java_server_socket.groovy

synchronized def print(String str) {
    System.out.print str
}
def bootServer() {
    def server = new ServerSocket(8080)
    Thread.start {
        while (true) {
            def soc
            try { soc = server.accept() }
            catch (SocketException e) { println "[server] SocketError: $e.message"; break }

            Thread.start {
                def s = soc
                s.withStreams { is, os ->
                    os << '[server echo] '
                    for (int a = is.read(); a != '\n'; a = is.read()) {
                        os.write(a)
                    }
                    os << '\n'
                }
            }
        }
    }
    server
}
def newEchoClient(String id, String message = null) {
    new Socket('localhost', 8080).withStreams { is, os ->
        def m = message ?: "This is client <$id>\n"
        print "client <$id>: $m"
        os << m
        print("client <$id>: $is.text")
    }
}
import java.util.concurrent.CountDownLatch
bootServer()?.withCloseable {
    println "server: $it"
    def n = 10
    def prepared = new CountDownLatch(n)
    def finished = new CountDownLatch(n)
    def startSignal = new CountDownLatch(1)
    (0..<n).each { id ->
        Thread.start {
            prepared.countDown()
            startSignal.await()
            newEchoClient(id.toString())
            finished.countDown()
        }
    }
    prepared.await()
    startSignal.countDown()
    finished.await()
}
null

~output.txt

server: ServerSocket[addr=0.0.0.0/0.0.0.0,localport=8080]
client <2>: This is client <2>
client <3>: This is client <3>
client <0>: This is client <0>
client <1>: This is client <1>
client <6>: This is client <6>
client <8>: This is client <8>
client <7>: This is client <7>
client <6>: [server echo] This is client <6>
client <9>: This is client <9>
client <5>: This is client <5>
client <4>: This is client <4>
client <2>: [server echo] This is client <2>
client <4>: [server echo] This is client <4>
client <0>: [server echo] This is client <0>
client <5>: [server echo] This is client <5>
client <3>: [server echo] This is client <3>
client <1>: [server echo] This is client <1>
client <8>: [server echo] This is client <8>
client <7>: [server echo] This is client <7>
client <9>: [server echo] This is client <9>
[server] SocketError: socket closed