Java no source how to do

How can Java modify and use the class methods in the jar without the source code?

The source code on your own hand and the jar on the line is inconsistent, when you do not know the specific situation, can not easily update the online jar file, but need minor changes to do?
1. Create a new Java project, call the previous jar as a third-party jar, and try to implement the modification in the new project.
2. If you do not want to add a new project (the original Jar is already in use and cannot modify the name), you can modify or add it in the original jar.

The first way to believe that everyone is familiar with the development work is in use,
The second method I used for the first time, the following summary records:
1. First familiarize yourself with the method information of the class to be used;
2. Follow the class in the jar, create a new Java class, make your own changes, and compile into a class file;
3. Copy the jar from the server to local, modify the suffix name to zip, open with WinRAR, and drag the class file from 2 to the appropriate directory;
4. Update the jar to run its newly-created Java class and invoke the corresponding class method in the original jar;

Java no source how to do