Last Value Caching Proxy in Java

import org.zeromq.ZContext;
import org.zeromq.ZFrame;
import org.zeromq.ZMQ;
import org.zeromq.ZMQ.PollItem;
import org.zeromq.ZMQ.Socket;

import java.util.HashMap;
import java.util.Map;

// Last value cache
// Uses XPUB subscription messages to re-send data

public class lvcache
{
public static void main(String[] args)
{
ZContext context = new ZContext();
Socket frontend = context.createSocket(ZMQ.SUB);
frontend.connect("tcp://*:5557");
Socket backend = context.createSocket(ZMQ.XPUB);
backend.bind("tcp://*:5558");

// Subscribe to every single topic from publisher
frontend.subscribe("".getBytes());

// Store last instance of each topic in a cache
Map<String, String> cache = new HashMap<String, String>();

// We route topic updates from frontend to backend, and
// we handle subscriptions by sending whatever we cached,
// if anything:
while (true) {
PollItem[] items = {
new PollItem(frontend, ZMQ.Poller.POLLIN),
new PollItem(backend, ZMQ.Poller.POLLIN),
};
if (ZMQ.poll(items, 1000) == -1)
break; // Interrupted

// Any new topic data we cache and then forward
if (items[0].isReadable()) {
String topic = frontend.recvStr();
String current = frontend.recvStr();

if (topic == null)
break;
cache.put(topic, current);
backend.sendMore(topic);
backend.send(current);
}
// When we get a new subscription, we pull data from the cache:
if (items[1].isReadable()) {
ZFrame frame = ZFrame.recvFrame(backend);
if (frame == null)
break;
// Event is one byte 0=unsub or 1=sub, followed by topic
byte[] event = frame.getData();
if (event [0] == 1) {
String topic = new String(event, 1, event.length -1);
System.out.printf ("Sending cached topic %s\n", topic);
String previous = cache.get(topic);
if (previous != null) {
backend.sendMore(topic);
backend.send(previous);
}
}
frame.destroy();
}
}
context.destroy();
}
}