how can my program wait until finish responce

I create web servlet my index page contain input text and submit button and this index page call servlet page (readXML) that take the input from the index page and write it in XML file then process this xml file then generate another XML file that contains processed text

after that (readXML) servlet call another servlet (coloring)

the problem is (readXML) takes time to process and generate the XML file so (readXML) call the other servlet (coloring) before finishing generate the XML file

are there any way to wait until the XML file generated ?