Add forFD functionality to UnixSocketChannel. Fixes #4.

src/main/java/jnr/unixsocket/UnixSocketChannel.java

@@ -64,6 +64,27 @@ public static final UnixSocketChannel[] pair() throws IOException {
         };
     }
 
+    /**
+     * Create a UnixSocketChannel to wrap an existing file descriptor (presumably itself a UNIX socket).
+     *
+     * @param fd the file descriptor to wrap
+     * @return the new UnixSocketChannel instance
+     */
+    public static final UnixSocketChannel fromFD(int fd) {
+        return fromFD(fd, SelectionKey.OP_READ | SelectionKey.OP_WRITE);
+    }
+
+    /**
+     * Create a UnixSocketChannel to wrap an existing file descriptor (presumably itself a UNIX socket).
+     *
+     * @param fd the file descriptor to wrap
+     * @param ops the SelectionKey operations the socket supports
+     * @return the new UnixSocketChannel instance
+     */
+    public static final UnixSocketChannel fromFD(int fd, int ops) {
+        return new UnixSocketChannel(fd, ops);
+    }
+
     private UnixSocketChannel() throws IOException {
         super(Native.socket(ProtocolFamily.PF_UNIX, Sock.SOCK_STREAM, 0),
                 SelectionKey.OP_CONNECT | SelectionKey.OP_READ | SelectionKey.OP_WRITE);

src/test/java/jnr/unixsocket/ForFDTest.java

@@ -0,0 +1,80 @@
+package jnr.unixsocket;
+
+import jnr.constants.platform.ProtocolFamily;
+import jnr.constants.platform.Sock;
+import junit.framework.Assert;
+import org.junit.Test;
+
+import java.io.File;
+import java.nio.ByteBuffer;
+import java.nio.CharBuffer;
+import java.nio.charset.StandardCharsets;
+
+import static junit.framework.Assert.assertEquals;
+import static junit.framework.Assert.assertNotNull;
+import static junit.framework.Assert.assertTrue;
+
+/**
+ * Created by headius on 11/24/15.
+ */
+public class ForFDTest {
+    private static final File SOCKADDR = new File("/tmp/jnr-unixsocket-forfd" + System.currentTimeMillis() + ".sock");
+    static { SOCKADDR.deleteOnExit(); }
+    private static final UnixSocketAddress ADDRESS = new UnixSocketAddress(SOCKADDR);
+    private static final String FOOBAR = "foobar";
+
+    private volatile Exception serverException;
+
+    @Test
+    public void testForFD() throws Exception {
+        int fd = 0;
+        UnixSocketChannel channel = null;
+
+        try {
+            final UnixServerSocketChannel server = UnixServerSocketChannel.open();
+            server.socket().bind(ADDRESS);
+
+            new Thread("accept thread") {
+                public void run() {
+                    UnixSocketChannel channel = null;
+
+                    try {
+                        channel = server.accept();
+                        channel.write(ByteBuffer.wrap(FOOBAR.getBytes()));
+                    } catch (Exception e) {
+                        serverException = e;
+                    } finally {
+                        try {channel.close();} catch (Exception e) {}
+                    }
+                }
+            }.start();
+
+            fd = Native.socket(ProtocolFamily.PF_UNIX, Sock.SOCK_STREAM, 0);
+
+            assertTrue("socket failed", fd > 0);
+
+            int ret = Native.connect(fd, ADDRESS.getStruct(), ADDRESS.getStruct().length());
+
+            assertTrue("connect failed", ret >= 0);
+
+            channel = UnixSocketChannel.fromFD(fd);
+
+            assertNotNull(channel);
+
+            ByteBuffer buf = ByteBuffer.allocate(1024);
+
+            channel.read(buf);
+
+            assertEquals(FOOBAR.length(), buf.position());
+
+            buf.flip();
+            String result = new String(buf.array(), buf.position(), buf.limit(), StandardCharsets.UTF_8);
+
+            assertEquals(FOOBAR, result);
+
+            if (serverException != null) throw serverException;
+        } finally {
+            channel.close();
+        }
+    }
+}