Problems with Manifest in JAR

It is widely known that JAR files are built on the ZIP file format. So any jar file can be opened with zip archiver, and, to create jar file, zip tools can be used. The only major difference is that jar files may have optional META-INF directory with MANIFEST.MF and other files related with meta-information.

Everything looks very simple. However, today I have faced one problem with finding manifest file in the jar. I have created zip file from the directory with the simple structure:
-META-INF
--MANIFEST.MF
-A.class
-B.class

After extension was changed to jar, this file was used by an external application, which tried to locate manifest in the file. This was a great surprise when the application was not able to find it. I tried to use jar util to prepare the package. When I pushed the result file to the application, it successfully found the manifest.

The situation was pretty strange - contents of both archives was identical and I knew that JAR files should not store any additional metainformation about manifests, but the experience showed the opposite. There were some doubts about possibly buggy logic in the client application, which read my jars. I looked through its source codes and found that it just uses standand JarInputStream from java.util.jar.

However, when I investigated this class, I noticed one thing that could be the cause of such behavior: logic responsible for finding manifest in the JAR assumes that META-INF directory should be located in the beginning of the archive. I investigated both my archives and it turned out that the one, created with zip tool placed this directory in the end. But the archive created with jar stored META-INF in the beginning, as it is expected.

So, if somebody still wants to create jar files with zip tools, do not forget to place META-INF in the beginning of the archive. Do not also forget to add two empty lines in the end of the manifest file, just in case.