Here is a very simple test which throws error on Mac:

import java.awt.Toolkit;
import javax.swing.JTree;

public final class JTreeTest {
    public static void main(final String... args) {
        System.setProperty("java.awt.headless", "true");
        System.out.println(Toolkit.getDefaultToolkit());
        final JTree tree = new JTree();
        System.out.println(tree.getPreferredSize());
    }
}

The error is:
Exception in thread "main" java.lang.Error: Could not find class: sun.awt.HeadlessGraphicsEnvironment
        at java.awt.GraphicsEnvironment.createGE(GraphicsEnvironment.java:116)
        at java.awt.GraphicsEnvironment.getLocalGraphicsEnvironment(GraphicsEnvironment.java:81)
        at sun.swing.SwingUtilities2.isLocalDisplay(SwingUtilities2.java:1457)
        at javax.swing.plaf.metal.MetalLookAndFeel.initComponentDefaults(MetalLookAndFeel.java:1556)
        at javax.swing.plaf.basic.BasicLookAndFeel.getDefaults(BasicLookAndFeel.java:148)
        at javax.swing.plaf.metal.MetalLookAndFeel.getDefaults(MetalLookAndFeel.java:1592)
        at javax.swing.UIManager.setLookAndFeel(UIManager.java:535)
        at javax.swing.UIManager.setLookAndFeel(UIManager.java:575)
        at javax.swing.UIManager.initializeDefaultLAF(UIManager.java:1343)
        at javax.swing.UIManager.initialize(UIManager.java:1432)
        at javax.swing.UIManager.maybeInitialize(UIManager.java:1420)
        at javax.swing.UIManager.getUI(UIManager.java:1002)
        at javax.swing.JTree.updateUI(JTree.java:714)
        at javax.swing.JTree.<init>(JTree.java:667)
        at javax.swing.JTree.<init>(JTree.java:564)
        at JTreeTest.main(JTreeTest.java:8)
Version of jdk:
java version "1.7.0_04-ea"
Java(TM) SE Runtime Environment (build 1.7.0_04-ea-b227)
Java HotSpot(TM) 64-Bit Server VM (build 23.0-b12, mixed mode)