A JAR file is actually just a ZIP file. It can contain anything - usually it contains compiled Java code (*.class), but sometimes also Java sourcecode (*.java).

However, Java can be decompiled - in case the developer obfuscated his code you won't get any useful class/function/variable names though.

Open a command prompt.

Go to the directory where you have your .java files

Create a directory build

Run java compilation from the command line

javac -d ./build *.java

if there are no errors, in the build directory you should have your class tree

move to the build directory and do a

jar cvf YourJar.jar *

For adding manifest check jar command line switches

Open the command prompt by typing cmd in the Windows 10 search box + S (Windows logo key+S).

1. Make sure you have Java installed on your system. Check this by typing java -version at the command prompt.

2. Run the .jar file using the command:

    java -jar \path\to\filename.jar  
   

   where \path\to\filename.jar in the above command should be replaced with the full path to the .jar file that you want to run.

If you double-click on a .jar file and your Java application does not start, your .jar association has been hijacked.

The workaround is to reinstall the Java Runtime Environment or fix the Windows Registry manually each time this problem occurs.

There is also a free, third-party tool called Jarfix to fix the jar association on Windows.