nullpointerexception - Android Things:在Raspberry PI 3上通过USB UART接收数据时出现NullPointerException

Question

在我的代码中，我在UartDeviceCallBack上注册了一个UartDevice，当我的外设通过串行发送数据时，我的应用程序崩溃并出现以下错误:

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)

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))? = null

    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")
        }

    }
}

有什么线索吗？

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))? = null

    private 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")
        }

    }
}

Answer 1

到行private var mDevice : UartDevice? = getDevice(..)被调用时，uartDeviceCallBack仍引用null，因为它的实例化还发生在一行上。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)