I was given this sample code from college today and it worked fine inside the college but when I run it (using Eclipse) on my home machine I get permission denied. The machine in college is Windows (7) and my computer at home is Linux (Ubuntu).
Why am I getting the following error?
Error in I/O
Permission denied
I'm using port 338.
Copy of the code :
import java.io.*;
import java.net.*;
import java.util.*;
public class Server
{
public static void main(String[] args)
{
try
{
// First create the input from the keyboard
BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
System.out.println("Server Program");
// Get the port to listen on
System.out.print("Enter port number to listen on: ");
String port_string = input.readLine();
// The port number needs to be an int, so convert the String to an int
int port = Integer.parseInt(port_string);
// Create a ServerSocket to listen on this address
ServerSocket server = new ServerSocket(port);
// Accept an incoming client connection on the server socket
Socket sock = server.accept();
// Create the output stream to the client
DataOutputStream network = new DataOutputStream(sock.getOutputStream());
// Send message
network.writeUTF("Welcome " + sock.getInetAddress().getHostName() + ". We are " + new Date() + "\n");
// Close sockets. This will cause the client to exit
sock.close();
server.close();
}
catch (IOException ioe)
{
System.err.println("Error in I/O");
System.err.println(ioe.getMessage());
System.exit(-1);
}
}
}
Ports under 1024 are on most modern OS's (Ubuntu included) privileged, and require you to run the program as administrator/root or with elevated privileges.
Try a higher port for testing at home and you should be ok.
You say your home machine is running Ubuntu.
On Ubuntu (and other Unix-like operating systems) normal users are not allowed to listen on ports less than port 1024.
Try running it with a port number >= 1024.
Related
I have developed a simply video game and I want to update it with the multiplayer functionality using websockets. I want to have two versions of the game. The first one to work as server and the other version as a client. I want to initialize the game from the server and to wait for the reaction of the client. My first question: is it possible to run server and client in the same machine (give as an input the same ip)? Secondly I am using the following code in order to create the sockets from the server side:
try {
serverSocket = new ServerSocket(10007);
}
catch (IOException e)
{
System.err.println("Could not listen on port: 10007.");
System.exit(1);
}
System.out.println ("Waiting for connection.....");
try {
clientSocket = serverSocket.accept();
}
catch (IOException e)
{
System.err.println("Accept failed.");
System.exit(1);
}
System.out.println ("Connection successful");
When I am trying to connect from the client it seems that the whole thing its not working as i am receiving the message waiting for connection.... My code for connecting the client is the following:
String serverHostname = new String("ip");
if (args.length > 0)
serverHostname = args[0];
System.out.println("Attemping to connect to host " +
serverHostname + " on port 10007.");
Socket echoSocket = null;
PrintWriter out = null;
BufferedReader in = null;
try {
echoSocket = new Socket(serverHostname, 10007);
out = new PrintWriter(echoSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(
echoSocket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Don't know about host: " + serverHostname);
System.exit(1);
} catch (IOException e) {
System.err.println("Couldn't get I/O for "
+ "the connection to: " + serverHostname);
System.exit(1);
}
What can it be the issue here?
Your Server Side Code is fine. just a little note here, the ServerSocket.accept(); method is ablocking call, meaning that program execution will halt there until a client connects to it.
Secondly i can see an issue from the first line of your client code below
if (args.length > 0)
serverHostname = args[0];
args[0] may not be an ip address, i'm not too sure about how java command line applications behave but in c++ for instance, args[0] in the right context is always the absolute path of the program executable. this may be the case in java too.
So you may be passing an ip address but will actually be passed in as args[1].
So I'm trying to communicate via client/server using sockets between a raspberry pi and a laptop. I've been able to send basic strings over using a simple python script on my pi to get the basic idea of how it worked. Now I got a little more advanced and started using OpenCV along with a usb camera to make a little security system that detects motion in the frame of the camera. I have the python script with the security system connect with the server and it is supposed to print out "Occupied" or "Unoccupied" in the console on my laptop in real time (eventually supposed to open a pop-up menu alerting that motion is detected) but it only prints out a long huge string of either "Occupied" or "Unoccupied" once I close the connection with my Pi. Why isn't it printing out in real time? Here is the java code on my laptop, unforunately my Pi is in school at the moment and I can't access the python code but I will post it tomorrow.
public class PyComms {
public static void main(String[] args) {
try{
ServerSocket server = new ServerSocket(4444);
System.out.println("Waiting for client on port 4444");
while(true){
Socket connected = server.accept();
System.out.println("CONNECTED WITH CLIENT");
BufferedReader inFromPi = new BufferedReader(new InputStreamReader(connected.getInputStream()));
while(true){
String fromclient = inFromPi.readLine();
if(fromclient.equalsIgnoreCase("Occupied")){
System.out.println("Client responded with "+fromclient + "\n");
}
else{
System.out.println("Client responded with "+fromclient + "\n");
connected.close();
}
}
}
}
catch(Exception e){
System.out.println(e);
}
}
}
The answer was exactly what #jtahlborn said. All I had to do was include a new line after each message was sent in the python code. I achieved this by doing something similar to client_socket.send(text+'\n')
I studied the Client-Server chapter of a Java book and I copied the code examples for a easy Client-Server interaction.
The server:
package knowledge;
import java.io.*;
import java.net.*;
public class DateTimeServer {
public static void main(String[] args) {
try {
int port = Integer.parseInt(args[0]);
ServerSocket server = new ServerSocket(port);
System.out.println("DateTimeServer laeuft");
Socket s = server.accept();
new DateTimeProtokoll(s).transact();
} catch (ArrayIndexOutOfBoundsException ae) {
System.out.println("Aufruf: java DateTimeServer <Port-Nr>");
} catch (IOException e) {
e.printStackTrace();
}
}
}
Protocol:
package knowledge;
import java.io.*;
import java.net.*;
import java.util.*;
import java.text.*;
public class DateTimeProtokoll {
static SimpleDateFormat time = new SimpleDateFormat(
"´Es ist gerade´H´.´mm´ Uhr.´");
static SimpleDateFormat date = new SimpleDateFormat(
"´Heute ist´EEEE´, der ´dd.MM.yy´");
Socket s;
BufferedReader vomClient;
PrintWriter zumClient;
public DateTimeProtokoll(Socket s) {
try {
this.s = s;
vomClient = new BufferedReader(new InputStreamReader(
s.getInputStream()));
zumClient = new PrintWriter(s.getOutputStream(), true);
} catch (IOException e) {
System.out.println("IO-Error");
e.printStackTrace();
}
}
public void transact() {
System.out.println("Protokoll gestartet");
try {
zumClient.println("Geben Sie DATE oder TIME ein");
String wunsch = vomClient.readLine();
Date jetzt = new Date();
if (wunsch.equalsIgnoreCase("date"))
zumClient.print(date.format(jetzt));
else if (wunsch.equalsIgnoreCase("time"))
zumClient.println(time.format(jetzt));
else
zumClient.println(wunsch + "ist als Kommando unzulaessig!");
s.close();
} catch (IOException e) {
System.out.println("IO-Error");
}
System.out.println("Protokoll beendet");
}
}
The Client:
package knowledge;
import java.net.*;
import java.io.*;
public class DateTimeClient {
public static void main(String[] args) {
String hostName="";
int port;
Socket c=null;
try{
hostName=args[0];
port= Integer.parseInt(args[1]);
c=new Socket(hostName,port);
BufferedReader vomServer=new BufferedReader(
new InputStreamReader(c.getInputStream()));
PrintWriter zumServer=new PrintWriter(c.getOutputStream(),true);
BufferedReader vonTastatur=new BufferedReader(
new InputStreamReader(System.in));
System.out.println("Server "+ hostName+":"+port+ "sagt:");
String text=vomServer.readLine();
System.out.println(text);
text=vonTastatur.readLine();
zumServer.println(text);
text=vomServer.readLine();
System.out.println(text);
c.close();
}
catch(ArrayIndexOutOfBoundsException ae){
System.out.println("Aufruf:");
System.out.println("java DateTimeClient <HostName><PortNr>");
}
catch(UnknownHostException ue){
System.out.println("Kein DNS-Eintrag fuer: "+hostName);
}
catch(IOException e){
System.out.println("IO-Error");
}
}
}
Here are some notes of my approach and my beliefs. Please disagree on wrong statements stated below:
1)I believe it is no problem to run Client as well as Server on the same (my) computer.
2)I use Eclipse, so I run Eclipse two times in two different workspaces.
3)My input for server program is (run configuration->arguments): 2222
4)My input for the client program is: 2223 my_ip
(my_ip is for example 127.0.0.1 I choosed to write my_ip instead because I am not sure if it is dangerous to reaveal my ip in public)
4b) also: "2223" "my_ip"
4c) and: {"2223","my_ip"}
5) also 2222 my_ip
(although the figure in my book suggests that the port numbers of client and server should be different, but you never know)
Also I get this very often this error message:
Address already in use sounds like client and server port numbers should be different. But I dont know and thats why I ask. Thank you for your help
(The page did not like my code so I took screenshots :/)
I replaced the images with code. Sorry for the inconvenience.
Restarting first the server with input 2222 and then the client with input 127.0.0.1 2222. After a while the client posts "IO-Error". Its the exception from the clien class (not from protocol class) right? Why is the exception triggered? Is the code working for someone? Thank you
1) Correct. Client and server can be on the same computer, no problem there.
2) Not required, but ok.
3) This will make the server listen on port 2222
4) 127.0.0.1 is just another way of saying "this computer". It is the same as "localhost". Your actual ip is irrelevant to the question anyway, 127.0.0.1 will suffice.
5) Your server is asking on which port to listen (the SOURCE port), your client is asking to which port to connect to (the TARGET port of the client). Of course that should be the SAME port, otherwise the client will try to send a message to port X while the server will listen on port Y.
Imagine the ip as a house address, for example "Mainstreet 12, MyCity". The port would be the appartment number then. Your server occupies appartment 2222, so of course the client needs to try to connect to appartment 2222, otherwise it will not reach any server.
The error is most likely just because you don't actually stop your old server program. Stop it (big red button in eclipse), otherwise it will "occupy" the given port (which will prevent any other program from listing at that port, thus you cannot have two servers running which are both listening on the same port). If we reuse my crude analogy: An appartment cannot contain two servers at the same time, so if one is already in appartment 2222, a second one trying to live there will fail.
I don't know exactly why, but usually server binds host as 127.0.0.1 or localhost or your IP like 192.168.1.100 but if one of that are not listed, than it might be failed to call. See more with netstat.exe on Windows.
The "Address already in use" exception is only because the last Java session still running, could not terminate for some reason, and if you use IDE like Eclipse, it will often happen. :)
Make sure that all java thread is terminated (with task manager in Windows). Note that Eclipse is a Java thread too! You would better run the server in console mode..
In real world, native sockets are rarely used, because there some higher level protocol and technology like HTTP, SOAP, WebService, microService, SOA etc. If native socket is not necessary, for example you communicate with microcontroller, you should use these technologies, because these are more robust and others can easily communicate with your interface.
I try to check if port 80 is available using the following method :
Sockets: Discover port availability using Java
I have a Java application that checks if port 80 is available, if so, it runs a small web server listening on port 80. It works great to detect if another Java application listens on port 80, e.g. if I run my application two times, the second instance will correctly tell me that the port 80 is being used.
The problem is that I have WAMP running and listening on port 80, and that if I run my Java application after I started WAMP, it won't tell me that the port 80 is busy. It seems that it only tells me if another Java application uses the port 80.
That goes beyond my understanding ... any help is greatly appreciated!
Code snippet:
int port = 80;
if(!Connection.isPortAvailable(port)) {
logger.info("Port " + port + " is already in use");
}
// in Connection class
public static boolean isPortAvailable(int port) {
ServerSocket ss = null;
DatagramSocket ds = null;
try {
ss = new ServerSocket(port);
ss.setReuseAddress(true);
ds = new DatagramSocket(port);
ds.setReuseAddress(true);
return true;
} catch (IOException e) {
} finally {
if (ds != null) {
ds.close();
}
if (ss != null) {
try {
ss.close();
} catch (IOException e) {
/* should not be thrown */
}
}
}
return false;
}
The correct answer to all questions of this nature is to try to use it and catch the exception. Not try to see if it's available and then try to use it and still have to handle the exception, which has several obvious problems:
There is a timing window between 'see' and 'try' during which the situation can change (both ways).
You still have to catch failures in the 'use' part anyway.
It is basically just trying to predict the future. This is supposed to be computer science, not fortune-telling.
This applies to most values of 'it', including network ports, files, any resource really.
I was able to reproduce your problem by running WampServer (verified that it was running by visiting localhost:80) and running a minimal java program given your example code.
The code in the try block did not throw an exception when WampServer was running. However, modify the first few lines of the try block like this
ss = new ServerSocket();
ss.bind(new InetSocketAddress("127.0.0.1", port));
and isPortAvailable will properly detect when WampServer is running and when it is not. Using "0.0.0.0" instead of "127.0.0.1" didn't work with WampServer, but did properly detect when IIS was running. You can check both by closing the first socket
ss = new ServerSocket();
ss.bind(new InetSocketAddress("0.0.0.0", port));
ss.close();
ss = new ServerSocket();
ss.bind(new InetSocketAddress("127.0.0.1", port));
I have an Eclipse plugin that needs to open a pair of sockets to a flash application running on the local machine. Flash requires a policy file (blob of XML) giving permissions to access the ports in question. Flash prefers to get this policy file over port 843, Java treats ports < 1024 as privileged ports and Mac OS X and Linux similarly restrict access to ports < 1024. I don't want to run my Eclipse plugin with root permissions, so serving up the policy file on port 843 is not an option. According to Adobe documentation, if Flash can't get the policy file on port 843, it falls back to requesting the policy file on the port to which it's trying to connect. The ActionScript code looks like this:
/**
* Connecting to some port to communicate with the debugger. We initiate the
* connection because Flex doesn't allow us to listen to any ports.
*/
private function initSockets():void
{
requestSocket = new Socket();
requestSocket.addEventListener(Event.CONNECT, requestConnected);
requestSocket.addEventListener(Event.CLOSE, closed);
requestSocket.addEventListener(ProgressEvent.SOCKET_DATA, processRequestData);
requestSocket.addEventListener(IOErrorEvent.IO_ERROR, ioError);
requestSocket.addEventListener(SecurityErrorEvent.SECURITY_ERROR, securityError);
requestSocket.connect("localhost", SCConstants.DEBUG_LESSON_REQUEST_PORT);
eventSocket = new Socket();
eventSocket.addEventListener(Event.CONNECT, eventConnected);
eventSocket.addEventListener(Event.CLOSE, closed);
eventSocket.addEventListener(ProgressEvent.SOCKET_DATA, processEventData);
eventSocket.addEventListener(IOErrorEvent.IO_ERROR, ioError);
eventSocket.addEventListener(SecurityErrorEvent.SECURITY_ERROR, securityError);
eventSocket.connect("localhost", SCConstants.DEBUG_LESSON_EVENT_PORT);
}
On the Eclipse plugin side I've inherited some code that works most of the time on OS X, but sometimes fails on Windows. Running on Wi-Fi rather than wired ethernet also tends to fail, although I have no idea why this should matter.
public Boolean connect() throws DebugException {
try {
try {
// connection code
fRequestServerSocket = new ServerSocket(requestPort);
fRequestServerSocket.setSoTimeout(ACCEPT_TIMEOUT);
fEventServerSocket = new ServerSocket(eventPort);
fEventServerSocket.setSoTimeout(ACCEPT_TIMEOUT);
TWBLogger.logInfo("Open socket request server:" + fRequestServerSocket);
TWBLogger.logInfo("Open socket event server:" + fEventServerSocket);
String policy = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
"<cross-domain-policy>\n" +
"<allow-access-from domain=\"*\" to-ports=\"5000,5001\" secure=\"false\" />\n" +
"</cross-domain-policy>\0";
// Because of the Flash security policy the first thing
// that will accept on the socket will be the Flash Player
// trying to verify us. The Flash player will request security
// policy file with the following string: <policy-file-request/>\0
// We will serve back the above policy file and then close the socket
// The next thing to accept is our process in the VM.
fRequestSocket = fRequestServerSocket.accept();
fRequestWriter = new PrintWriter(fRequestSocket.getOutputStream());
fRequestReader = new BufferedReader(new InputStreamReader(fRequestSocket.getInputStream()));
// Wait some time before giving flash the policy file. Otherwise they don't get it. ;(
// 3 is too much ... ;(
Thread.sleep(100);
fRequestWriter.print(policy);
fRequestWriter.flush();
fRequestSocket.close();
// this should be the real connection
fRequestSocket = fRequestServerSocket.accept();
TWBLogger.logInfo("Open socket request:" + fRequestSocket);
fRequestWriter = new PrintWriter(fRequestSocket.getOutputStream());
fRequestReader = new BufferedReader(new InputStreamReader(fRequestSocket.getInputStream()));
// the same situation for the EventSocket
fEventSocket = fEventServerSocket.accept();
fEventReader = new BufferedReader(new InputStreamReader(fEventSocket.getInputStream()));
TWBLogger.logInfo("Open socket event:" + fEventSocket);
} catch (SocketTimeoutException e) {
TWBLogger.logWaring("Connection to the Client Timed out.");
cleanSockets();
return false;
requestFailed("Connection to the VM timed out. Please close any other running lessons that you debug and try again", e);
} catch (SocketSecurityException e) {
requestFailed("Security error occured when connecting to the VM", e);
} catch (Exception e) {
if (!fTerminated)
requestFailed("Error occured when connecting to the VM. Please close any other running lessons that you debug.", e);
}
} catch (DebugException e) {
// close the sockets so that we can debug another application
cleanSockets();
throw e;
}
// our VM is single threaded
fThread = new TWBThread(this);
fThreads = new IThread[] {fThread};
// start listening for events from the VM
fEventDispatch = new EventDispatchJob();
fEventDispatch.schedule();
// start listening for breakpoints
IBreakpointManager breakpointManager = getBreakpointManager();
breakpointManager.addBreakpointListener(this);
breakpointManager.addBreakpointManagerListener(this);
return true;
}
This code looks wrong. It doesn't wait for the message from Flash and instead just jams the policy response into the port. As I said, it works most of the time, but it fails sometimes and doesn't seem to comply with Adobe's documentation.
I tried listening for request packets on each port and sending a port specific response. I watched socket traffic using WireShark on the loopback interface (Mac OS X). I saw policy requests coming in and responses getting sent, but Flash still gave me Security Sandbox Violation on both ports.
I also tried adding this line at the beginning of initSockets shown above:
Security.loadPolicyFile("xmlsocket://localhost:5002");
Then I added code in my plugin to listen on port 5002 and send the following master policy file content:
private final static String FLASH_POLICY_RESPONSE =
"<?xml version=\"1.0\"?>\n" +
"<!DOCTYPE cross-domain-policy SYSTEM \"/xml/dtds/cross-domain-policy.dtd\">\n" +
"<cross-domain-policy>\n" +
"<site-control permitted-cross-domain-policies=\"master-only\"/>\n" +
"<allow-access-from domain=\"*\" to-ports=\"5000,5001\"/>\n" +
"</cross-domain-policy>\0";
Again I saw the request come in and the response go out, but Flash didn't seem to respond to it. I didn't get the Security Sandbox Violation errors, but there was also no traffic over the ports.
Can anyone enlighten me on the correct approach to opening sockets between Java and Flash?
I found the solution to this. I made a mistake early on and used BufferedReader.readLine to read the policy request. This isn't appropriate since policy requests are null terminated, not new line terminated. This was confusing since it does return when the underlying stream closes. Thus I got the request and sent a response, but the response was sent after the ActionScript code had already decided that the request had failed.
On the Java side I used the following code to establish communication on the ports:
// Create server sockets.
fRequestServerSocket = new ServerSocket(REQUEST_PORT);
fRequestServerSocket.setSoTimeout(ACCEPT_TIMEOUT);
TWBLogger.logInfo("Open socket request server:" + fRequestServerSocket);
fEventServerSocket = new ServerSocket(EVENT_PORT);
fEventServerSocket.setSoTimeout(ACCEPT_TIMEOUT);
TWBLogger.logInfo("Open socket event server:" + fEventServerSocket);
// Serve up the Flash policy file.
serveFlashPolicy();
// Connect request socket.
fRequestSocket = fRequestServerSocket.accept();
TWBLogger.logInfo("Open socket request:" + fRequestSocket);
fRequestWriter = new PrintWriter(fRequestSocket.getOutputStream());
fRequestReader = new BufferedReader(new InputStreamReader(fRequestSocket.getInputStream()));
// Connect event socket.
fEventSocket = fEventServerSocket.accept();
TWBLogger.logInfo("Open socket event:" + fEventSocket);
fEventReader = new BufferedReader(new InputStreamReader(fEventSocket.getInputStream()));
Serving up the policy file is handled as follows:
private void serveFlashPolicy() {
ServerSocket serverSocket = null;
Socket socket = null;
TWBLogger.logInfo("Waiting for flash policy request on port " + FLASH_POLICY_PORT);
try {
serverSocket = new ServerSocket(FLASH_POLICY_PORT);
serverSocket.setSoTimeout(ACCEPT_TIMEOUT);
socket = serverSocket.accept();
PrintWriter writer = new PrintWriter(socket.getOutputStream());
BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
StringBuilder request = new StringBuilder();
int c;
while (0 < (c = reader.read())) {
request.append((char) c);
}
String policyRequest = request.toString();
if (policyRequest.startsWith(FLASH_POLICY_REQUEST)) {
writer.print(FLASH_POLICY_RESPONSE);
writer.print("\0");
writer.flush();
}
} catch (IOException e) {
TWBLogger.logWaring("IOException on port " + FLASH_POLICY_PORT + ": " + e.toString());
e.printStackTrace();
} finally {
if (null != socket) {
try {
socket.close();
} catch (Exception e) {
// Ignore
}
}
if (null != serverSocket) {
try {
serverSocket.close();
} catch (Exception e) {
// Ignore
}
}
}
TWBLogger.logInfo("Flash policy complete on port " + FLASH_POLICY_PORT);
}
The Flash policy response looks like this:
private final static String FLASH_POLICY_RESPONSE =
"<?xml version=\"1.0\"?>\n" +
"<!DOCTYPE cross-domain-policy SYSTEM \"/xml/dtds/cross-domain-policy.dtd\">\n" +
"<cross-domain-policy>\n" +
"<allow-access-from domain=\"*\" to-ports=\"5000,5001\"/>\n" +
"</cross-domain-policy>";
The site-control tag I had previously been sending is only allowed in master policy files served from port 843.