Something You May Not Know about Jar Command: Part 2

4. You can choose not to have manifest file when creating a jar file, using M option:

jar cvfM no-meta.jar A.class
adding: A.class(in = 405) (out= 279)(deflated 31%)

Note: it's upper-case M. Lower-case m has a different meaning.

5. You can specify your own manifest file when creating a jar file, using m option:

jar cvfm my-meta.jar my-meta-inf\my.mf A.class
added manifest
adding: A.class(in = 405) (out= 279)(deflated 31%)

The option used here cvfm tells the jar command that the destination file (f) will come next and then custom manifest file (m). I can also specify them in a different order:

jar cvmf my-meta-inf\my.mf my-meta.jar A.class
added manifest
adding: A.class(in = 405) (out= 279)(deflated 31%)

It's lower-case m. Upper-case M has a different meaning.

6.META-INF/MANIFEST.MF file in source files is always ignored.

jar cvf ignore.jar A.class META-INF
added manifest
adding: A.class(in = 405) (out= 279)(deflated 31%)
ignoring entry META-INF/
adding: META-INF/LICENSE.txt(in = 2657) (out= 1185)(deflated 55%)
ignoring entry META-INF/MANIFEST.MF

When it comes to manifest files for new jar file, you only have 3 options:

• Do not specify any manifest-related options and use the default MANIFEST.MF
  


• Use option M not to include a manifest file
  


• Use option m to use a custom manifest file. Inside the jar file, this file will always be named MANIFEST.MF under META-INF directory. Outside of the jar file, this custom manifest file can be anywhere and have any name.
  

7. If you unjar an archive, do nothing, and then jar it up again, the resulted new archive may not equal to the original one. And applications using this new archive may not work correctly because of the wrong manifest file. The original jar may have a custom manifest file, which is expaned into META-INF/MANIFEST.MF. But when you jar these files up again without using cvfm option, this custom manifest file is ignored and a default MANIFEST.MF is included.

Tags: java, JDK, jar, meta-inf, manifest.mf