Develop Reference

Running a live Java Server on Amazon AWS

I am developing a Client-Server application with several other programmers, in Java. At this point in time I do not want to be running the code locally. I want to be able to connect to the Server from any machine.
I wrote a test server and test client, just to make sure that things are working properly. But they are not. I am using Amazon AWS EC2 Linux that comes with Java. I am able to compile and run my Server after I SSH into the EC2, but the Client on my local disk is just not connecting. Here is the code.
// Code found online (https://cs.lmu.edu/~ray/notes/javanetexamples/)
import java.io.IOException;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
public class TestServer {
public static void main(String[] args) throws Exception {
try (ServerSocket listener = new ServerSocket(50000)) {
System.out.println("The capitalization server is running...");
System.out.println(listener.getInetAddress());
ExecutorService pool = Executors.newFixedThreadPool(20);
while (true) {
pool.execute(new Capitalizer(listener.accept()));
}
}
}
private static class Capitalizer implements Runnable {
private Socket socket;
Capitalizer(Socket socket) {
this.socket = socket;
}
#Override
public void run() {
System.out.println("Connected: " + socket);
try {
Scanner in = new Scanner(socket.getInputStream());
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
while (in.hasNextLine()) {
out.println(in.nextLine().toUpperCase());
}
} catch (Exception e) {
System.out.println("Error:" + socket);
} finally {
try { socket.close(); } catch (IOException e) {}
System.out.println("Closed: " + socket);
}
}
}
}
// Code found online (https://cs.lmu.edu/~ray/notes/javanetexamples/)
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;
public class TestClient {
public static void main(String[] args) throws Exception {
try (Socket socket = new Socket("ADDRESS HERE", 50000)) {
System.out.println("Enter lines of text then Ctrl+D or Ctrl+C to quit");
Scanner scanner = new Scanner(System.in);
Scanner in = new Scanner(socket.getInputStream());
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
while (scanner.hasNextLine()) {
out.println(scanner.nextLine());
System.out.println(in.nextLine());
}
}
}
}
In place of "ADDRESS HERE" in the Client, I have tried the private IP and public IP of my Amazon EC2 instance. I have also tried the public DNS name. Nothing seems to work. There is just no connection from the Client to the Server. In fact, "Enter lines of text then Ctrl+D or Ctrl+C to quit" never prints.
All help is appreciated. Thank you.

Allow your IP address to send request to the EC2. For this, you need to go to your Security Group and add your IP there. Follow these steps-
GO to your AWS console.
Click on EC2, then under Resources you will find Security Groups.
Select your security group.
Follow the steps in the given image.

Since you're able to connect to EC2 instance via SSH, your Security Group allows this.
Now you need to allow requests from the client in this Security Group. You will either need to provide a concrete IP, IP range or allow all IPs (not recommended) in the group.
You can find how to do this here.

Redirect Client in Java-Run Server

I am creating a Java HTTP server that checks to make sure a client is not banned before redirecting to the main server. I have already created everything for the server that is needed, I just don't know how to redirect to another port that is running the main server. Here is my code:
package netlyaccesscontrol;
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
public class AllowedCheck {
public static void main(String[] args) {
String line = null;
try {
FileReader reader = new FileReader("Banned.txt");
BufferedReader buffer = new BufferedReader(reader);
ServerSocket s = new ServerSocket(80);
Socket c = s.accept();
String clientIP = c.getInetAddress().toString();
while ((line = buffer.readLine()) != null) {
if (clientIP == line) {
s.close();
} else {
// redirect to main server here
}
}
} catch (FileNotFoundException ex) {
System.out.println("The banned IP address file does not exist.");
} catch (IOException ex) {
ex.printStackTrace();
}
}
}

The redirection that you are thinking of is something supported by HTTP and the browsers. There's a specific HTTP response code that tells the caller to redirect and a way to specify it.
Raw sockets are a low-level network protocol that is not going to support redirection as you expect. The most you might be able to do is have this program be a proxy and, upon success, push all incoming data/outgoing responses to/from the ultimate server. But what you have here is by no means going to cut it.

Java Socket Connection Failure

I have a server code in Java which I run on my machine and my friend has a client code which runs on his machine. When he enters my IP so as to connect to my server and get the date, connection fails and nothing happens. Note that when I run server and client programs on my own machine and enter localhost as the address, connection is successful and I get the date message correctly. I'm looking for possible errors and problems causing this.
Server code in Java:
import java.io.IOException;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Date;
public class Server {
public static void main(String[] args) throws IOException {
ServerSocket listener = new ServerSocket(9999);
try {
while (true) {
Socket socket = listener.accept();
try {
PrintWriter out =
new PrintWriter(socket.getOutputStream(), true);
out.println(new Date().toString());
} finally {
socket.close();
}
}
}
finally {
listener.close();
}
}
}
Client code in Java:
import javax.swing.*;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.Socket;
public class Client {
public static void main(String[] args) throws IOException {
String serverAddress = JOptionPane.showInputDialog(
"Enter IP Address of a machine that is\n" +
"running the date service on port 9999:");
Socket s = new Socket(serverAddress, 9999);
BufferedReader input =
new BufferedReader(new InputStreamReader(s.getInputStream()));
String answer = input.readLine();
JOptionPane.showMessageDialog(null, answer);
System.exit(0);
}
}

Some routers might isolate computers in different networks. Try it with both computers on Wifi or both wired to the router. Are your IPs on the same network? Can you see your friend's computer on the network? There might also be some security configurations on your router.
Other than that and firewall issue (which you have disabled), the code looks like it should work fine.

Passing Data from a Java program to a Python program and getting results back

What is the preferred way of passing data (a list of string) from a Java program to a Python script. The python script performs some processing on the data and then I need to get the results back in my Java program.
Is there is a framework that allows you to do this easily?
EDIT: More specific requirements.
My Java program is a scheduler (runs every X minutes and Y seconds ) that connects to an external service and gets the RAW data and send it to python.
I can rewrite everything in Python but that will take a me good amount of time. I was looking if there is a way to reuse what I already have.
I want to use an existing Python script with minimal change. My python script uses a bunch of external libraries (e.g., numpy)
The data passed from Java to Python is in Json format and the data returned by Python is also Json.
Using sockets is an options but then I've to run server processes.

I hacked this together a couple of months ago when I was faced with an similar problem. I avoided Jython because I wanted separate processes. The Java code is the server as it listens for requests but it doesn't re-connect on failure. The concept is is that the classes are extended threads that have a socket member so the send and receive commands can block the object threads and leave the host threads unaffected.
Python Code:
import StringIO
import re
import select
import socket
import sys
import threading
class IPC(threading.Thread):
def __init__(self, line_filter = None):
threading.Thread.__init__(self)
self.daemon = True
self.lock = threading.Lock()
self.event = threading.Event()
self.event.clear()
self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.recv_buffer_size = 8192
self.buffer = StringIO.StringIO()
if(line_filter == None):
self.line_filter = lambda x: x
else:
self.line_filter = line_filter
def run(self):
self.sock.connect(("localhost", 32000))
data = True
while data:
try:
data = self.sock.recv(self.recv_buffer_size)
except socket.error, e:
print e
self.sock.close()
break
self.lock.acquire()
self.buffer.write(data)
self.lock.release()
self.event.set()
def readlines(self):
self.lock.acquire()
self.buffer.seek(0)
raw_lines = self.buffer.readlines()
self.buffer.truncate(0)
self.lock.release()
lines = map(self.line_filter, raw_lines)
return lines
proc_control = IPC()
while True:
proc_control.event.wait()
data = proc_control.readlines()
if(data):
# Do Stuff
proc_control.event.clear()
Java Code:
SocketIPC.java:
package project;
import java.net.Socket;
import java.net.ServerSocket;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.PrintWriter;
import java.io.OutputStreamWriter;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;
public class SocketIPC {
public PrintWriter out;
public BufferedReader in;
Socket socket = null;
ServerSocket serverSocket = null;
ConnectionListener connlisten = null;
DataListener datalisten = null;
Thread connlisten_thread = null;
Thread datalisten_thread = null;
CommandObject ipc_event_cmd = null;
// Server thread accepts incoming client connections
class ConnectionListener extends Thread {
private int port;
ConnectionListener(int port) {
this.port = port;
}
#Override
public void run() {
try {
serverSocket = new ServerSocket(port);
socket = serverSocket.accept();
out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())), true);
in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
datalisten = new DataListener();
datalisten_thread = new Thread(datalisten);
datalisten_thread.start();
} catch (Exception e) {
System.err.println("SocketIPC creation error: " + e.getMessage());
}
}
}
// Server thread accepts incoming client connections
class DataListener extends Thread {
String data_str = null;
DataListener() {
}
#Override
public void run() {
try {
while(true) {
data_str = recv();
ipc_event_cmd.buffer.add(data_str);
ipc_event_cmd.execute();
}
} catch (Exception e) {
System.err.println("SocketIPC reading error: " + e.getMessage());
}
}
public String read() {
String ret_string = null;
if(!ipc_event_cmd.buffer.isEmpty()) {
ret_string = ipc_event_cmd.buffer.remove(0);
}
return ret_string;
}
}
public SocketIPC(int port) {
ipc_event_cmd = new CommandObject();
connlisten = new ConnectionListener(port);
connlisten_thread = new Thread(connlisten);
connlisten_thread.start();
}
public void send(String msg) {
if (out != null) {
out.println(msg);
}
}
public void flush() {
if (out != null) {
out.flush();
}
}
public void close() {
if (out != null) {
out.flush();
out.close();
try {
in.close();
socket.close();
serverSocket.close();
} catch (Exception e) {
System.err.println("SocketIPC closing error: " + e.getMessage());
}
}
}
public String recv() throws Exception {
if (in != null) {
return in.readLine();
} else {
return "";
}
}
public void set_cmd(CommandObject event_cmd) {
if (event_cmd != null) {
this.ipc_event_cmd = event_cmd;
}
}
}
CommandObject.java:
package project;
import java.util.List;
import java.util.ArrayList;
public class CommandObject {
List<String> buffer;
public CommandObject() {
this.buffer = new ArrayList<String>();
}
public void execute() {
}
}
DoStuff.java:
package project;
import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.HashMap;
import java.util.Random;
public class DoStuff extends CommandObject {
public DoStuff () {
}
#Override
public void execute() {
String tmp_string = null;
while (!buffer.isEmpty()) {
tmp_string = buffer.remove(0);
// Do Stuff
}
}
}

Sounds like a job for Jython! Jython is an embeddedable Python runtime written in Java. As long as you don't need to run your Python script in another process (e.g., want to be able to kill it, may use lots of memory, etc.), this is the best way by far.

If you are trying to work Java and python together then make your life simple with Jython.
Jython, successor of JPython, is an implementation of the Python
programming language written in Java. Jython programs can import and
use any Java class. Except for some standard modules, Jython programs
use Java classes instead of Python modules. Jython includes almost all
of the modules in the standard Python programming language
distribution, lacking only some of the modules implemented originally
in C.
Assuming you have java lib in your python path. Here is a code snippet to give you an idea how simple it is to use the java classes:
'''
Import JavaUtilities class from a java package
'''
from com.test.javalib import JavaUtilities
'''
Call a java method
'''
response = JavaUtilities.doSomething();

Please have a look Jython,which is best for communication between java and Python.

Get default gateway in java

I want to fetch default gateway for local machine using java. I know how to get it by executing dos or shell commands, but is there any another way to fetch?
Also need to fetch primary and secondary dns ip.

My way is:
try(DatagramSocket s=new DatagramSocket())
{
s.connect(InetAddress.getByAddress(new byte[]{1,1,1,1}), 0);
return NetworkInterface.getByInetAddress(s.getLocalAddress()).getHardwareAddress();
}
Because of using datagram (UDP), it isn't connecting anywhere, so port number may be meaningless and remote address (1.1.1.1) needn't be reachable, just routable.

In Windows with the help of ipconfig:
import java.awt.Desktop;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URI;
public final class Router {
private static final String DEFAULT_GATEWAY = "Default Gateway";
private Router() {
}
public static void main(String[] args) {
if (Desktop.isDesktopSupported()) {
try {
Process process = Runtime.getRuntime().exec("ipconfig");
try (BufferedReader bufferedReader = new BufferedReader(
new InputStreamReader(process.getInputStream()))) {
String line;
while ((line = bufferedReader.readLine()) != null) {
if (line.trim().startsWith(DEFAULT_GATEWAY)) {
String ipAddress = line.substring(line.indexOf(":") + 1).trim(),
routerURL = String.format("http://%s", ipAddress);
// opening router setup in browser
Desktop.getDesktop().browse(new URI(routerURL));
}
System.out.println(line);
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
Here I'm getting the default gateway IP address of my router, and opening it in a browser to see my router's setup page.

There is not an easy way to do this. You'll have to call local system commands and parse the output, or read configuration files or the registry. There is no platform independent way that I'm aware of to make this work - you'll have to code for linux, mac and windows if you want to run on all of them.
See How can I determine the IP of my router/gateway in Java?
That covers the gateway, and you could use ifconfig or ipconfig as well to get this. For DNS info, you'll have to call a different system command such as ipconfig on Windows or parse /etc/resolv.conf on Linux or mac.

There is currently no standard interface in Java to obtain the default gateway or the DNS server addresses. You will need a shell command.

I'm not sure if it works on every system but at least here I found this:
import java.net.InetAddress;
import java.net.UnknownHostException;
public class Main
{
public static void main(String[] args)
{
try
{
//Variables to find out the Default Gateway IP(s)
String canonicalHostName = InetAddress.getLocalHost().getCanonicalHostName();
String hostName = InetAddress.getLocalHost().getHostName();
//"subtract" the hostName from the canonicalHostName, +1 due to the "." in there
String defaultGatewayLeftover = canonicalHostName.substring(hostName.length() + 1);
//Info printouts
System.out.println("Info:\nCanonical Host Name: " + canonicalHostName + "\nHost Name: " + hostName + "\nDefault Gateway Leftover: " + defaultGatewayLeftover + "\n");
System.out.println("Default Gateway Addresses:\n" + printAddresses(InetAddress.getAllByName(defaultGatewayLeftover)));
} catch (UnknownHostException e)
{
e.printStackTrace();
}
}
//simple combined string out the address array
private static String printAddresses(InetAddress[] allByName)
{
if (allByName.length == 0)
{
return "";
} else
{
String str = "";
int i = 0;
while (i < allByName.length - 1)
{
str += allByName[i] + "\n";
i++;
}
return str + allByName[i];
}
}
}
For me this produces:
Info:
Canonical Host Name: PCK4D-PC.speedport.ip
Host Name: PCK4D-PC
Default Gateway Leftover: speedport.ip
Default Gateway Addresses:
speedport.ip/192.168.2.1
speedport.ip/fe80:0:0:0:0:0:0:1%12