Adding a router class to handle messages that expect responses.

mojo/bindings/java/src/org/chromium/mojo/bindings/Message.java

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.mojo.bindings;
 
 import org.chromium.mojo.system.Handle;
 import org.chromium.mojo.system.MessagePipeHandle;
 import org.chromium.mojo.system.MessagePipeHandle.ReadMessageResult;
 import org.chromium.mojo.system.MojoResult;
 
 import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
 import java.util.List;
 
 import javax.annotation.Nullable;
 
 /**
  * A raw message to be sent/received from a {@link MessagePipeHandle}.
  */
 public final class Message {
 
     /**
      * The data of the message.
      */
     public final ByteBuffer buffer;
 
     /**
      * The handles of the message.
      */
     public final List<? extends Handle> handles;
 
+    private MessageHeader mHeader = null;
+    private Message mPayload = null;
+
     /**
      * Constructor.
      *
      * @param buffer The buffer containing the bytes to send. This must be a direct buffer.
      * @param handles The list of handles to send.
      */
     public Message(ByteBuffer buffer, List<? extends Handle> handles) {
         assert buffer.isDirect();
         this.buffer = buffer;
         this.handles = handles;
@@ skipping 11 matching lines @@
             return result.getMojoResult();
         }
         ByteBuffer buffer = ByteBuffer.allocateDirect(result.getMessageSize());
         result = handle.readMessage(buffer, result.getHandlesCount(),
                 MessagePipeHandle.ReadFlags.NONE);
         if (receiver != null && result.getMojoResult() == MojoResult.OK) {
             receiver.accept(new Message(buffer, result.getHandles()));
         }
         return result.getMojoResult();
     }
+
+    /**
+     * Returns the header of the given message. This should only be called on message that are known
+     * to have a header. This will throw a {@link DeserializationException} it the start of the

[rmcilroy, 2014/07/10 19:03:57]

/s/it/if

[qsr, 2014/07/11 11:42:07]

Done.

+     * message is not a valid header.
+     */
+    public MessageHeader getHeader() {

[rmcilroy, 2014/07/10 19:03:57]

I'm not sure I like the idea of a Message optionally having header/payload. 
Could you have a new class - MessageWithHeader, which either extends or wraps
Message, and modify acceptWithResponder to take a MessageWithHeader.  You can
then have a single place where you convert between Message and MessageWithHeader
(namely Router.handleIncomingMessage) and ensure type safety the rest of the
time.

[qsr, 2014/07/11 11:42:07]

Done, except that I need to use header in all the message receiver... As well as
when reading a message from a pipe. The only place where I do not want header is
when serializing/deserializing a struct.

+        if (mHeader == null) {
+            mHeader = new org.chromium.mojo.bindings.MessageHeader(this);
+        }
+        return mHeader;
+    }
+
+    /**
+     * Returns the payload of the message. This should only be called on message that are known to
+     * have a header.
+     */
+    public Message getPayload() {
+        if (mPayload == null) {
+            ByteBuffer truncatedBuffer = ((ByteBuffer) buffer.position(
+                    getHeader().getSize())).slice();
+            truncatedBuffer.order(ByteOrder.nativeOrder());
+            mPayload = new Message(truncatedBuffer, handles);
+        }
+        return mPayload;
+    }
+
+    /**
+     * Reset the cache header. Used by {@link MessageHeader} when it modified the message buffer.
+     */
+    void resetHeader() {

[rmcilroy, 2014/07/10 19:03:57]

Having to remember to resetHeader also seems error prone.  If we have a separate
class then we could have setRequestId be a method on MessageWithHeader, which
could then internally ensure that the cached header/payload is invalidated
appropriately, WDYT?

[qsr, 2014/07/11 11:42:07]

Done.

+        mHeader = null;
+        mPayload = null;
+    }
+
 }