JXMultiSplitPane is somewhat of a hybrid component using MultiSplitLayout which is somewhere between GridLayout and GridBagLayout. MultiSplitLayout divides its components into horizontal or vertical Splits which are composed of Leaf components(which accept components) and Dividers(which separate them).
Vertical splits can be created by setting the Split property rowLayout to false.
The code above produces the below layout:
Here is the code for a slightly more complex layout: