How eclipse or myeclipse-associates the jar package with the corresponding source code in eclipse or MyEclipse

Today, summarize how the jar package and the corresponding source code are associated in eclipse or MyEclipse:
1. Why should I associate the jar package with the corresponding source code:
We have encountered in the process of developing the application does not understand the class or method is always accustomed to find the corresponding API, um, well, this is a good note, but you do not think this is more cumbersome? If the corresponding jar package and corresponding source code, we just need to move the mouse to the corresponding class or method will have the appropriate English prompt:
Before association:

After association:

2. How to associate the jar package with the corresponding source code:

How eclipse or myeclipse-associates the jar package with the corresponding source code in eclipse or MyEclipse