如何从 Java 中的端口提供 HTTP 内容

Question

When I run ActiveMQ by executing the batch file in its bin/ directory, I am able to go to its admin/management console by opening a web browser and going to http://localhost:8161/admin/ .

This has me curious.

This is my local sandbox and I do not have any web server ( httpd or otherweise) installed. So how is it that ActiveMQ is able to "register" a port on my machine, and listen to it exclusively?

If I try to go to http://localhost:8162/admin/ (notice the different port #), I get an unable to connect error.

Somewhere, somehow, AMQ is saying "map this URI ( localhost:8161 ) to some root directory on this machine". As a programmer, I'm interested in how something like this works.

Answer 1

A Java process is able to use any port (>= 1024 on linux) as a web server or for any other purpose. You don't need a separate web server to do this

Answer 2

I suggest you read up on sockets: here . All a web server is, is a socket listener that handles the HTTP protocol. HTTP protocol is here .

Web servers often handle a lot of other things, but that is the basics. If you want a small program that also runs a web server I suggest not re-inventing the wheel. Try incorporating jetty into your server.

Answer 3

ActiveMQ starts an embedded Jetty server , which listens for HTTP connections on that port. You don't need any other server running. It's all done from Java. If you dig down deep enough, you'll find some variety of ServerSocket at the bottom of it all. You can learn all about sockets and listening on ports in the Java Tutorial .

Answer 4

At its simplest level, ActiveMQ is creating a ServerSocket instance within itself and listening for connections using this server socket. A socket is always bound to a port.

Answer 5

One: this port is greater than (or equal to) 1024, so it means a "non root" user can listen on it.

Second: you can bind to ports from dedicated addresses only. This means ActiveMQ may have only opened that port on 127.0.0.1 (localhost). Try and see if you can open that URL from your external interface's IP address: chances are you cannot.

If you are under a Unix system, you can check what program listens on which port by using netstat -ltpn .

The basic system call for binding to a port is listen(2) .