In the developer documentation you can find the Developer Guide, and the FAQ. The example implementation shows you in details how to extend the KNIME Analytics Platform with your own nodes. If you develop your own nodes, please also pay attention to the KNIME Noding Guidlines, which specify how KNIME nodes should behave.
Please note: With the 3.6 release, we retired our KNIME SDK in favor of a more flexible setup. You can now find all information you need to get started with the development of new extensions for KNIME Analytics Platform on our KNIME SDK Setup page.
Accessing KNIME's source code
If you want to browse source code of KNIME Analytics Platform, KNIME Extensions or KNIME Integrations, the easiest approach is checkout the code of many of our extensions on GitHub. Alternatively, you can include the source code in your target platform (see KNIME SDK Setup page). For the latter simply select the option "Include source if available" in the dialog where you select the target platform's contents from the update site (see screenshot).
Once you add a dependency to an extension you can browse the source code from within Eclipse.