要进行改造，首先应认识到，GWT的每一个控件，对应的其实都是HTML的实现，GWT也支持我们拿到html元素，然后对其做修改，例如，对于窗体控件，可以通过geCaption()方法，拿到窗体头部对象，然后将其转为HTML对象，也就是

HTML caption = (HTML) getCaption()。拿到这个caption这个HTML对象之后，就能调用getElement().appenChild()方法，在上面添加其他元素，例如添加一个关闭的按钮。之后再给这个关闭按钮添加点击事件就OK啦~

具体实现如下

public class MyDialogBox extends DialogBox
{

	private Label closeBtn;

	private Label captiontLabel;

	public MyDialogBox()
	{

		super();
		this.createCaptionWithClose();

		this.setModal(true);
		
		this.setAutoHideOnHistoryEventsEnabled(true);
		this.setAnimationEnabled(true);
		this.setGlassEnabled(true);

	}

	private void createCaptionWithClose()
	{
		closeBtn = new Label("×");
		closeBtn.setStylePrimaryName("close");

		FlexTable captionLayoutTable = new FlexTable();
		captionLayoutTable.setWidth("100%");
		captiontLabel = new Label("");
		captionLayoutTable.setWidget(0, 0, captiontLabel);
		captionLayoutTable.setWidget(0, 1, closeBtn);
		captionLayoutTable.getCellFormatter().setHorizontalAlignment(0, 1, HasHorizontalAlignment.HorizontalAlignmentConstant.endOf(HasDirection.Direction.LTR));

		HTML caption = (HTML) getCaption();
		caption.getElement().appendChild(captionLayoutTable.getElement());

		caption.addClickHandler(new ClickHandler()
		{
			@Override
			public void onClick(ClickEvent event)
			{
				EventTarget target = event.getNativeEvent().getEventTarget();
				Element targetElement = (Element) target.cast();

				if (targetElement == closeBtn.getElement())
				{
					closeBtn.fireEvent(event);
				}
			}
		});

		addCloseHandler(new ClickHandler()
		{
			@Override
			public void onClick(ClickEvent event)
			{
				hide();
			}
		});
	}

	public void addCloseHandler(ClickHandler handler)
	{
		closeBtn.addClickHandler(handler);
	}
}