The SpringLayout was added in Java 1.4. It supports a combination of absolute positioning and point-to-point attachments between components (kind of like the "glue" of the BoxLayout). SpringLayout is useful in GUI builder applications because it's easy to translate manual user placement into these kinds of constraints.
SpringLayout uses Spring objects to attach edges of components. Springs have a range of motion and can be anchored at a coordinate or at a component's edge. Any two component edges can also be connected by a Spring. The two edges may belong to two different components, in which case, the spring constrains the way the components are placed relative to one another, or they may belong to the same component, in which case, the spring constrains the width or height of the component.
We don't cover SpringLayout here, but you can read about it in O'Reilly's Java Swing.