比较两个几乎相似的xml文件，然后添加第一个xml文件中缺少的标签

I have to xml files say abc.xml & 123.xml which are almost similar, i mean has the same content, but the second one i.e, 123.xml has more content than the earlier one. I want to read both the files using Java, and compare whether the content present in abc.xml for each tag is same as that in 123.xml, and if 123.xml has some additional tags then add it into abc.xml same as remove tags from 123.xml if abc.xml doesnt have it.I have to xml files say abc.xml & 123.xml which