Is it possible to intercept outgoing tcp requests of a java jar?

The problem I am trying to solve is figuring out all the external dependencies of an "Etl" java jar. The jar can call out to any number of external web services but there is no real insight to what they are calling out to.

We launch these jars from another java application(e.g. "java -jar jarname.jar"), so that's why I think we might be able to do something to intercept all http requests from the launch jar using a java agent or something like that.

In all, is it possible to intercept all outgoing tcp requests when launching a java jar? I say tcp because we are looking for all outgoing connections database, rabbitmq, http.

The usual strategy is to use tools like wireshark which can be configured to tell you all the details (including the data itself), in a nice user interface, and you can trivially filter on 'only all traffic caused by this specific process'.

You CAN also do this with an agent, but you're signing yourself up for a ton of code work for an end result that is going to be vastly inferior to what tools like wireshark can tell you.

If you want to go the agent route, find every entrypoint call that could possibly set up a TCP connection. Hopefully, that's just Socket, but in practice I bet there is more, and I don't think there's a list out there. Use ASM or bytebuddy or some other bytecode rewrite tool to 'load' every class being defined 'in transit', inspect it, and rewrite any calls to e.g. the Socket constructor to your own utility method that logs this, or wraps this so you can keep track of it. This will be quite a sizable project.

You don't really need an agent for this, though; if you want to know when connections happen, just, in your main, set up a SecurityManager - allow everything, but do some logging in the relevant methods. For example:

class MyNetworkInspectingManager extends SecurityManager {
    @Override public void checkAccept(String host, int port) {
        log.warn("accepting connection on " + host + ":" + port);
    }

    @Override public void checkConnect(String host, int port, Object context) {
        log.warn("opening connection to " + host + ":" + port);
    }

    @Override public void checkConnect(String host, int port) {
        log.warn("opening connection to " + host + ":" + port);
    }
}

and possibly, also override checkListen, checkMulticast.

then, in your main:

public static void main(String[] args) throws Exception {
    System.setSecurityManager(new MyNetworkInspectingManager());
}

Still far less useful than wireshark, but it's an hour or so to set this up, vs. the weeks+ to make the agent solution work out.