diff --git a/runtime/src/main/java/org/capnproto/BufferedWritableByteChannel.java b/runtime/src/main/java/org/capnproto/BufferedWritableByteChannel.java
new file mode 100644
index 0000000..60aa3aa
--- /dev/null
+++ b/runtime/src/main/java/org/capnproto/BufferedWritableByteChannel.java
@@ -0,0 +1,8 @@
+package org.capnproto;
+
+import java.nio.ByteBuffer;
+import java.nio.channels.WritableByteChannel;
+
+public interface BufferedWritableByteChannel extends WritableByteChannel {
+    public ByteBuffer getWriteBuffer();
+}
diff --git a/runtime/src/main/java/org/capnproto/BufferedWritableByteChannelWrapper.java b/runtime/src/main/java/org/capnproto/BufferedWritableByteChannelWrapper.java
new file mode 100644
index 0000000..e2c8a18
--- /dev/null
+++ b/runtime/src/main/java/org/capnproto/BufferedWritableByteChannelWrapper.java
@@ -0,0 +1,45 @@
+package org.capnproto;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.WritableByteChannel;
+
+public final class BufferedWritableByteChannelWrapper implements BufferedWritableByteChannel {
+
+    public final WritableByteChannel inner;
+    public final ByteBuffer buf;
+
+    public BufferedWritableByteChannelWrapper(WritableByteChannel w) {
+        this.inner = w;
+        this.buf = ByteBuffer.allocate(8192);
+    }
+
+    public final int write(ByteBuffer buf) throws IOException {
+        int available = this.buf.remaining();
+        int size = buf.remaining();
+        if (size <= available) {
+            this.buf.put(buf);
+        } else if (size <= this.buf.capacity()) {
+            //# Too much for this buffer, but not a full buffer's worth,
+            //# so we'll go ahead and copy.
+            //this.buf.put(buf); // XXX
+            //this.inner.write();
+        } else {
+            //# Writing so much data that we might as well write
+            //# directly to avoid a copy.
+        }
+        throw new Error("unimplemented");
+    }
+
+    public final ByteBuffer getWriteBuffer() {
+        return this.buf;
+    }
+
+    public final void close() throws IOException {
+        this.inner.close();
+    }
+
+    public final boolean isOpen() {
+        return this.inner.isOpen();
+    }
+}