使用usb4java读取数据时，USB没有返回数据，为什么？

Question

现在

transferred

获得正确的数据大小，但缓冲区中没有字节，为什么？

public static void handle(short vid, short pid) {
        Device device = findDevice(vid, pid);
        Pair pair = findBulkInEndpoint(device, LibUsb.TRANSFER_TYPE_BULK);
        if (pair == null) {
            System.out.println("No bulk - in endpoint found.");
            return;
        }
        int result;
        DeviceHandle handle = new DeviceHandle();
        result = LibUsb.open(device, handle);
        if (result != LibUsb.SUCCESS) {
            throw new LibUsbException("Unable to open device", result);
        }
        try {
            result = LibUsb.claimInterface(handle, pair.interfaceDescriptor.iInterface());
            if (result != LibUsb.SUCCESS) {
                throw new LibUsbException("Unable to claim interface", result);
            }
            boolean running = true;
            while (running) {
                ByteBuffer buffer = BufferUtils.allocateByteBuffer(1024).order(ByteOrder.LITTLE_ENDIAN);
                IntBuffer transferred = IntBuffer.allocate(1);
                result = LibUsb.bulkTransfer(handle, pair.endpointDescriptor.bEndpointAddress(), buffer, transferred, 10000);
                if (result == 0) {
                    int len = transferred.get();
                    System.out.println("Read " + len);
                    buffer.flip();
                    byte[] bytes = new byte[len];
                    buffer.get(bytes, 0, len);  //limit=0, position=0, throw BufferUnderflowException
                    System.out.println(new String(bytes));
                } else {
                    throw new LibUsbException("Bulk transfer error", result);
                }
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        } finally {
            LibUsb.close(handle);
        }
    }

Answer 1

您可能想尝试JavaDoesUSB。使用起来更加方便。

下面的代码假设接口号和端点号是已知的，这是已知设备的情况。如果需要的话也可以检测到。

int INTERFACE_NO = 1;
int ENDPOINT_IN = 1;

public static void handle(int vid, int pid) {

    var device = Usb.findDevice(vid, pid).get();
    device.open();
    device.claimInterface(INTERFACE_NO);

    boolean running = true;
    while (running) {
        var bytes = device.transferIn(ENDPOINT_IN);
        System.out.println(new String(bytes, StandardCharsets.UTF_8));

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }

    device.close();
}

请注意，JavaDoesUSB 是使用 Foreign Function & Memory API 构建的。所以它至少需要 Java 22。