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A problem about Java UDP communication with multi thread

I'm studying java network programming now and i wrote a program that the client sends 30 times current time to Server and the Server will create a new thread to parse the received UDP packet and give feedback to Client if Server receives the UDP packet.
The question is, after I run my code, the Server can receive the UDP packet and create a new thread, but it seems like DatagramSocket and DatagramPacket don't pass to the thread. Thence the thread can't give a feedback to Client and Client will wait all the time after send the first UDP packet.
My code is here:
Server
public class MulUDPServer {
public static void main(String[] args) {
DatagramSocket socket = null;
DatagramPacket receivedPacket;
final int PORT = 10010;
byte[] b = new byte[1024];
receivedPacket = new DatagramPacket(b, b.length);
try {
socket = new DatagramSocket(PORT);
System.out.println("Server start!");
while (true) {
// receive the packet from server
socket.receive(receivedPacket);
// to check if Server get the packet
System.out.println(new String(receivedPacket.getData(), 0, receivedPacket.getLength()));
// start the thread to handle the packet we have got
Thread thread = new Thread(new LogicThread(socket, receivedPacket));
thread.start();
}
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
// close the connection
socket.close();
} catch (Exception e) {
}
}
}
Thread
public class LogicThread implements Runnable {
DatagramSocket socket = null;
DatagramPacket receivedPacket = null;
public LogicThread(DatagramSocket socket, DatagramPacket receivedPacket) {
this.socket = socket;
this.receivedPacket = receivedPacket;
}
public void run() {
try {
// to test if a thread have been set up
System.out.println("a thread have been set up");
byte[] data = receivedPacket.getData();
int len = receivedPacket.getLength();
// get the client IP
InetAddress clientAddress = receivedPacket.getAddress();
// get the client port
int clientPort = receivedPacket.getPort();
// print the info about received packet
System.out.println("Client's IP：" + clientAddress.getHostAddress());
System.out.println("Client's port：" + clientPort);
System.out.println("The info：" + new String(data, 0, len));
// feedback to Client
byte[] b = "OK".getBytes();
DatagramPacket sendPacket = new DatagramPacket(b, b.length, clientAddress, clientPort);
// send
socket.send(sendPacket);
} catch (Exception e) {
e.printStackTrace();
}
}
}
Client
public class MulUDPClient {
public static void main(String[] args) {
DatagramSocket socket = null;
DatagramPacket sendPacket;
DatagramPacket receivedPacket;
String serverHost = "localhost";
int serverPort = 10010;
try {
socket = new DatagramSocket();
InetAddress address = InetAddress.getByName(serverHost);
byte[] b = new byte[1024];
receivedPacket = new DatagramPacket(b, b.length);
System.out.println("Client ready!");
for (int i = 0; i < 30; i++) {
// get the current time
Date d = new Date();
String content = d.toString();
byte[] data = content.getBytes();
sendPacket = new DatagramPacket(data, data.length, address, serverPort);
socket.send(sendPacket);
System.out.println("already send time");
Thread.sleep(10);
// receive packet from Server
socket.receive(receivedPacket);
byte[] response = receivedPacket.getData();
int len = receivedPacket.getLength();
String s = new String(response, 0, len);
System.out.println("the feedback from Server：" + s);
}
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
// close the connection
socket.close();
} catch (Exception e) {
}
}
}
}
the **Result** after run the Server and Client separately in two terminals:
Server
Server start!
Fri Nov 23 14:52:02 CST 2018
a thread have been set up
Client
Client ready!
already send time
From the result we can know, the Client have send a UDP packet and the Server parse it correctly and create a thread. Then the program is waiting...
It puzzle me a few days, Can anyone help me to solve it? :). Thanks!
Edit
Client
**CAN NOT** work
for (int i = 0; i < 30; i++) {
Thread writerWorker = new WriterWorker(socket);
writerWorker.start();
Thread readerWorker = new ReaderWorker(socket);
readerWorker.start();
}
**CAN** work
for (int i = 0; i < 30; i++) {
Date d = new Date();
String content = d.toString();
byte[] data = content.getBytes();
sendPacket = new DatagramPacket(data, data.length, address, serverPort);
socket.send(sendPacket);
Thread.sleep(10);
Thread readerWorker = new ReaderWorker(socket);
readerWorker.start();
}
WriterWorker
public class WriterWorker extends Thread {
DatagramSocket socket;
String serverHost = "localhost";
int serverPort = 10000;
DatagramPacket sendPacket;
public WriterWorker(DatagramSocket socket) {
this.socket = socket;
}
#Override
public void run() {
try {
InetAddress address = InetAddress.getByName(serverHost);
Date d = new Date();
String content = d.toString();
byte[] data = content.getBytes();
sendPacket = new DatagramPacket(data, data.length, address, serverPort);
socket.send(sendPacket);
System.out.println("already send time");
} catch (Exception e) {
// TODO: handle exception
}
}
}

there is a flaw in your logic, when you do socket.receive(receivedPacket); this will let the worker wait until some datagramm packets does arrive...
the right way to handle asynchron communication via sockets would be the following (here an example for client side)
socket = new DatagramSocket();
Thread readerWorker = new ReaderWorker(socket);
readerWorker.start();
Thread writerWorker = new WriterWorker(socket);
writerWorker.start();
that way you would seperate reading and writing into different threads and the blocking methode socket.receive(...) would not stop your writing thread...
each worker would implement it's own worker loop
writer loop:
while(true){
if (sendPacket!= null){
socket.send(sendPacket);
}
Thread.sleep(10);
}
reader loop:
while(true){
socket.receive(receivedPacket);
handlePacket(receivedPacket);
}
NOTE:
that code was written totally out of my mind i didn't check proper syntax

delete the "Thread.sleep(10)" in client

Why is DatagramPacket object get stuck when using one of it's methods? [duplicate]

This question already has an answer here:
Can not access object's properties(methods) from within the run method! Java Multithreading
(1 answer)
Closed 5 years ago.
I am using a server and a client to share information via UDP packets.
The client assembles a packet and sends it to the server.
The server receives the packet , and issues a thread to handle it.
In the threads code I try to use the packet but I get stuck for every method I use: getAddress() , getData() , etc.
When I try to use the packet's methods in the Server's code - it doesn't get stuck. Only in the Threads'.
I don't understand why I get stuck when using the packet in the thread's code
Here is the code and it compiles:
The Client
public class ExchangeClientProgram
{
public final double ERROR = 0;
private DatagramPacket packet;
private DatagramSocket socket;
private InetAddress hostAddress;
private int port;
private byte [] buf;
public ExchangeClientProgram(String hostIp , int port) throws SocketException, UnknownHostException
{
this.port = port;
socket = new DatagramSocket();
hostAddress = InetAddress.getByName(hostIp);
}
public boolean sendRequestPacket(ExchangeRequest exchangeRequest)
{
try
{
buf = ExchangeServerProgram.convertObjectToByteArr(exchangeRequest);
if(null != buf)
{
packet = new DatagramPacket(buf, buf.length, hostAddress , port);
socket.send(packet);
return true;
}
else
JOptionPane.showMessageDialog(null, "Sorry, this exchange cannot be trasnmitted to server");
} catch (IOException e){e.printStackTrace();}
return false;
}
}
The Server
public class ExchangeServerProgram extends Thread
{
public static final int DEFAULT_SERVER_PORT = 4444;
public static final int DEFAULT_BUFFER_SIZE = 1024;
private DatagramSocket socket;
private DatagramPacket packet;
private boolean listening = false;// default initial value
private byte[] buffer = new byte[DEFAULT_BUFFER_SIZE];
public ExchangeServerProgram() throws SocketException
{
socket = new DatagramSocket(DEFAULT_SERVER_PORT);
packet = new DatagramPacket(buffer , buffer.length);
}
public void run()
{
listening = true;
while(listening)
{
try
{
socket.receive(packet);
packet.getAddress(); // this line BEFORE the thread starts works fine.
new ExchangeClientRequestHandlerThread(packet ).start(); // inside this thread the trouble starts
// when using the SAME line from above
} catch (IOException e){e.printStackTrace();}
}
}
public static byte [] convertObjectToByteArr(Object obj)
{
ByteArrayOutputStream bos = new ByteArrayOutputStream();
byte [] buffer = null;
ObjectOutput out = null;
try {
out = new ObjectOutputStream(bos);
out.writeObject(obj);
out.flush();
buffer = bos.toByteArray();
}
catch (IOException e){e.printStackTrace(); }
finally {
try
{
bos.close();
} catch (IOException e){e.printStackTrace(); }
}
return buffer;
}
}
The Thread Handling The Packet, here is where I get stuck
public class ExchangeClientRequestHandlerThread extends Thread
{
private DatagramPacket packet;
public ExchangeClientRequestHandlerThread(DatagramPacket packet)
{
this.packet = packet;
}
public void run()
{
if(null == packet)
return;
packet.getAddress(); //... get stuck here
System.out.println("doesn't get to this code line");
}
}
ExchangeRequest
public class ExchangeRequest implements Serializable
{
private static final long serialVersionUID = -1355753051547829379L;
private String coinFrom;
private String coinTo;
private double amount;
private int requestId;
public ExchangeRequest(String coinFrom, String coinTo, double amount , int requestId)
{
this.coinFrom = coinFrom;
this.coinTo = coinTo;
this.amount = amount;
this.requestId = requestId;
}
//getters & setters
}
Main
public class Main
{
public static void main(String [] args)
{
try
{
ExchangeServerProgram server = new ExchangeServerProgram();
server.start(); // starts the server
String inputIpAddress = "localhost";
ExchangeClientProgram clientProgram = new ExchangeClientProgram(inputIpAddress, ExchangeServerProgram.DEFAULT_SERVER_PORT);
ExchangeRequest request = new ExchangeRequest("usd", "euro", 4, 1111); // 1111 is just an ID number for the message
clientProgram.sendRequestPacket(request);
while(true)
{
// this while loop is just for the example
// here I am waiting for received packet
// via "socket.receive();"
}
} catch (SocketException | UnknownHostException e)
{
e.printStackTrace();
}
}
}

Threading is your problem. You have one thread calling receive(packet), and when you receive it you pass it to another thread for processing, making the first thread call receive(packet) again.
The problem is that receive synchronizes on packet and methods in DatagramPacket are synchronized, so while the first thread is blocked on receive(packet) you can't call any of the DatagramPacket methods without it blocking the thread.
Possible solutions include processing the packet in the original thread or creating a new DatagramPacket for each receive().

convert byte stream to string from socket in server side java

I serialized an object to bytes and send to the server side.
in the server side i got the byte stream but i want to print the object/string i got from the server in order to verify i got it well
server side:
CarServerSocket = new ServerSocket(4441);
System.out.println("Server is ready and waiting for connection from client..\n");
try {
while (true) {
carSocket = CarServerSocket.accept();
System.out.println("Server Connected");
final DataInputStream bytesIR = new DataInputStream(carSocket.getInputStream());
bytesIRLength = bytesIR.readInt();
while (bytesIRLength > 0) {
byte[] messageIn = new byte[bytesIRLength];
bytesIR.readFully(messageIn,0,messageIn.length);
bytesIR.readUTF();
}
}
}catch(EOFException e ){
System.out.println("\ngot all objects from client.\ndisconnecting server...");
CarServerSocket.close();
carSocket.close();
}
}
Cliend side - serialization
objectOut.writeObject(CarList[i]); // converting object to bytes.
objectOut.flush();
objectInBytes = byteOut.toByteArray();
System.out.println("Sending car object #"+i);
dOut.writeInt(objectInBytes.length); // getting object bytes size.
dOut.write(objectInBytes); // sending object in bytes.
I tired to use: toString(), readUTF()... but no luck.
can anyone please advise how i solve it?
Thank you.

You can try to read data from your InputStream with some kind of InputStreamReader, something like this :
CarServerSocket = new ServerSocket(4441);
System.out.println("Server is ready and waiting for connection from client..\n");
try {
while (true) {
carSocket = CarServerSocket.accept();
System.out.println("Server Connected");
StringBuilder yourData = new StringBuilder();
new BufferedReader(new InputStreamReader(carSocket.getInputStream()))
.lines().forEach(stringBuilder::append);
System.out.println(yourData.toString());
}catch(EOFException e ){
System.out.println("\ngot all objects from client.\ndisconnecting server...");
CarServerSocket.close();
carSocket.close();
}
}

objectOut.writeObject(CarList[i]); // converting object to bytes.
objectOut.flush();
objectInBytes = byteOut.toByteArray();
System.out.println("Sending car object #"+i);
dOut.writeInt(objectInBytes.length); // getting object bytes size.
dOut.write(objectInBytes); // sending object in bytes.
All this is completely pointless. It just wastes time and space. Just use ObjectOutputStream.writeObject() directly to the socket at the sender, and ObjectInputStream.readObject() directly from the socket at the receiver.

You need to use ObjectInputStream to deserialize objects. Ok, so your object is entirely contained in a datagram that you've already received. You just need to turn the data buffer into an ObjectInputStream. Coding from the hip, this would be something like ...
try( ByteArrayInputStream bais = new ByteArrayInputStream(messageIn);
ObjectInputStream ois = new ObjectInputStream(bais)) {
Object o = ois.readObject();
}
Edit: Here is some complete code showing this working.
public class ByteStream {
public static void main(String[] args) throws IOException {
Server server = new Server(4441);
new Thread(server).start();
Client client = new Client(4441);
new Thread(client).start();
}
}
class Client implements Runnable {
private final Socket socket;
Client(int port) throws UnknownHostException, IOException {
socket = new Socket("localhost", port);
}
#Override
public void run() {
MyObject send = new MyObject();
send.x = 10;
send.msg = "X = ";
try {
try (DataOutputStream dos = new DataOutputStream(socket.getOutputStream());
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos)) {
oos.writeObject(send);
oos.flush();
byte[] objectInBytes = baos.toByteArray();
int length = objectInBytes.length;
System.out.println("Client: sending 'objectInBytes' length = " + length);
dos.writeInt(length);
dos.write(objectInBytes);
} finally {
socket.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
class Server implements Runnable {
private final ServerSocket serverSocket;
Server(int port) throws IOException {
serverSocket = new ServerSocket(port);
}
#Override
public void run() {
try {
try (Socket socket = serverSocket.accept();
DataInputStream bytesIR = new DataInputStream(socket.getInputStream())) {
int length = bytesIR.readInt();
byte[] messageIn = new byte[length];
bytesIR.readFully(messageIn);
System.out.println("Server: got datagram length = " + length);
process(messageIn);
} finally {
serverSocket.close();
}
} catch (IOException | ClassNotFoundException e) {
e.printStackTrace();
}
}
private void process(byte[] messageIn) throws IOException, ClassNotFoundException {
try (ByteArrayInputStream bais = new ByteArrayInputStream(messageIn);
ObjectInputStream ois = new ObjectInputStream(bais)) {
Object o = ois.readObject();
System.out.println(o.getClass() + ": " + o);
}
}
}
class MyObject implements Serializable {
private static final long serialVersionUID = -1478875967217194114L;
double x;
String msg;
public String toString() { return msg + x; }
}
And the output:
Client: sending 'objectInBytes' length = 75
Server: got datagram length = 75
class MyObject: X = 10.0

Implementing a simple UDP network that continuously broadcasts to its neighbours

I have 3 nodes A, B and C with their respective port numbers.
I'm trying to write a java program that takes in 3 arguments:
its node name and its 2 neighboring node's ports and broadcasts a string "Hello I'm A" to them (so A would broadcast to B and C). It will do this every 3 seconds.
This program will be run in 3 separate instances.
Upon receiving a string it will print what node it has received it from "Received string" (example for Port B).
I have difficulties implementing this, I have heard of something called multicasting with UDP though. Here is my work so far, what am I doing wrong?
class UDP {
public static void main(String[] args) throws Exception {
String nodeName = args[0];
int neighbourPort1 = Integer.valueOf(args[1]);
int neighbourPort2 = Integer.valueOf(args[2]);
while(true) {
Thread.sleep(3000); //every 3 seconds
//Continously broadcast and listen to neighbour1
DatagramSocket socket1 = null;
try {
//CREATE SOCKET TO NEIGHBOUR1
InetAddress host = InetAddress.getByName("localhost");
socket1 = new DatagramSocket();
socket1.connect(host, neighbour1);
//CREATE DATAGRAMS FOR SENDING
String message = "Hello I'm " + nodeName;
byte[] sendData = message.getBytes();
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, host, port);
socket1.send(sendPacket);
//CREATE DATAGRAMS FOR RECEIVING
byte[] receiveData = new byte[100]; //is there a way to determine the needed space?
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
socket1.receive(receivePacket);
System.out.println("Received string");
} catch(Exception e) { }
//Do the same for neighbour2, code is basically identical except for variables
DatagramSocket socket2 = null;
try {
//CREATE SOCKET TO NEIGHBOUR2
InetAddress host = InetAddress.getByName("localhost");
socket2 = new DatagramSocket();
socket2.connect(host, neighbour2);
//FOR SENDING DATAGRAMS
String message = "Hello I'm " + nodeName;
byte[] sendData = message.getBytes();
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, host, port);
socket2.send(sendPacket);
//FOR RECEIVING DATAGRAMS
byte[] receiveData = new byte[100]; //is there a way to determine the needed space?
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
socket2.receive(receivePacket);
System.out.println("Received string");
} catch(Exception e) { }
}
}
}
I know I'm close to the solution. I'm able to broadcast properly but it's the constantly listening part that gets me.

I think its best to use a separate Thread to listen for data on your own port.
A sends data to B and blocks until it gets a packet from B.
B sends data to C and blocks until it gets a packet from C.
C sends data to A and blocks until it gets a packet from A.
Every node is waiting for each other. Just send the packets and wait 3 seconds.
The other thread is going to listen only.
public class UDP {
public static void main(String[] args) throws Exception {
final String nodeName = args[0];
final int ownPort = Integer.valueOf(args[1]);
final int neighbourPort1 = Integer.valueOf(args[2]);
final int neighbourPort2 = Integer.valueOf(args[3]);
// Don't create a new socket every time
DatagramSocket neighbour1 = new DatagramSocket();
DatagramSocket neighbour2 = new DatagramSocket();
neighbour1.connect(InetAddress.getLocalHost(), neighbourPort1);
neighbour2.connect(InetAddress.getLocalHost(), neighbourPort2);
// You have to LISTEN
new Thread() {
#Override
public void run() {
try {
DatagramSocket socket = new DatagramSocket(ownPort);
byte[] buffer = new byte[socket.getReceiveBufferSize()];
DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
while (true) {
// Blocks until it gets a packet
socket.receive(packet);
System.out.println("Received string");
}
// socket.close();
} catch (final Exception e) {
e.printStackTrace();
}
}
}.start();
while (true) {
Thread.sleep(3000);
sendPacket(neighbour1, nodeName);
sendPacket(neighbour2, nodeName);
}
// If you're not using an infinite loop:
// neighbour1.close();
// neighbour2.close();
}
private static void sendPacket(DatagramSocket to, String from) throws Exception {
String message = "Hello I'm " + from;
byte[] data = message.getBytes();
DatagramPacket packet = new DatagramPacket(data, data.length);
to.send(packet);
}
}

Here is a simple frame for a server with two threads, one writing and one reading, place your network code at the right places.
package testing;
import java.util.Scanner;
public class ThreadTest {
public class MyListenerThread extends Thread {
#Override
public void run() {
/*
* Open Datagram ...
*/
while (true) {
/*
* Read data ...
*/
Scanner scanner = new Scanner(System.in);
System.out.println("read: " + scanner.next());
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public class MySenderThread extends Thread {
#Override
public void run() {
/*
* Open Multicast ...
*/
while (true) {
/*
* Send ...
*/
System.out.println("Send ...");
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public void start() {
MyListenerThread listener = new MyListenerThread();
MySenderThread sender = new MySenderThread();
listener.start();
sender.start();
}
public static void main(String[] args) {
ThreadTest server = new ThreadTest();
server.start();
}
}

Is multithreading possible in a simple java server using udp connectionless protocol?

Is multi-threading possible in a simple java server using udp connectionless protocol? give an example!!

Multi-threading is actually simpler with UDP because you don't have to worry about connection state. Here is the listen loop from my server,
while(true){
try{
byte[] buf = new byte[2048];
DatagramPacket packet = new DatagramPacket( buf, buf.length, address );
socket.receive( packet );
threadPool.execute( new Request( this, socket, packet ));
.......
The threadPool is a ThreaPoolExecutor. Due to the short-lived nature of UDP sessions, thread pool is required to avoid the overhead of repeatedly thread creation.

Yes, it's possible through the use of the DatagramChannel class in java.nio. Here's a tutorial (It does not address the multithreading, but that is a separate issue anyway).

Here is one example try putting your own ip to get the hard-coded message back
package a.first;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;
public class Serv {
/**
* #param args
* #throws IOException
*/
public static void main(String[] args) throws IOException {
Listner listner = new Listner();
Thread thread = new Thread(listner);
thread.start();
String messageStr = "Hello msg1";
int server_port = 2425;
DatagramSocket s = new DatagramSocket();
InetAddress local = InetAddress.getByName("172.20.88.223");
int msg_length = messageStr.length();
byte[] message = messageStr.getBytes();
DatagramPacket p = new DatagramPacket(message, msg_length, local,
server_port);
System.out.println("about to send msg1");
s.send(p);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
e.printStackTrace();
}
messageStr = "Hello msg2";
msg_length = messageStr.length();
message = messageStr.getBytes();
p = new DatagramPacket(message, msg_length, local,
server_port);
System.out.println("about to send msg2");
s.send(p);
}
}
class Listner implements Runnable
{
#Override
public void run() {
String text = null;
while(true){
text = null;
int server_port = 2425;
byte[] message = new byte[1500];
DatagramPacket p = new DatagramPacket(message, message.length);
DatagramSocket s = null;
try{
s = new DatagramSocket(server_port);
}catch (SocketException e) {
e.printStackTrace();
System.out.println("Socket excep");
}
try {
s.receive(p);
}catch (IOException e) {
e.printStackTrace();
System.out.println("IO EXcept");
}
text = new String(message, 0, p.getLength());
System.out.println("message = "+text);
s.close();
}
}
}