DatagramSocket's are Java's mechanism for network communication via UDP instead of TCP. UDP is still
layered ontop of IP. You can use Java's
DatagramSocket both for sending and receiving UPD datagrams.
UDP vs. TCP
UDP works a bit differently from TCP. When you send data via TCP you first create a connection. Once the TCP connection is established TCP guarantess that your data arrives at the other end, or it will tell you that an error occurred.
With UDP you just send packets of data (datagrams) to some IP address on the network. You have no guarantee that the data will arrive. You also have no guarantee about the order which UDP packets arrive in at the receiver. This means that UDP has less protocol overhead (no stream integrity checking) than TCP.
UDP is appropriate for data transfers where it doesn't matter if a packet is lost in transition. For instance, imagine a transfer of a live TV-signal over the internet. You want the signal to arrive at the clients as close to live as possible. Therefore, if a frame or two are lost, you don't really care. You don't want the live broadcast to be delayed just to make sure all frames are shown at the client. You'd rather skip the missed frames, and move directly to the newest frames at all times.
This could also be the case with a surveillance camera broadcasting over the internet. Who cares what happened in the past, when you are trying to monitor the present. You don't want to end up being 30 seconds behind reality, just because you want to show all frames to the person monitoring the camera. It is a bit different with the storage of the camera recordings. You may not want to lose a single frame when recording the images from the camera to disk. You may rather want a little delay, than not have those frames to go back and examine, if something important occurs.
Sending Data via a DatagramSocket
To send data via Java's
DatagramSocket you must first create a
Here is how that is done:
byte buffer = new byte; InetAddress address = InetAddress.getByName("jenkov.com"); DatagramPacket packet = new DatagramPacket( buffer, buffer.length, address, 9000);
The byte buffer (the byte array) is the data that is to be sent in the UDP datagram. The length of the above buffer, 65508 bytes, is the maximum amount of data you can send in a single UDP packet.
The length given to the
DatagramPacket constructor is the length of the
data in the buffer to send. All data in the buffer after that amount of data is ignored.
InetAddress instance contains the address of the node (e.g. server) to send the
UDP packet to. The
InetAddress class represents an IP address (Internet Address).
method returns an
InetAddress instance with the IP address matching the given host name.
The port parameter is the UDP port the server to receiver the data is listeing on. UDP and TCP ports are not the same. A computer can have different processes listening on e.g. port 80 in UDP and in TCP at the same time.
To send the
DatagramPacket you must create a
DatagramSocket targeted at sending
data. Here is how that is done:
DatagramSocket datagramSocket = new DatagramSocket();
To send data you call the
send() method, like this:
Here is a full example:
DatagramSocket datagramSocket = new DatagramSocket(); byte buffer = "0123456789".getBytes(); InetAddress receiverAddress = InetAddress.getLocalHost(); DatagramPacket packet = new DatagramPacket( buffer, buffer.length, receiverAddress, 80); datagramSocket.send(packet);
Receiving Data via a DatagramSocket
Receiving data via a
DatagramSocket is done by first creating a
and then receiving data into it via the
Here is an example:
DatagramSocket datagramSocket = new DatagramSocket(80); byte buffer = new byte; DatagramPacket packet = new DatagramPacket(buffer, buffer.length); datagramSocket.receive(packet);
Notice how the
DatagramSocket is instantiated with the parameter value 80 passed
to its constructor. This parameter is the UDP port the
DatagramSocket is to receive
UDP packets on. As mentioned earlier, TCP and UDP ports are not the same, and thus do not overlap.
You can have two different processes listening on both TCP and UDP port 80, without any conflict.
Second, a byte buffer and a
DatagramPacket is created. Notice how the
has no information about the node to send data to, as it does when creating a
for sending data. This is because we are going to use the
DatagramPacket for receiving data,
not sending it. Thus no destination address is needed.
receive() method is called. This method blocks
DatagramPacket is received.
The data received is located in the
DatagramPacket's byte buffer. This buffer can be obtained
byte buffer = packet.getData();
How much data was received in the buffer is up to you to find out. The protocol you are using should specify either how much data is sent per UDP packet, or specify an end-of-data marker you can look for instead.
A real server program would probably call the
receive() method in a loop, and pass all
DatagramPacket's to a pool of worker threads, just like a TCP server does with
incoming connections (see Java Multithreaded Servers for more details).