Download the JDK.dmg file, jdk-12. Before the file can be downloaded, you must accept the license agreement. From either the browser Downloads window or from the file browser, double-click the.dmg file to start it. Learn how to use the JDK to write applications for a wide range of environments. Hack on the JDK itself, right here in the OpenJDK Community: Browse the code on the web, clone a Mercurial repository to make a local copy, and contribute a patch to fix a bug, enhance an existing component, or define a new feature.