The low-level API of usb4java closely follows the C API of the libusb project. All global functions and constants of libusb are defined as static members of the class org.usb4java.LibUsb. All structures of libusb are defined in separate classes which are named similar to the original struct names but without underscores, with camel-case names and with the libusb prefix removed. For example the struct libusb_device_handle is defined in the class DeviceHandle. Struct members are represented by static methods in the corresponding class.
The following notable differences exists between the libusb 1.0 API and the usb4java API:
Before using any usb4java functionality you must initialize libusb:
Context context = new Context();
int result = LibUsb.init(context);
if (result != LibUsb.SUCCESS) throw new LibUsbException("Unable to initialize libusb.", result);Specifiying a context is optional. If your application only needs a single libusb context then you can specify null as context.
Before your application terminates you should deinitialize libusb:
LibUsb.exit(context);
Related libusb documentation:
Your program most likely wants to communicate with a specific device so first of all you have to find it. You have to get a list of all connected USB devices and then check the vendor/product ids. Here is a method which can be used for this purpose:
public Device findDevice(short vendorId, short productId)
{
// Read the USB device list
DeviceList list = new DeviceList();
int result = LibUsb.getDeviceList(null, list);
if (result < 0) throw new LibUsbException("Unable to get device list", result);
try
{
// Iterate over all devices and scan for the right one
for (Device device: list)
{
DeviceDescriptor descriptor = new DeviceDescriptor();
result = LibUsb.getDeviceDescriptor(device, descriptor);
if (result != LibUsb.SUCCESS) throw new LibUsbException("Unable to read device descriptor", result);
if (descriptor.idVendor() == vendorId && descriptor.idProduct() == productId) return device;
}
}
finally
{
// Ensure the allocated device list is freed
LibUsb.freeDeviceList(list, true);
}
// Device not found
return null;
}In your application it might be a little bit more complicated. Maybe you have more than one device of the same type so you may need a list of devices. Or you have to identify your device by the product or vendor string descriptor instead of just checking the ID (In case you are using a shared vendor/product ID). But this example should bring you on the right track.
Related libusb documentation:
For the real USB communication you must open a new device handle and you must close it again when you are finished communicating with the device. Example:
DeviceHandle handle = new DeviceHandle();
int result = LibUsb.open(device, handle);
if (result != LibUsb.SUCCESS) throw new LibUsbException("Unable to open USB device", result);
try
{
// Use device handle here
}
finally
{
LibUsb.close(handle);
}When you want to communicate with an interface or with endpoints of this interface then you have to claim it before using it and you have to release it when you are finished. Example:
int result = LibUsb.claimInterface(handle, interfaceNumber);
if (result != LibUsb.SUCCESS) throw new LibUsbException("Unable to claim interface", result);
try
{
// Use interface here
}
finally
{
result = LibUsb.releaseInterface(handle, interfaceNumber);
if (result != LibUsb.SUCCESS) throw new LibUsbException("Unable to release interface", result);
}It is possible that the interface you want to communicate with is already used by a kernel driver. In this case you have to detach the kernel driver from the interface before claiming it. Example:
// Check if kernel driver must be detached
boolean detach = LibUsb.hasCapability(LibUsb.CAP_SUPPORTS_DETACH_KERNEL_DRIVER)
&& LibUsb.kernelDriverActive(handle, interfaceNumber);
// Detach the kernel driver
if (detach)
{
int result = LibUsb.detachKernelDriver(handle, interfaceNumber);
if (result != LibUsb.SUCCESS) throw new LibUsbException("Unable to detach kernel driver", result);
}
// Communicate with the device
...
// Attach the kernel driver again if needed
if (detach)
{
int result = LibUsb.attachKernelDriver(handle, interfaceNumber);
if (result != LibUsb.SUCCESS) throw new LibUsbException("Unable to re-attach kernel driver", result);
}Please note that detaching kernel drivers is not supported on Windows.
For the actual USB communication you usually have to create a direct byte buffer for the data to send or receive.
This examples sends 8 bytes to a claimed interface using a control transfer:
ByteBuffer buffer = ByteBuffer.allocateDirect(8);
buffer.put(new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 });
int transfered = LibUsb.controlTransfer(handle,
(byte) (LibUsb.REQUEST_TYPE_CLASS | LibUsb.RECIPIENT_INTERFACE),
(byte) 0x09, (short) 2, (short) 1, buffer, timeout);
if (transfered < 0) throw new LibUsbException("Control transfer failed", transfered);
System.out.println(transfered + " bytes sent");This example sends 8 bytes to endpoint 0x03 of the claimed interface using a bulk transfer:
ByteBuffer buffer = ByteBuffer.allocateDirect(8);
buffer.put(new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 });
IntBuffer transfered = IntBuffer.allocate(1);
int result = LibUsb.bulkTransfer(handle, 0x03, buffer, transfered, timeout);
if (result != LibUsb.SUCCESS) throw new LibUsbException("Control transfer failed", transfered);
System.out.println(transfered.get() + " bytes sent");Related libusb documentation:
Asynchronous I/O is a little bit more complex than synchronous I/O. That's because libusb doesn't start its own thread to handle the actual background tasks. Instead you have to create you own worker thread like this:
class EventHandlingThread extends Thread
{
/** If thread should abort. */
private volatile boolean abort;
/**
* Aborts the event handling thread.
*/
public void abort()
{
this.abort = true;
}
@Override
public void run()
{
while (!this.abort)
{
int result = LibUsb.handleEventsTimeout(null, 250000);
if (result != LibUsb.SUCCESS)
throw new LibUsbException("Unable to handle events", result);
}
}
}This simple thread implementation doesn't use a specific libusb context so it specified null as context. If you need contexts then you may want to pass it to the thread somehow.
The thread must be started after you have initialized libusb:
EventHandlingThread thread = new EventHandlingThread(); thread.start();
And it must be stopped before deinitializing libusb:
thread.abort(); thread.join();
So now with this thread running in the background you can use the asynchronous functions of libusb. If you don't like this thread and your program already has some kind of application loop then you can also simply call LibUsb.handleEventsTimeout(null, 0) inside the loop. This call returns immediately if there are no events to process.
An actual asynchronous transfer is submitted like this (In this case an outgoing bulk transfer to endpoint 0x03):
ByteBuffer buffer = BufferUtils.allocateByteBuffer(8);
buffer.put(new byte[] { 1, 2, 3, 4, 5, 6, 7, 8 });
Transfer transfer = LibUsb.allocTransfer();
LibUsb.fillBulkTransfer(transfer, handle, 0x03, buffer, callback, null, timeout);
int result = LibUsb.submitTransfer(transfer);
if (result != LibUsb.SUCCESS) throw new LibUsbException("Unable to submit transfer", result);The callback is an object implementing the TransferCallback interface. Here is an example of such a callback:
TransferCallback callback = new TransferCallback()
{
@Override
public void processTransfer(Transfer transfer)
{
System.out.println(transfer.actualLength() + " bytes sent");
LibUsb.freeTransfer(transfer);
}
};Related libusb documentation: