CODEWITHSUNDEEP
java
Saturn
Saturn

Reputation: 18149

How to append html to a website response before it reaches the browser in Java?

Recently I used a Mac application called Spotflux. I think it's written in Java (because if you hover over its icon it literally says "java"...).

It's just a VPN app. However, to support itself, it can show you ads... while browsing. You can be browsing on chrome, and the page will load with a banner at the bottom.

Since it is a VPN app, it obviously can control what goes in and out of your machine, so I guess that it simply appends some html to any website response before passing it to your browser.

I'm not interested in making a VPN or anything like that. The real question is: how, using Java, can you intercept the html response from a website and append more html to it before it reaches your browser? Suppose that I want to make an app that literally puts a picture at the bottom of every site you visit.

Upvotes: 4

Views: 98

Answers (2)

FrobberOfBits
FrobberOfBits

Reputation: 18022

There are many ways to do this. Probably the easiest would be to proxy HTTP requests through a web proxy like RabbIT (written in java). Then just extend the proxy to mess with the response being sent back to the browser.

In the case of Rabbit, this can either be done with custom code, or with a special Filter. See their FAQ.

WARNING: this is not as simple as you think. Adding an image to the bottom of every screen will be hard to do, depending on what kind of HTML is returned by the server. Depending on what CSS, javascript, etc that the remote site uses, you can't just put the same HTML markup in and expect it to act the same everywhere.

Upvotes: 1

Neville Kuyt
Neville Kuyt

Reputation: 29639

This is, of course, a hypothetical answer - I don't really know how Spotflux works.

However, I'm guessing that as part of its VPN, it installs a proxy server. Proxy servers intercept HTTP requests and responses, for a variety of reasons - most corporate networks use proxy servers for caching, monitoring internet usage, and blocking access to NSFW content.

As a proxy server can see all HTTP traffic between your browser and the internet, it can modify that HTTP; often, a proxy server will inject an HTTP header, for instance; injecting an additional HTML tag for an image would be relatively easy.

Here's a sample implementation of a proxy server in Java.

Upvotes: 3

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