Java proxy server - edit response before sending to browser, what to use?

Question

Java

It is necessary to raise a java proxy server to work with http requests/responses.
1) The user accesses the site through a proxy, he is given a JSON array with information that needs to be processed.
2) After that, the user goes to the next page - this is where you need to intercept the response from the server, edit the html page (Fill all inputs and selects with the data that we have already processed from the json array) and give it to the browser so that the user can safely reap button "done"
Questions:

• What is the library that allows you to raise an http / socks proxy server, with the ability to edit requests / responses on the fly? - at the moment I'm looking towards Netty, maybe there is something better / easier?
  
• Where to store json array? (It is planned that several people can sit on the same site through a proxy, so the variable is not an option)