP Compressed ZIP archive.
We have found 4 software records in our database eligible for .jar to .zip file format conversion.
Microsoft Windows software - convert jar to zip on Windows
A free file archiver developed by Igor Pavlov featuring higher compression ratios with LZMA and LZMA2 compression.
It is mainly available for Windows platform (for Mac and Linux are available command line tools) and is the primary program to handle 7z archives. However, also supports various compression file formats.
Packing / unpacking: 7z, XZ, BZIP2, GZIP, TAR, ZIP and WIM
Unpacking only: AR, ARJ, CAB, CHM, CPIO, CramFS, DMG, EXT, FAT, GPT, HFS, IHEX, ISO, LZH, LZMA, MBR, MSI, NSIS, NTFS, QCOW2, RAR, RPM, SquashFS, UDF, UEFI, VDI, VHD, VMDK, WIM, XAR and Z
Convert from jar file
to zip file
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned jar to zip. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
ALZip is compression utility that supports about 40 archive file formats. ALzip can open ALZ, ACE, EGG, RAR, ZIP etc.. Also can open disk images, such as ISO and BIN, LCD. ALZip is able to recovery passwords in protected ZIP archives.
FilZip will compress and decompress ACE, ARC, ARJ, BH, XXE, ZOO, ZIP, and others. FilZip includes a simple six-option context menu interface to unarchive files. Easily selected options include extracting in place, to a new directory, e-mailing or testing it. FilZip includes the many archive options expected in current tools.
Apple macOS / Mac OS X software - convert jar to zip on OS X
iPack is a full-featured compressed archives manager that allows users to compress 7z, zip, jar, and rar archives and decompress 7z, zip, jar, rar, cab, deb, iso, arj, gz and bz2 archives.