Teach ThreadBuffer to read from a URL

Author: Matthew L Daniel

src/main/java/com/maxmind/db/ThreadBuffer.java

@@ -2,15 +2,51 @@
 
 import java.io.Closeable;
 import java.io.File;
+import java.io.InputStream;
 import java.io.IOException;
 import java.io.RandomAccessFile;
+import java.net.URL;
+import java.net.URLConnection;
 import java.nio.ByteBuffer;
+import java.nio.channels.Channels;
 import java.nio.channels.FileChannel;
 import java.nio.channels.FileChannel.MapMode;
+import java.nio.channels.ReadableByteChannel;
 
 import com.maxmind.db.Reader.FileMode;
 
 final class ThreadBuffer extends ThreadLocal<ByteBuffer> implements Closeable {
+    /**
+     * Construct a ThreadBuffer from the provided URL.
+     * @param source the URL that I will read in and use as the source of my
+     *               bytes.
+     * @return a newly constructed instance based on the contents of your URL.
+     * @throws IOException if unable to read from your source, or if your
+     * source does not return all of its contents (short read).
+     */
+    public static ThreadBuffer newInstance(URL source) throws IOException {
+        final URLConnection conn = source.openConnection();
+        conn.connect();
+
+        final InputStream stream = conn.getInputStream();
+        final int length = conn.getContentLength();
+        final ByteBuffer buffer = ByteBuffer.allocate(length);
+        final ReadableByteChannel channel = Channels.newChannel(stream);
+        int bytesRead = 0;
+        while (true) {
+            final int br = channel.read(buffer);
+            if (-1 == br) {
+                throw new IOException(String.format(
+                        "Short read from %s, wanted %d got %d", source, length, bytesRead));
+            }
+            bytesRead += br;
+            if (length == bytesRead) {
+                break;
+            }
+        }
+        return new ThreadBuffer(buffer);
+    }
+
     // DO NOT PASS THESE OUTSIDE THIS CLASS. Doing so will remove thread
     // safety.
     private final ByteBuffer buffer;
@@ -49,4 +85,4 @@ public void close() throws IOException {
             this.raf.close();
         }
     }
-}
+}