[Java] JSON for Message Payloads (#254)

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

docker/java/build.gradle

@@ -17,6 +17,9 @@ dependencies {
     implementation 'io.nats:jnats-json:2.0.0'
     implementation 'io.nats:jwt-java:2.0.0'
     implementation 'io.nats:jnats:2.19.2-SNAPSHOT'
+
+    // JSON serialization/deserialization
+    implementation 'com.fasterxml.jackson.core:jackson-databind:2.17.2'
 }
 
 apply plugin: 'java'

examples/messaging/json/java/Main.java

@@ -0,0 +1,94 @@
+package example;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.nats.client.Connection;
+import io.nats.client.Nats;
+import io.nats.client.Dispatcher;
+
+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)) {
+
+            // Construct a payload and serialize it.
+            // Using Jackson in this example, but any other JSON library can be used as well
+            // (and even any other message format since the payload is just bytes).
+            ObjectMapper objectMapper = new ObjectMapper();
+            Payload payload = new Payload("bar", 27);
+            byte[] messageBytes = objectMapper.writeValueAsBytes(payload);
+
+            CountDownLatch latch = new CountDownLatch(2);
+
+            // Create a message dispatcher for handling messages in a
+            // separate thread and then subscribe to the target subject.
+            Dispatcher dispatcher = nc.createDispatcher((msg) -> {
+
+                // Attempt to deserialize the payload.
+                // If deserialization fails, alternate handling can be performed.
+                try {
+                    Payload deserializedPayload = objectMapper.readValue(msg.getData(), Payload.class);
+
+                    System.out.printf("received valid JSON payload: %s\n", deserializedPayload);
+                } catch (IOException e) {
+                    System.out.printf("received invalid JSON payload: %s\n",
+                            new String(msg.getData(), StandardCharsets.UTF_8));
+                } finally {
+                    latch.countDown();
+                }
+            });
+
+            dispatcher.subscribe("foo");
+
+            // Publish the serialized payload.
+            nc.publish("foo", messageBytes);
+            nc.publish("foo", "not json".getBytes(StandardCharsets.UTF_8));
+
+            // Await the dispatcher thread to have received all the messages before the program quits.
+            latch.await();
+
+        } catch (InterruptedException | IOException e) {
+            e.printStackTrace();
+        }
+    }
+}
+
+class Payload {
+    private final String foo;
+    private final int bar;
+
+    @JsonCreator
+    Payload(@JsonProperty("foo") String foo,
+            @JsonProperty("bar") int bar) {
+        this.foo = foo;
+        this.bar = bar;
+    }
+
+    public String getFoo() {
+        return foo;
+    }
+
+    public int getBar() {
+        return bar;
+    }
+
+    @Override
+    public String toString() {
+        return "Payload{" +
+                "foo='" + foo + '\'' +
+                ", bar=" + bar +
+                '}';
+    }
+}