Adding a router class to handle messages that expect responses.

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

Index: mojo/bindings/java/src/org/chromium/mojo/bindings/Message.java
diff --git a/mojo/bindings/java/src/org/chromium/mojo/bindings/Message.java b/mojo/bindings/java/src/org/chromium/mojo/bindings/Message.java
index f826a8e034c9215a8a8e6496e05c716619a32d0d..14bdbf1e7cbc1eb376ea6c39590804f41a4293d9 100644
--- a/mojo/bindings/java/src/org/chromium/mojo/bindings/Message.java
+++ b/mojo/bindings/java/src/org/chromium/mojo/bindings/Message.java
@@ -10,6 +10,7 @@ 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;
@@ -29,6 +30,9 @@ public final class Message {
      */
     public final List<? extends Handle> handles;
 
+    private MessageHeader mHeader = null;
+    private Message mPayload = null;
+
     /**
      * Constructor.
      *
@@ -60,4 +64,39 @@ public final class Message {
         }
         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;
+    }
+
 }