How to modify the class file in the jar package and then re-play the jar package

1. Import into Eclipse, modify and then export the jar
2.jar CF File.jar *.class package all class files in the current directory into a new jar file
3. For patches. There have been jars for example: a bug was found. Then, to modify one of the classes, you need to package it in a second, for this case, just put the corresponding class in the specified directory for example: File.jar has a class demo fix bug need to upload again, The path he corresponds to is com.demo.Demo;
Then just put Demo.class in the project classes directory, create a com/demo/folder
The class in the jar is first loaded with the Demo.class in the classes, which overwrites the classes in the jar file.

How to modify the class file in the jar package and then re-play the jar package