[Java] Iterating Over Multiple Subscriptions (#253)

Signed-off-by: Maurice van Veen <github@mauricevanveen.com>

examples/messaging/iterating-multiple-subscriptions/java/Main.java

@@ -0,0 +1,58 @@
+package example;
+
+import io.nats.client.Connection;
+import io.nats.client.Dispatcher;
+import io.nats.client.Nats;
+
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.util.concurrent.CountDownLatch;
+
+public class Main {
+    public static void main(String[] args) {
+        String natsURL = System.getenv("NATS_URL");
+        if (natsURL == null) {
+            natsURL = "nats://127.0.0.1:4222";
+        }
+
+        // Initialize a connection to the server. The connection is AutoCloseable
+        // on exit.
+        try (Connection nc = Nats.connect(natsURL)) {
+
+            int total = 80;
+            CountDownLatch latch = new CountDownLatch(total);
+
+            // Create a message dispatcher. A dispatcher is a process that runs on
+            // its own thread, receives incoming messages via a FIFO queue,
+            // for subjects registered on it. For each message it takes from
+            // the queue, it makes a blocking call to the MessageHandler
+            // passed to the createDispatcher call.
+            Dispatcher dispatcher = nc.createDispatcher((msg) -> {
+                System.out.printf("Received %s: %s\n",
+                        msg.getSubject(),
+                        new String(msg.getData(), StandardCharsets.UTF_8));
+                latch.countDown();
+            });
+
+            // Subscribe directly on the dispatcher for multiple subjects.
+            dispatcher.subscribe("s1");
+            dispatcher.subscribe("s2");
+            dispatcher.subscribe("s3");
+            dispatcher.subscribe("s4");
+
+            for (int i = 0; i < total / 4; i++) {
+                nc.publish("s1", String.valueOf(i).getBytes(StandardCharsets.UTF_8));
+                nc.publish("s2", String.valueOf(i).getBytes(StandardCharsets.UTF_8));
+                nc.publish("s3", String.valueOf(i).getBytes(StandardCharsets.UTF_8));
+                nc.publish("s4", String.valueOf(i).getBytes(StandardCharsets.UTF_8));
+                Thread.sleep(100);
+            }
+
+            // Await the dispatcher thread to have received all the messages before the program quits.
+            latch.await();
+
+        } catch (InterruptedException | IOException e) {
+            e.printStackTrace();
+        }
+    }
+}