diff --git a/runtime/src/main/java/org/capnproto/Serialize.java b/runtime/src/main/java/org/capnproto/Serialize.java
new file mode 100644
index 0000000..3f06e39
--- /dev/null
+++ b/runtime/src/main/java/org/capnproto/Serialize.java
@@ -0,0 +1,29 @@
+package org.capnproto;
+
+import java.io.IOException;
+import java.nio.channels.GatheringByteChannel;
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+
+public final class Serialize {
+    public static void writeMessage(GatheringByteChannel outputChannel,
+                                    MessageBuilder message) throws IOException {
+        ByteBuffer[] segments = message.getSegmentsForOutput();
+        int tableSize = (segments.length + 2) & (~1);
+
+        ByteBuffer table = ByteBuffer.allocate(4 * tableSize);
+        table.order(ByteOrder.LITTLE_ENDIAN);
+
+        table.putInt(0, segments.length - 1);
+
+        for (int i = 0; i < segments.length; ++i) {
+            table.putInt(4 * (i + 1), segments[i].limit() / 8);
+        }
+
+        // Any padding is already zeroed.
+
+        outputChannel.write(table);
+
+        outputChannel.write(segments);
+    }
+}