第一步， 把第三方库加到workspace :   https://stackoverflow.com/questions/370814/how-to-set-a-breakpoint-in-eclipse-in-a-third-party-library

   The most sure-fire way to do this (and end up with something that's actually useful) is to download the source (you say that it is open-source), and set up another "Java Project" pointing at that source.

To do that, get the source downloaded and unzipped somewhere on your system. Click "File"->"New"->"Java Project". In the next dialog, give it a project name and select "Create Project from Existing Source". Browse to the root location of the open source library.

Supposing that all the additional libraries that are required by the project and such are included in the project you downloaded, Eclipse will figure everything out and set the build path up for you.

You'll need to remove the open source jar from your project's build path, and add this new project to the build path of your project.

Now, you can just treat this as your code, and debug at will.

第二步 ， include 新添加的项目 

1. Right Click > Project.
2. Click Project Properties.
3. Click Java Build Path.
4. Click the Projects Tab.
5. Click the Add Button.
6. Select the Project.
7. Click OK.

第三步 ： 这时调试， 如果断点在 第三方库 内部， 会发生 "Source not found" 错误， 

　　　　所以 ， 需要在  debug configuration ---> Source ---> Source Lookup Path 

　　　　add 第三方工程 。