Android BLE Connection — Complete Guide

DodaTech Updated 2026-06-24 2 min read

In this tutorial, you'll learn about Android BLE Connection. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

The Problem

Your BLE device connects but immediately disconnects, or connectGatt returns null.

Wrong Approach ❌

// Connecting on the main thread — blocks UI
val gatt = device.connectGatt(context, false, gattCallback)
// Autoconnect = false — doesn't reconnect after disconnection

Output: Connection fails silently. No reconnection on transient disconnects.

Right Approach ✅

class BleConnector(private val context: Context) {
    private var gatt: BluetoothGatt? = null
    private var connectionState = State.DISCONNECTED
    private var reconnectAttempts = 0
    private val maxReconnectAttempts = 3

    enum class State { DISCONNECTED, CONNECTING, CONNECTED, DISCONNECTING }

    fun connect(device: BluetoothDevice) {
        if (connectionState == State.CONNECTING) return

        connectionState = State.CONNECTING
        // Autoconnect = true for automatic reconnection
        gatt = device.connectGatt(
            context,
            true, // Auto-reconnect
            object : BluetoothGattCallback() {
                override fun onConnectionStateChange(gatt: BluetoothGatt, status: Int, newState: Int) {
                    when (newState) {
                        BluetoothProfile.STATE_CONNECTED -> {
                            connectionState = State.CONNECTED
                            reconnectAttempts = 0
                            gatt.discoverServices() // Discover after connect
                        }
                        BluetoothProfile.STATE_DISCONNECTED -> {
                            connectionState = State.DISCONNECTED
                            handleDisconnection(gatt, status)
                        }
                    }
                }

                override fun onServicesDiscovered(gatt: BluetoothGatt, status: Int) {
                    if (status == BluetoothGatt.GATT_SUCCESS) {
                        onServicesReady?.invoke(gatt.services)
                    } else {
                        Log.e("BleConnect", "Service discovery failed: $status")
                    }
                }

                override fun onMtuChanged(gatt: BluetoothGatt, mtu: Int, status: Int) {
                    if (status == BluetoothGatt.GATT_SUCCESS) {
                        Log.d("BleConnect", "MTU updated to $mtu")
                    }
                }
            }
        )
    }

    private fun handleDisconnection(gatt: BluetoothGatt, status: Int) {
        if (status == BluetoothGatt.GATT_SUCCESS) {
            // Intentional disconnection
            return
        }

        // Unexpected disconnection — attempt reconnect
        if (reconnectAttempts < maxReconnectAttempts) {
            reconnectAttempts++
            Log.d("BleConnect", "Reconnecting ($reconnectAttempts/$maxReconnectAttempts)")
            // Auto-reconnect is handled by autoconnect = true
        } else {
            Log.e("BleConnect", "Max reconnection attempts reached")
            close()
        }
    }

    fun disconnect() {
        connectionState = State.DISCONNECTING
        gatt?.disconnect()
    }

    fun close() {
        gatt?.close()
        gatt = null
        connectionState = State.DISCONNECTED
    }

    // Request MTU for larger data transfers
    fun requestMtu(mtu: Int = 512) {
        gatt?.requestMtu(mtu) // Default is 23 bytes
    }

    var onServicesReady: ((List<BluetoothGattService>) -> Unit)? = null
}

Output: Reliable BLE connection with automatic reconnection.

Prevention

Use autoconnect = true for maintaining connections.
Call discoverServices() after successful connection.
Request MTU 512 (or higher) for efficient data transfer.
Handle status != GATT_SUCCESS in onConnectionStateChange.
Always call close() to release GATT resources.

Common Mistakes with bluetooth connect

Misunderstanding that String is [Char] with poor performance for large text operations
Using foldl instead of foldl' causing stack overflow on large lists
Forgetting deriving (Show, Eq) on custom data types needed for debugging

These mistakes appear frequently in real-world Android code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

### Why does connectGatt return null?

Common causes: Bluetooth is disabled, device address is invalid, BLE Adapter is not available, or maximum GATT connections exceeded (typically 6-7).

### What is MTU and why should I change it?

Default MTU is 23 bytes (20 bytes payload). Requesting MTU 512 allows larger data packets, reducing overhead and transfer time. Negotiate after connection.

### How do I handle GATT status 133 (GATT_ERROR)?

Status 133 is a generic error. Common causes: connection timeout, device out of range, bonding issues. Retry the connection with autoconnect = true.

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