how and where does GWT define the mechanism of xml-2-widget converting mechanism?

for example,in a ui.xml file,we use <g:at> and <g:label> within a <g:AbsolutePanel> tag,and use <g:anyWidget> or <htmltag> in a <HTMLPanel> tag.
so where does GWT define the relationship of xml-2-widget converting when injecting?

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