The JDK source code is inside the src.zip, this article shows you how to get it on Windows, Ubuntu (Linux) and Mac OSX.

1. Windows

In Windows, visit the Oracle website and download the JDK (not JRE).
oracle-jdk-download
Install the JDK and the src.zip is inside the JDK installed folder, for example :
C:\Program Files\Java\jdk1.7.0_40\src.zip

2. Ubuntu (*nix)

In Linux, for example, Ubuntu, you can get the source code from OpenJDK.
Search “openjdk”
$ apt-cache search openjdk
 
default-jdk - Standard Java or Java compatible Development Kit
default-jdk-doc - Standard Java or Java compatible Development Kit (documentation)
default-jre - Standard Java or Java compatible Runtime
openjdk-7-demo - Java runtime based on OpenJDK (demos and examples)
openjdk-7-doc - OpenJDK Development Kit (JDK) documentation
openjdk-7-jdk - OpenJDK Development Kit (JDK)
openjdk-7-jre - OpenJDK Java runtime, using Hotspot JIT
openjdk-7-source - OpenJDK Development Kit (JDK) source files
java-package - Utility for creating Java Debian packages
Install the source code.
$sudo apt-get install openjdk-7-source

3. Mac OSX

In Mac OSX, get the JDK source code from Apple Developer, and download and install the Java for OS X Developer Package.
Later, find the source code here.
/Library/Java/JavaVirtualMachines/${jdk.version}.jdk/Contents/Home/src.jar
Note
Refer to this detail guide on how to download JDK Source Code For Mac OS X

References

  1. Oracle JavaSE download link
  2. Wikipedia : OpenJDK
  3. How To Install JDK On Ubuntu
  4. Download JDK Source Code For Mac OS X