Android事物（在Raspberry PI 3上通过USB UART接收数据时出现NullPointerException）

少年恃险若平地，独倚长剑凌清秋。这篇文章主要讲述Android事物：在Raspberry PI 3上通过USB UART接收数据时出现NullPointerException相关的知识，希望能为你提供帮助。
在我的代码中，我在UartDeviceCallBack上注册了UartDevice，当我的外围设备通过串口发送数据时，我的应用程序崩溃，出现以下错误：

E/androidRuntime: FATAL EXCEPTION: main Process: co.foodles.posapp, PID: 1655 java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.google.android.things.pio.UartDeviceCallback.onUartDeviceDataAvailable(com.google.android.things.pio.UartDevice)' on a null object reference at com.google.android.things.pio.UartDeviceImpl$UartDeviceCallbackDispatch.dispatchInterruptEvent(UartDeviceImpl.java:245) at com.google.android.things.pio.CallbackDispatch.onFileDescriptorEvents(CallbackDispatch.java:149) at android.os.MessageQueue.dispatchEvents(MessageQueue.java:284) at android.os.MessageQueue.nativePollOnce(Native Method) at android.os.MessageQueue.next(MessageQueue.java:325) at android.os.Looper.loop(Looper.java:142) at android.app.ActivityThread.main(ActivityThread.java:6494) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

【Android事物（在Raspberry PI 3上通过USB UART接收数据时出现NullPointerException）】这是我的Uart设备的类：

class RFideas80581AK0(peripheralManagerService : PeripheralManagerService): Closeable {companion object { private val LOG_TAG = RFideas80581AK0::class.java.simpleName private val UART_DEVICE_NAME = "USB1-1.2:1.0" private val BAUDRATE = 57600 private val DATA_SIZE = 8 private val PARITY = UartDevice.PARITY_NONE private val STOP_BIT = 1 }private var mDevice : UartDevice? = getDevice(peripheralManagerService, UART_DEVICE_NAME, BAUDRATE, DATA_SIZE, PARITY, STOP_BIT)private val uartDeviceCallBack = CustomUartCallBack()var readCallBack : ((String) -> (Unit))? = nullprivate fun getDevice(peripheralManagerService: PeripheralManagerService, name : String, baudrate : Int, dataSize : Int, parity : Int, stopBit : Int) : UartDevice? = try { peripheralManagerService.openUartDevice(name).also { it.setBaudrate(baudrate) it.setDataSize(dataSize) it.setParity(parity) it.setStopBits(stopBit) it.registerUartDeviceCallback(uartDeviceCallBack) Log.d(LOG_TAG, "configured device ${it.name}") } } catch (e : IOException){ Log.e(LOG_TAG, "error while opening UartDevice on address $UART_DEVICE_NAME",e) null }override fun close() { Log.d(LOG_TAG, "close") mDevice = mDevice?.let { try { it.unregisterUartDeviceCallback(uartDeviceCallBack) it.close() } catch (e : IOException){ Log.e(LOG_TAG, "error while closing UartDevice",e) throw e } null } }private fun convertAsciiNumbersToHexString(asciiNumbers : String) : String { var hexString = BigInteger(asciiNumbers).toString(16) if (hexString.length %2 == 1) hexString = "0"+hexString return hexString.windowed(2, 2).reversed().joinToString("").toUpperCase() }override fun readUartBuffer(uartDevice: UartDevice) : String { val maxCount = 128 val buffer = ByteArray(maxCount) uartDevice.read(buffer, maxCount) return convertAsciiNumbersToHexString(String(buffer).takeWhile { it.isDigit() }) }inner class CustomUartCallBack : UartDeviceCallback(){override fun onUartDeviceDataAvailable(uart: UartDevice): Boolean { try { readCallBack?.invoke(readUartBuffer(uart)) } catch (e : IOException){ Log.e(LOG_TAG, "error while reading uart buffer",e) } return true }override fun onUartDeviceError(uart: UartDevice, error: Int) { Log.e(LOG_TAG, "error $error in CustomUartCallBack") }} }

任何线索？
感谢Onik，这是正确的实现：

class RFideas80581AK0(peripheralManagerService : PeripheralManagerService): Closeable {companion object { private val LOG_TAG = RFideas80581AK0::class.java.simpleName private val UART_DEVICE_NAME = "USB1-1.2:1.0" private val BAUDRATE = 57600 private val DATA_SIZE = 8 private val PARITY = UartDevice.PARITY_NONE private val STOP_BIT = 1 }private val uartDeviceCallBack = CustomUartCallBack()var readCallBack : ((String) -> (Unit))? = nullprivate var mDevice : UartDevice? = getDevice(peripheralManagerService, UART_DEVICE_NAME, BAUDRATE, DATA_SIZE, PARITY, STOP_BIT)private fun getDevice(peripheralManagerService: PeripheralManagerService, name : String, baudrate : Int, dataSize : Int, parity : Int, stopBit : Int) : UartDevice? = try { peripheralManagerService.openUartDevice(name).also { it.setBaudrate(baudrate) it.setDataSize(dataSize) it.setParity(parity) it.setStopBits(stopBit) it.registerUartDeviceCallback(uartDeviceCallBack) Log.d(LOG_TAG, "configured device ${it.name}") } } catch (e : IOException){ Log.e(LOG_TAG, "error while opening UartDevice on address $UART_DEVICE_NAME",e) null }override fun close() { Log.d(LOG_TAG, "close") mDevice = mDevice?.let { try { it.unregisterUartDeviceCallback(uartDeviceCallBack) it.close() } catch (e : IOException){ Log.e(LOG_TAG, "error while closing UartDevice",e) throw e } null } }private fun convertAsciiNumbersToHexString(asciiNumbers : String) : String { var hexString = BigInteger(asciiNumbers).toString(16) if (hexString.length %2 == 1) hexString = "0"+hexString return hexString.windowed(2, 2).reversed().joinToString("").toUpperCase() }override fun readUartBuffer(uartDevice: UartDevice) : String { val maxCount = 128 val buffer = ByteArray(maxCount) uartDevice.read(buffer, maxCount) return convertAsciiNumbersToHexString(String(buffer).takeWhile { it.isDigit() }) }inner class CustomUartCallBack : UartDeviceCallback(){override fun onUartDeviceDataAvailable(uart: UartDevice): Boolean { try { readCallBack?.invoke(readUartBuffer(uart)) } catch (e : IOException){ Log.e(LOG_TAG, "error while reading uart buffer",e) } return true }override fun onUartDeviceError(uart: UartDevice, error: Int) { Log.e(LOG_TAG, "error $error in CustomUartCallBack") }} }

答案当private var mDevice : UartDevice? = getDevice(..)行被调用时，uartDeviceCallBack引用了null，因为它的实例化进一步发生了一条线。更改行的顺序：

private val uartDeviceCallBack = CustomUartCallBack() private var mDevice : UartDevice? = getDevice(peripheralManagerService, UART_DEVICE_NAME, BAUDRATE, DATA_SIZE, PARITY, STOP_BIT)