To extract the code source files

mkdir src
cd src
jar -xf ../stanford-corenlp-<version>-sources.jar 
cd ..

The code would get extracted to the src directory


Recompiling the code base:

ant
cd classes ; jar -cfm ../stanford-corenlp-<version>.jar ../META-INF/MANIFEST.MF edu ; cd ..

For running ant, make sure you have ant installed and ANT_HOME set.