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RESTful web applications using WAN IP instead of localhost

I am developing a web application in Java by using RESTful web services and Tomcat. So far I was using the localhost in the URI: http://localhost:8080/3.ServerAPI/rest/Variable. But what if I want to use the real IP?
I have tried that on a local network by replacing the localhost with the LAN IP and it works fine: http://192.168.1.2:8080/3.ServerAPI/rest/Variable
The application at this address received the Variable.
If I want to send this through the internet as long as I know I have to use the WAN IP: http://188.39.25.247:8080/3.ServerAPI/rest/Variable
My question is, if I use the last URI with the WAN IP do I need also to port forward to the LAN IP by configuring the router or it is going to work like when I used the LAN IP ??
Thanks in advance

It depends on your network setup really.
You may have to enable port forwarding on your router to direct the request to the machine that your server is on, also make sure that the router allows connections to port 8080.
I had to do this recently when working with callbacks on external APIs. Seem to remember that I had to enable port forwarding on my router to get it to work. Wasn't too difficult though, just check router instructions on how to do it - like I say, depends on network setup though.
Hope this helps.

I've seen something similar to this and the problem was due to the port being used by another application.

Connect to http server behind vpn with java

I would like to connect to an REST Web Service through a VPN. Is there a way in Java to establish an pptp, l2tp ipsec connection to the VPN gateway an tunnel the HTTP request, without using the Operation System functions? This is important because I will connected to several rest services from a servlet. This Services could be behind different VPNs and I do not want to connect the network of the server with this VPNs.
Do anyone know about an API for that?

If you want to connect to a server behind a private VPN, from the outside, nothing you can do on you app can/will allow you to do connect. Unless you launch a VPN client and programmatically connect your network, to that VPN server, your java app will just sit there waiting for a socket on http connect.
Your question is technically incorrect (not from the SO point of view).
Look for a VPN client library that will pop up a dialog and take username/pwd.

A VPN has the purpose of connecting networks. If you want to reach another system via a VPN you will have to establish a network connection.
a Java API for all of this protocols will be (nearly) impossible, since VPN is handled by OS drivers and not on the application level (where java has its place) in most cases.
If you don't want to have your physical server being connected with those VPNs, you could perhaps set up a virtual system with virtualbox or vmware (or others) which handles all those connections and use it as a proxy. But this is no java issue than.

Here is a simple Java API that allows you to use Nord. I've made several bash scripts that also allow me to start, end and cycle NordIKE-VPN sessions. I have not used this yet, but I am intending on repurposing it for use with Android.
https://github.com/yaniferhaoui/NordVPN-Public-Java-API

Amazon EC2 server TCP Socket Connection

I have developed a Java server using Eclipse that accepts TCP socket connection from android client, performs some computations, and returns the result to the android phone using this socket. I tried it on Wi-Fi.
I want now to move the Java server to the cloud - basically amazon EC2. Is this possible? I am just using a simple tcp socket connection. I have checked and couldn't find an example but came across "elastic beanstalk". Any help is appreciated, maybe a link or tutorial with such an example.
can i convert my java project to .war and use it or can i install eclipse on the cloud and run it as i do locally?

It is definitely possible. And you don't have to convert your project to a .war, unless you want to.
All you have to do is:
Pick the Amazon Image (AMI) you want to use - Amazon Linux is a good place to start, but there are plenty of other options, including Ubuntu and Windows.
Set up a security group - you need to set an incoming rule for your server's port number. It is pretty easy to do this from the Amazon web-based console.
Start a machine and assign it to the security group you created. Again, this is easily accomplished from the amazon web console.
Once the machine is up, log in (using ssh for Linux or Remote Desktop for windows) and install your server.
A few things to remember:
Since you are now running on a public server, sooner or later your server will be attacked. EVERYONE gets attacked. If all you are opening is your single application port, make sure it is secured.
An Amazon server has a private and public IP. Your client application will connect to the public IP.
Servers can fail, and new servers get new public IPs! You need to prepare for this. Either make the IP in the client configurable, or look into something like Amazon Elastic IPs or dynamic DNS.

How to serve HTTP content from a port in Java

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.

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

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.

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.

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.

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).

Whats my hostname on my local MySQL server?

I just set up a MySQL server on my PC for testing Java with JDBC.
At the moment "localhost" works perfectly as hostname for my applications, when running them on the same system.
However what would be the hostname for my MySQL server for applications that are running on different computers? Something like "my_ip:port" would work? I was thinking of writing an applet, which I could upload on a web server and try to connect to my database here. Is it possible to achieve that?

Something like "my_ip:port" would work?
If the MySQL instance has bound to your public interface, and if your firewall allows it, yes. If you connect to the 'net via a router that does NAT (for instance, a combined DSL modem and wireless router allowing you to connect multiple computers), you'll have to set up forwarding rules in the router to tell it which of the local machines to forward requests to.
You don't have to use an IP address. Your machine will also probably have a host name of some kind (either one you've assigned or, if you connect through an ISP, more likely one they've assigned). That would work too.
I was thinking of writing an applet, which I could upload on a web server and try to connect to my database here. Is it possible to achieve that?
With a signed Java applet, yes; otherwise, no. That's because the security sandbox that Java applets run in doesn't let them access servers other than the one they were loaded from (the web server).
A much better approach is to have your client-side code (Java applet, or just DHTML+Ajax stuff) talk to server-side code on the web server, which in turn talks to your DB. That way, the DB is never directly exposed to the outside world, and you don't have to do things like signed applets.

You can always use the ip address of the server running mysql as the hostname or its fully qualified domain name.

That should work, but you also should consider port-forwarding through your firewall.
Go here to get your IP: http://www.whatsmyip.org/
The port is the port mysql is setup on.