This article shows you how to attach the JDK source code in Eclipse IDE.
In Eclipse, select Windows -> Preferences -> Java -> Installed JREs , expands rt.jar, select “Source attachment” and find the src.zip from your disk drive.
attach-jdk-source-code-eclipse
P.S JDK source code is inside the src.zip.
Note 
You may interest at this article – where to download JDK source code.

References

  1. Oracle JavaSE download link
  2. download the JDK source code
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