How to scan and connect to a Bluetooth device (BLE)

Purpose. Discover nearby Bluetooth devices, connect to one of them, and exchange data — by stream (Classic/SPP) or by GATT characteristics (BLE: read, write, receive notifications).

⚠️ Honesty note (read before using)

In the core module, the BLE layer is a skeleton over an in-memory cache, not the real radio:

• GattCharacteristic.read() returns the cached value (CompletableFuture.completedFuture(value)), not a hardware read.
• GattCharacteristic.write(byte[]) only stores the value locally.
• enableNotifications(...) registers the listener, but the data is triggered by notifyValueChanged(...).
• BluetoothDevice.discoverServices(...) returns an empty list in core.

The real radio (actual scan/connection/GATT) is provided by the platform launchers (Android/iOS). Treat the code below as the public API; the effective behavior depends on the launcher the application runs on.

APIs used

• BluetoothManager.getInstance() throws IOException — singleton.
• scanForDevices(long duration, TimeUnit unit, ScanCallback cb) — asynchronous discovery; ScanCallback.onSuccess(List<BluetoothDevice>) / onError(String).
• connect(BluetoothDevice device, ConnectionCallback cb) — asynchronous connection; ConnectionCallback.onSuccess(BluetoothConnection) / onError(String).
• BluetoothConnection (AutoCloseable) — read() / read(int max) → byte[]; write(byte[]) / write(String) → int.
• GATT: GattCharacteristic.read() → CompletableFuture<byte[]>; write(byte[]) → CompletableFuture<Void>; enableNotifications(Consumer<byte[]>) → CompletableFuture<Void>.
• BluetoothManager.isBluetoothEnabled(), getPairedDevices(...), getLocalDevice().

Complete example (copy, paste, and run)

Remember the honesty note above: in the core module the BLE layer is a skeleton over an in-memory cache — the real radio comes from the platform launchers (Android/iOS). The example below is the public API; the effective behavior depends on the launcher the application runs on.

package com.exemplo;

import java.util.concurrent.*;              // TimeUnit
import dev.crossfly.ui.*;                   // Window, Component, Button, Label
import dev.crossfly.ui.layout.*;            // VerticalLayout
import dev.crossfly.device.bluetooth.*;     // BluetoothManager, BluetoothDevice

/** Runnable main screen. Point the launcher (classname) and run: gradlew :lwjgl3:run */
public class MainWindow extends Window
{
   @Override
   public void onShow() throws Exception
   {
      super.onShow();
      setLayout(new VerticalLayout());

      Label resultado = new Label("");
      add(LAY_AUTO, resultado);

      Button acao = new Button("Escanear Bluetooth");
      acao.onAction(c ->
      {
         try
         {
            BluetoothManager bt = BluetoothManager.getInstance();

            // Scan (callbacks outside the paint thread → onUI when touching the UI)
            bt.scanForDevices(10, TimeUnit.SECONDS, new BluetoothManager.ScanCallback()
            {
               public void onSuccess(java.util.List<BluetoothDevice> devices)
               {
                  Component.onUI(() -> resultado.setText("Encontrados: " + devices.size()));
               }
               public void onError(String error)
               {
                  Component.onUI(() -> resultado.setText("Erro: " + error));
               }
            });
         }
         catch (java.io.IOException e)
         {
            resultado.setText("Bluetooth indisponível");   // Bluetooth system unavailable
         }
      });
      add(LAY_AUTO, acao);
   }
}

The snippet below shows only the core. To run it, place it in the MainWindow skeleton above.

Minimal snippet

// verified at source: BluetoothManager.java (getInstance, scanForDevices, connect); BluetoothConnection.java (read/write); GattCharacteristic.java (read/write/enableNotifications)
import java.util.concurrent.*;             // TimeUnit
import dev.crossfly.device.bluetooth.*;    // BluetoothManager, BluetoothDevice, BluetoothConnection, GattCharacteristic
import dev.crossfly.ui.*;                  // Component (onUI)

try {
   BluetoothManager bt = BluetoothManager.getInstance();

   // 1) Scan (callbacks outside the paint thread → onUI when touching the UI)
   bt.scanForDevices(10, TimeUnit.SECONDS, new BluetoothManager.ScanCallback() {
      public void onSuccess(java.util.List<BluetoothDevice> devices) {
         Component.onUI(() -> listarDispositivos(devices));
      }
      public void onError(String error) {
         Component.onUI(() -> mostrarErro(error));
      }
   });

   // 2) Connect and exchange data over a stream (Classic/SPP)
   BluetoothDevice alvo = /* chosen by the user */;
   bt.connect(alvo, new BluetoothManager.ConnectionCallback() {
      public void onSuccess(BluetoothConnection conn) {
         try (conn) {                        // AutoCloseable
            conn.write("PING".getBytes());
            byte[] resposta = conn.read();    // up to 4096 bytes
         } catch (java.io.IOException e) { /* handle it */ }
      }
      public void onError(String error) {
         Component.onUI(() -> mostrarErro(error));
      }
   });

   // 3) BLE / GATT — read and subscribe to notifications of a characteristic
   GattCharacteristic ch = /* obtained from the device's service */;
   ch.enableNotifications(data ->
      Component.onUI(() -> atualizarValor(data)));   // Consumer<byte[]>
   ch.read().thenAccept(valor ->
      Component.onUI(() -> mostrarValor(valor)));     // CompletableFuture<byte[]>
} catch (java.io.IOException e) {
   // Bluetooth system unavailable
}

Go deeper

• Bluetooth/BLE guide: Bluetooth & BLE