Getting started¶

Run this example in the browser

Let's publish a note and receive it back from a relay. You'll need a key to sign with, a relay connection and a subscription for the incoming event.

In this guide, you will learn how to set up the project, then publish a note and receive it back from a relay.

Set up the project¶

Add org.ngengine:nostr4j and the matching platform adapter to your build. For the coordinates and requirements of each platform, follow the platform setup.

Publish and receive a note¶

Save the following as QuickStart.java and run it. Use a temporary identity: connect to a relay, subscribe to that identity's notes before publishing, then wait briefly so the subscription receives the note before the program exits.

import org.ngengine.nostr4j.NostrFilter;
import org.ngengine.nostr4j.NostrPool;
import org.ngengine.nostr4j.NostrSubscription;
import org.ngengine.nostr4j.event.SignedNostrEvent;
import org.ngengine.nostr4j.event.UnsignedNostrEvent;
import org.ngengine.nostr4j.keypair.NostrKeyPair;
import org.ngengine.nostr4j.signer.NostrKeyPairSigner;
import org.ngengine.platform.AsyncTask;
import org.ngengine.platform.NGEPlatform;
import org.ngengine.platform.jvm.JVMAsyncPlatform;

public class QuickStart {

    public static void main(String[] args) throws Exception {
        // snippet:start
        NGEPlatform.set(new JVMAsyncPlatform());

        NostrKeyPair keys = new NostrKeyPair();
        NostrPool pool = new NostrPool();
        NostrSubscription subscription = null;
        try {
            pool.ensureRelay("wss://relay.ngengine.org").await();

            NostrFilter filter = new NostrFilter();
            filter.withKind(1);
            filter.withAuthor(keys.getPublicKey());

            subscription = pool.subscribe(filter);
            subscription.addEventListener((sub, event, stored) -> {
                System.out.println("Received: " + event.getContent());
            });
            AsyncTask.awaitAll(subscription.open());

            UnsignedNostrEvent draft = new UnsignedNostrEvent();
            draft.withKind(1);
            draft.withContent("Hello from Nostr4j!");

            NostrKeyPairSigner signer = new NostrKeyPairSigner(keys);
            SignedNostrEvent note = signer.sign(draft).await();
            pool.publish(note).await();
            System.out.println("Published: " + note.getId());

            // Keep this standalone program alive briefly to receive the note.
            Thread.sleep(5_000);
        } finally {
            if (subscription != null) subscription.close();
            pool.close();
            keys.close();
        }
        // snippet:end
    }
}

After publish(note).await() returns, at least one relay has accepted the event. Seeing it in the subscription is a separate confirmation: a relay has sent the event back to you. In a running application, keep the pool open for as long as you need it.

For a persistent account, load an existing key or use a browser or remote signer. You can then change the filter to read other authors and choose how many relay acknowledgements to wait for when publishing.