If your xml is written by java.lang.String you can just using HttpClient in this way
public void post() throws Exception{
HttpClient client = new DefaultHttpClient();
HttpPost post = new HttpPost("http://www.baidu.com");
String xml = "<xml>xxxx</xml>";
HttpEntity entity = new ByteArrayEntity(xml.getBytes("UTF-8"));
post.setEntity(entity);
HttpResponse response = client.execute(post);
String result = EntityUtils.toString(response.getEntity());
}
pay attention to the Exceptions.
BTW, the example is written by the httpclient version 4.x