JCH
Java Community | Help. Code. Learn.•4mo ago
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Week 89 — What is the purpose of the `DatagramSocket` class and how can it be used?

Question of the Week #89
What is the purpose of the DatagramSocket class and how can it be used?
3 Replies
0x150
0x150•4mo ago
While Socket allows communicating via TCP, DatagramSocket can be used for sending and receiving UDP packets. It is possible to receive UDP packets by first creating a DatagramPacket object with a reference to a byte[] and the number of bytes it is allowed to load into that array and then calling DatagramSocket#receive(DatagramPacket) on the socket. Similarly, UDP packets can be sent by creating a DatagramPacket with the data to send and calling DatagramSocket#send(DatagramPacket) with it. For example, it is possible to create a server that is listening for UDP packets and always sending them back: 
try(DatagramSocket sock = new DatagramSocket(1337)){
    byte[] data = new byte[1024];
    while(!Thread.currentThread().isInterrupted()){
        //receive a packet
        DatagramPacket resceivingPacket = new DatagramPacket(data, data.length);
        sock.receive(resceivingPacket);
        //send the same content back
        DatagramPacket sendingPacket = new DatagramPacket(data, resceivingPacket.getLength(), resceivingPacket.getAddress(), resceivingPacket.getPort());
        sock.send(sendingPacket);
    }
}
try(DatagramSocket sock = new DatagramSocket(1337)){
    byte[] data = new byte[1024];
    while(!Thread.currentThread().isInterrupted()){
        //receive a packet
        DatagramPacket resceivingPacket = new DatagramPacket(data, data.length);
        sock.receive(resceivingPacket);
        //send the same content back
        DatagramPacket sendingPacket = new DatagramPacket(data, resceivingPacket.getLength(), resceivingPacket.getAddress(), resceivingPacket.getPort());
        sock.send(sendingPacket);
    }
}
0x150
0x150•4mo ago
It is then also possible to write a client that sends some sample data to the server and receives the response: 
try(DatagramSocket sock = new DatagramSocket()){//if no port is specified, it will use use a random port
    byte[] dataToSend = new byte[] {13, 37, 1, 2, 3, 123};
    //send a packet to localhost on UDP port 1337
    DatagramPacket sendingPacket = new DatagramPacket(dataToSend, dataToSend.length, InetAddress.getByName("localhost"), 1337);
    sock.send(sendingPacket);
    //receive a packet
    byte[] receivedData = new byte[1024];
    DatagramPacket resceivingPacket = new DatagramPacket(receivedData, receivedData.length);
    sock.receive(resceivingPacket);
    //print received data to console
    System.out.println(Arrays.toString(Arrays.copyOf(receivedData, resceivingPacket.getLength())));//[13, 37, 1, 2, 3, 123]
}
try(DatagramSocket sock = new DatagramSocket()){//if no port is specified, it will use use a random port
    byte[] dataToSend = new byte[] {13, 37, 1, 2, 3, 123};
    //send a packet to localhost on UDP port 1337
    DatagramPacket sendingPacket = new DatagramPacket(dataToSend, dataToSend.length, InetAddress.getByName("localhost"), 1337);
    sock.send(sendingPacket);
    //receive a packet
    byte[] receivedData = new byte[1024];
    DatagramPacket resceivingPacket = new DatagramPacket(receivedData, receivedData.length);
    sock.receive(resceivingPacket);
    //print received data to console
    System.out.println(Arrays.toString(Arrays.copyOf(receivedData, resceivingPacket.getLength())));//[13, 37, 1, 2, 3, 123]
}
📖 Sample answer from dan1st
0x150
0x150•4mo ago
Java DatagramSocket and DatagramPacket classes are used for connection-less socket programming using the UDP instead of TCP.
Submission from mitsuki087248

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