USB Host

Most people's experience with Android and USB is the phone acting as a USB device — plugging it into a computer to transfer files. USB Host mode flips that relationship: the Android device becomes the USB host, and it talks to an attached USB accessory — a scanner, a scale, a controller — the same way a desktop computer would.

Checking for USB Host Support

Access starts with the UsbManager system service:

UsbManager usbManager = (UsbManager) getSystemService(Context.USB_SERVICE);
HashMap<String, UsbDevice> deviceList = usbManager.getDeviceList();

for (UsbDevice device : deviceList.values()) {
    Log.d("UsbHost", "Found device: " + device.getDeviceName()
        + " (vendorId=" + device.getVendorId() + ")");
}

Requesting Permission for a Specific Device

Before you can open and communicate with a particular UsbDevice, the user must grant permission for that specific device:

private static final String ACTION_USB_PERMISSION = "com.recipebox.USB_PERMISSION";

private void requestUsbAccess(UsbDevice device) {
    PendingIntent permissionIntent = PendingIntent.getBroadcast(
        this, 0, new Intent(ACTION_USB_PERMISSION), PendingIntent.FLAG_MUTABLE);

    IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
    registerReceiver(usbPermissionReceiver, filter);

    usbManager.requestPermission(device, permissionIntent);
}

private final BroadcastReceiver usbPermissionReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        if (ACTION_USB_PERMISSION.equals(intent.getAction())) {
            UsbDevice device = intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);
            boolean granted = intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false);
            if (granted && device != null) {
                // Safe to open a connection to this device now
            }
        }
    }
};

The Alternative: UsbAccessory Mode

Everything above assumes the Android device is the host, and something else is the attached device. The USB API also supports the opposite relationship through UsbAccessory: here, the Android device is the USB accessory, and some other hardware (often a microcontroller board) acts as the host driving the connection. Which mode applies depends entirely on which side of the USB cable your Android device sits on for the hardware you're integrating with.

Quick Reference