public void setUI(SeparatorUI ui) {
    synchronized(this) {
        if (!separatorUIInitialized) {
            macOsXSeparatorUI = createCustomUI(this);
            separatorUIInitialized = true;
        }
    }

    if (macOsXSeparatorUI == null) {
        super.setUI(ui);
    } else {
        super.setUI(macOsXSeparatorUI);
        setForeground(UIManager.getColor(MAC_OS_X_SEPARATOR_COLOR_KEY));
    }
}
 

private static SeparatorUI createCustomUI(JComponent separator) {
    if (MAC_OS_X_SEPARATOR_UI_CLASS != null) {
        try {
            Method m = MAC_OS_X_SEPARATOR_UI_CLASS.getDeclaredMethod(
                       "createUI", JComponent.class); // NOI18N
            return (SeparatorUI)m.invoke(null, separator);
        } catch (Throwable e) {
            return null;
        }
    } else {
        return null;
    }
}
 

public void setUI(SeparatorUI ui) {
    synchronized(this) {
        if (!separatorUIInitialized) {
            macOsXSeparatorUI = createCustomUI(this);
            separatorUIInitialized = true;
        }
    }

    if (macOsXSeparatorUI == null) {
        super.setUI(ui);
    } else {
        super.setUI(macOsXSeparatorUI);
        setForeground(UIManager.getColor(MAC_OS_X_SEPARATOR_COLOR_KEY));
    }
}
 

private static SeparatorUI createCustomUI(JComponent separator) {
    if (MAC_OS_X_SEPARATOR_UI_CLASS != null) {
        try {
            Method m = MAC_OS_X_SEPARATOR_UI_CLASS.getDeclaredMethod(
                       "createUI", JComponent.class); // NOI18N
            return (SeparatorUI)m.invoke(null, separator);
        } catch (Throwable e) {
            return null;
        }
    } else {
        return null;
    }
}