Adding a router class to handle messages that expect responses.

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

Index: mojo/bindings/java/src/org/chromium/mojo/bindings/MessageWithHeader.java
diff --git a/mojo/bindings/java/src/org/chromium/mojo/bindings/MessageWithHeader.java b/mojo/bindings/java/src/org/chromium/mojo/bindings/MessageWithHeader.java
new file mode 100644
index 0000000000000000000000000000000000000000..9e7e28ca092d35c48ad6ae6503b17fd5c7c6c323
--- /dev/null
+++ b/mojo/bindings/java/src/org/chromium/mojo/bindings/MessageWithHeader.java
@@ -0,0 +1,95 @@
+// 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.MessagePipeHandle;
+import org.chromium.mojo.system.MessagePipeHandle.ReadMessageResult;
+import org.chromium.mojo.system.MojoResult;
+
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+
+/**
+ * A wrapper around {@link Message} that handles parsing the message header.

[rmcilroy, 2014/07/16 15:06:44]

I think maybe: 

Represents a {@link Message} which contains a {@link MessageHeader}.  Deals with
parsing the {@link MessageHeader} for a message.

[qsr, 2014/07/16 15:58:38]

Done.

+ */
+public class MessageWithHeader {
+
+    private final Message mBaseMessage;
+    private final MessageHeader mHeader;
+    private Message mPayload;
+
+    /**
+     * Reinterpret the given message as a message with header.

[rmcilroy, 2014/07/16 15:06:44]

Reinterpret the given |message| as a message with the given |header|.  The
|message| must contain the |header| as the start of its raw data.

[qsr, 2014/07/16 15:58:38]

Done.

+     */
+    public MessageWithHeader(Message baseMessage, MessageHeader header) {
+        assert header.equals(new org.chromium.mojo.bindings.MessageHeader(baseMessage));
+        this.mBaseMessage = baseMessage;
+        this.mHeader = header;
+    }
+
+    /**
+     * Reinterpret the given message as a message with header.

[rmcilroy, 2014/07/16 15:06:44]

Reinterpret the given |message| as a message with a header.  The
|message| must contain a header as the start of it's raw data, which will be
parsed by this constructor.

[qsr, 2014/07/16 15:58:38]

Done.

+     */
+    public MessageWithHeader(Message baseMessage) {
+        this(baseMessage, new org.chromium.mojo.bindings.MessageHeader(baseMessage));
+    }
+
+    /**
+     * Returns the header of the given message. This will throw a {@link DeserializationException}
+     * if the start of the message is not a valid header.
+     */
+    public MessageHeader getHeader() {
+        return mHeader;
+    }
+
+    /**
+     * Returns the payload of the message.
+     */
+    public Message getPayload() {
+        if (mPayload == null) {
+            ByteBuffer truncatedBuffer = ((ByteBuffer) mBaseMessage.buffer.position(
+                    getHeader().getSize())).slice();
+            truncatedBuffer.order(ByteOrder.nativeOrder());
+            mPayload = new Message(truncatedBuffer, mBaseMessage.handles);
+        }
+        return mPayload;
+    }
+
+    /**
+     * Returns the raw message.
+     */
+    public Message getMessage() {
+        return mBaseMessage;
+    }
+
+    /**
+     * Set the request identifier on the message.
+     */
+    void setRequestId(long requestId) {
+        mHeader.setRequestId(mBaseMessage.buffer, requestId);
+    }
+
+    /**
+     * Read a message, and pass it to the given |MessageReceiver| if not null. If the
+     * |MessageReceiver| is null, the message is lost.
+     *
+     * @param receiver The {@link MessageReceiver} that will receive the read {@link Message}. Can
+     *            be <code>null</code>, in which case the message is discarded.
+     */
+    public static int readAndDispatchMessage(MessagePipeHandle handle, MessageReceiver receiver) {
+        // TODO(qsr) Allow usage of a pool of pre-allocated buffer for performance.
+        ReadMessageResult result = handle.readMessage(null, 0, MessagePipeHandle.ReadFlags.NONE);
+        if (result.getMojoResult() != MojoResult.RESOURCE_EXHAUSTED) {
+            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 MessageWithHeader(new Message(buffer, result.getHandles())));
+        }
+        return result.getMojoResult();
+    }
+}