Modifying a Manifest File

You can modify the contents of a JAR file in 2 ways:

Use the m option to add custom information to the manifest during creation of a JAR file. The m option is described in this section.
Use the u option to update the contents of an existing JAR file, including its manifest.

The u option is covered in the section called Updating a JAR File.

The Jar tool puts a default manifest with pathname META-INF/MANIFEST.MF into any JAR file you create.
You can enable special JAR file functionality, such as specifying an entry point, by modifying the default manifest.
The Jar tool's m option allows you to add information to the default manifest during creation of a JAR file.
You first prepare a text file containing the information you wish to add to the default manifest.
The basic command has this format:

jar cmf manifest-addition jar-file input-file(s)

The options and arguments used in this command:

The m option indicates that you want to merge information from an existing manifest file into the manifest file of the JAR file you're creating.
manifest-addition is the name (or path and name) of the existing text file whose contents you want included in the JAR file's manifest.
jar-file is the name that you want the resulting JAR file to have.
The input-file(s) argument is a space-separated list of one or more files that you want to be placed in your JAR file.

The c, m, and f options can appear in any order, but there must not be any whitespace between them.

Example

To indicate which class is the project's entry point, add a Main-Class header to the JAR file's manifest. The header takes the form:
Main-Class: classname

The header's value, classname, is the name of the class that's the project's entry point.

Prepare a text file, say mainClass, consisting of single line with the Main-Class header and value.
For example, if your project's main class is Main, your text file has this line:

Main-Class: Main

Merge the information in file mainClass into a JAR file's manifest with a command such as this:

jar cmf mainClass <project_name>.jar class

where class is the directory with your class files.

You now can run your project from the command line:

java -jar <project_name>.jar

Warning: The manifest text file must end with a new line or carriage return.

I have success only with jar files whose Main-Class (e.g., teamname.projectname.Main) is at the top level of the jar (e.g., not inside a directory, such as class).