add support for directly serializing a MessageReader

Previously, you would need to copy the message into a MessageBuilder first.

runtime/src/main/java/org/capnproto/Serialize.java

@@ -28,6 +28,10 @@ import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 import java.util.ArrayList;
 
+/**
+ * Serialization using the standard (unpacked) stream encoding:
+ * https://capnproto.org/encoding.html#serialization-over-a-stream
+ */
 public final class Serialize {
 
     static ByteBuffer makeByteBuffer(int bytes) {
@@ -186,9 +190,8 @@ public final class Serialize {
         return bytes / Constants.BYTES_PER_WORD;
     }
 
-    public static void write(WritableByteChannel outputChannel,
+    private static void writeSegmentTable(WritableByteChannel outputChannel,
-                             MessageBuilder message) throws IOException {
+                              ByteBuffer[] segments) throws IOException {
-        ByteBuffer[] segments = message.getSegmentsForOutput();
         int tableSize = (segments.length + 2) & (~1);
 
         ByteBuffer table = ByteBuffer.allocate(4 * tableSize);
@@ -204,6 +207,34 @@ public final class Serialize {
         while (table.hasRemaining()) {
             outputChannel.write(table);
         }
+    }
+
+    /**
+     * Serializes a MessageBuilder to a WritableByteChannel.
+     */
+    public static void write(WritableByteChannel outputChannel,
+                             MessageBuilder message) throws IOException {
+        ByteBuffer[] segments = message.getSegmentsForOutput();
+        writeSegmentTable(outputChannel, segments);
+
+        for (ByteBuffer buffer : segments) {
+            while(buffer.hasRemaining()) {
+                outputChannel.write(buffer);
+            }
+        }
+    }
+
+    /**
+     * Serializes a MessageReader to a WritableByteChannel.
+     */
+    public static void write(WritableByteChannel outputChannel,
+                             MessageReader message) throws IOException {
+        ByteBuffer[] segments = new ByteBuffer[message.arena.segments.size()];
+        for (int ii = 0 ; ii < message.arena.segments.size(); ++ii) {
+            segments[ii] = message.arena.segments.get(ii).buffer.duplicate();
+        }
+
+        writeSegmentTable(outputChannel, segments);
 
         for (ByteBuffer buffer : segments) {
             while(buffer.hasRemaining()) {

runtime/src/main/java/org/capnproto/SerializePacked.java

@@ -21,6 +21,9 @@
 
 package org.capnproto;
 
+/**
+ * Serialization using the packed encoding: https://capnproto.org/encoding.html#packing
+ */
 public final class SerializePacked {
 
     public static MessageReader read(BufferedInputStream input) throws java.io.IOException {
@@ -42,16 +45,42 @@ public final class SerializePacked {
         return Serialize.read(packedInput, options);
     }
 
+    /**
+     * Serializes a MessageBuilder to a BufferedOutputStream.
+     */
     public static void write(BufferedOutputStream output,
                              MessageBuilder message) throws java.io.IOException {
         PackedOutputStream packedOutputStream = new PackedOutputStream(output);
         Serialize.write(packedOutputStream, message);
     }
 
+    /**
+     * Serializes a MessageReader to a BufferedOutputStream.
+     */
+    public static void write(BufferedOutputStream output,
+                             MessageReader message) throws java.io.IOException {
+        PackedOutputStream packedOutputStream = new PackedOutputStream(output);
+        Serialize.write(packedOutputStream, message);
+    }
+
+    /**
+     * Serializes a MessageBuilder to an unbuffered output stream.
+     */
     public static void writeToUnbuffered(java.nio.channels.WritableByteChannel output,
                                        MessageBuilder message) throws java.io.IOException {
         BufferedOutputStreamWrapper buffered = new BufferedOutputStreamWrapper(output);
         write(buffered, message);
         buffered.flush();
     }
+
+    /**
+     * Serializes a MessageReader to an unbuffered output stream.
+     */
+    public static void writeToUnbuffered(java.nio.channels.WritableByteChannel output,
+                                         MessageReader message) throws java.io.IOException {
+        BufferedOutputStreamWrapper buffered = new BufferedOutputStreamWrapper(output);
+        write(buffered, message);
+        buffered.flush();
+    }
+
 }

runtime/src/test/java/org/capnproto/SerializeTest.java

@@ -55,6 +55,10 @@ public class SerializeTest {
     {
       MessageReader messageReader = Serialize.read(new ArrayInputStream(ByteBuffer.wrap(exampleBytes)));
       checkSegmentContents(exampleSegmentCount, messageReader.arena);
+
+      byte[] outputBytes = new byte[exampleBytes.length];
+      Serialize.write(new ArrayOutputStream(ByteBuffer.wrap(outputBytes)), messageReader);
+      Assert.assertArrayEquals(exampleBytes, outputBytes);
     }
 
     // ------