Getting a FileDescriptor reference

Anything that implements the junixsocket interface FileDescriptorAccess (all AFSockets, for example), can return the corresponding FileDescriptor:

AFSocket socket = ...;
FileDescriptor fd = socket.getFileDescriptor();

You can also get the file descriptor from some classes in the Java API, for example:

FileInputStream in = ...;
FileDescriptor fd = in.getFD();

RandomAccessFile raf = ...;
FileDescriptor fd = in.getFD();

// Standard file descriptors
FileDescriptor stdin = FileDescriptor.in;
FileDescriptor stdout = FileDescriptor.out;
FileDescriptor stderr = FileDescriptor.err;

Sending and receiving File Descriptors

A very useful feature of Unix Domain Sockets is the ability to send and receive file descriptors between processes.

This, for example, enables non-privileged processes to access otherwise restricted files: A privileged process (e.g., running as root, or as a UID with special access rights) opens the restricted file, and then exposes the file handle to another process via AF_UNIX sockets.

File descriptors are sent as so-called “ancillary messages” along with regulary payload. They cannot be sent alone, so make sure that you send at least some data, even if it's just one byte.

AFUNIXSocket socket = ...

FileInputStream fin = new FileInputStream(file);
socket.setOutboundFileDescriptors(fin.getFD());
// you can also send more than one FD at the same time, just make sure they're all part of the same call to setOutboundFileDescriptors. 

// Ancillary messages are sent _along_ regular in-band messages, so we have to send something here.
os.write("Some message".getBytes("UTF-8"));

AFUNIXSocket socket = ...

// set ancillary receive buffer to a reasonable size (disabled by default!)
socket.setAncillaryReceiveBufferSize(1024);

InputStream in = socket.getInputStream();
// do you regular socket IO here
in.read(...)

// If there were any file descriptors sent as ancillary messages, check them right after a call to read
FileDescriptor[] descriptors = socket.getReceivedFileDescriptors();
if (descriptors != null) {
  for (FileDescriptor fd : descriptors) {
    FileInputStream fin = new FileInputStream(fd);
    // do something with the stream
  }
}

FileInputStream fin = ...;

RemoteFileInput rfi = new RemoteFileInput(socketFactory, fin);
// rfi can now be used for inter-process communication via RMI:
someRMIService.someMethod(rfi);

// on the receiving side:
public void someMethod(RemoteFileInput rfi) throws IOException {
    FileInputStream fin = rfi.asFileInputStream();
    // ...
    fin.close(); // closes the stream for this process only (different file handle).
} 

“Casting” FileDescriptor objects to Socket etc.

When you receive a FileDescriptor from another process, you want to use it as if it was created locally. While the Java standard API does not provide us with much more than a FileDescriptor-taking constructor for java.io.FileInputStream/FileOutputStream, junixsocket gives you more control via FileDescriptorCast:

First, you instantiate a FileDescriptorCast instance using the FileDescriptor of your choice, then you specify as what class you want to access it:

FileDescriptor fd = ...;
// NOTE: check `fd.valid()` or an `IOException` may be thrown.
Class<T> desiredClass = ...;
T instance = FileDescriptorCast.using(fd).as(desiredClass);

For example, if you want to simply read from this file descriptor, use:

InputStream in = FileDescriptorCast.using(fd).as(InputStream.class);

If the file descriptor is a Socket, you can use:

Socket sock = FileDescriptorCast.using(fd).as(Socket.class);
// or:
AFSocket sock = FileDescriptorCast.using(fd).as(AFSocket.class);
// or:
AFUNIXSocket sock = FileDescriptorCast.using(fd).as(AFUNIXSocket.class);
// etc.

If you want to access the native file descriptor value as an integer (only where supported), you can use:

int fdVal = FileDescriptorCast.using(fd).as(Integer.class); // won't work for all types on Windows

Note that if the specified FileDescriptor is incompatible with the target class, a ClassCastException is thrown. Also be aware that this technically isn't a cast, since a different object reference is returned.

In order to check if a file descriptor can be “cast” to a particular class, you can use FileDescriptorCast#isAvailable and FileDescriptorCast#availableTypes:

FileDescriptorCast fdc = FileDescriptorCast.using(fd);
if (fdc.isAvailable(Socket.class)) {
	Socket socket = fdc.as(Socket.class);
} else {
	throw new IllegalStateException("Cannot cast to Socket, only to: " + fdc.availableTypes());
}

Sending a FileDescriptor via ProcessBuilder.Redirect

You can pass a FileDescriptor as standard input (“stdin”) to a process launched with ProcessBuilder:

FileDescriptor fd = ...;
ProcessBuilder pb = ...;
pb.redirectInput(FileDescriptorCast.using(fd).as(ProcessBuilder.Redirect.class));
Process p = pb.start();

if (AFSocket.supports(AFSocketCapability.CAPABILITY_FD_AS_REDIRECT)) {...}