Corrupt jar file

This will happen when you doubleclick a JAR file in Windows explorer, but the JAR is by itself actually not an executable JAR. A real executable JAR should have at least a class with a main() method and have it referenced in MANIFEST.MF.

In Eclispe, you need to export the project as Runnable JAR file instead of as JAR file to get a real executable JAR.

Or, if your JAR is solely a container of a bunch of closely related classes (a library), then you shouldn't doubleclick it, but open it using some ZIP tool. Windows explorer namely by default associates JAR files with java.exe, which won't work for those kind of libary JARs.

This regularly occurs when you change the extension on the JAR for ZIP, extract the zip content and make some modifications on files such as changing the MANIFEST.MF file which is a very common case, many times Eclipse doesn't generate the MANIFEST file as we want, or maybe we would like to modify the CLASS-PATH or the MAIN-CLASS values of it.

The problem occurs when you zip back the folder.

A valid Runnable/Executable JAR has the next structure:

myJAR (Main-Directory)    |-META-INF (Mandatory)             |-MANIFEST.MF (Mandatory Main-class: com.MainClass)    |-com          |-MainClass.class (must to implement the main method, mandatory)    |-properties files (optional)    |-etc (optional)

If your JAR complies with these rules it will work doesn't matter if you build it manually by using a ZIP tool and then you changed the extension back to .jar

Once you're done try execute it on the command line using:

java -jar myJAR.jar 

When you use a zip tool to unpack, change files and zip again, normally the JAR structure changes to this structure which is incorrect, since another directory level is added on the top of the file system making it a corrupted file as is shown below:

**myJAR (Main-Directory)    |-myJAR (creates another directory making the file corrupted)**          |-META-INF (Mandatory)                   |-MANIFEST.MF (Mandatory Main-class: com.MainClass)          |-com               |-MainClass.class (must to implement the main method, mandatory)          |-properties files (optional)          |-etc (optional)

:)

The problem might be that there are more than 65536 files in your JAR: Why java complains about jar files with lots of entries? The fix is described in this question's answer.