option u updates an existing JAR file entries or its manifest file.

The following command adds a C.class file to an existing test.jar file:

jar uf test.jar C.class

To change the Main-Class entry in its manifest file to HelloWorld class, use the following command:

jar ufe test.jar HelloWorld
Option Meaning
u indicates that you are updating a JAR file
f indicates that you are specifying the JAR file name, which is test.jar
e indicates that you are specifying the Main-Class entry's value as HelloWorld for the MANIFEST.MF file in test.jar file.

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