Add support for byte order to LengthFieldPrepender

Motivation:

While the LengthFieldBasedFrameDecoder supports a byte order the LengthFieldPrepender does not.
That means that I can simply add a LengthFieldBasedFrameDecoder with ByteOrder.LITTLE_ENDIAN to my pipeline
but have to write my own Encoder to write length fields in little endian byte order.

Modifications:

Added a constructor that takes a byte order and all other parameters.
All other constructors delegate to this one with ByteOrder.BIG_ENDIAN.
LengthFieldPrepender.encode() uses this byte order to write the length field.

Result:

LengthFieldPrepender will write the length field in the defined byte order.

codec/src/main/java/io/netty/handler/codec/LengthFieldPrepender.java

@@ -18,7 +18,9 @@ package io.netty.handler.codec;
 import io.netty.buffer.ByteBuf;
 import io.netty.channel.ChannelHandler.Sharable;
 import io.netty.channel.ChannelHandlerContext;
+import io.netty.util.internal.ObjectUtil;
 
+import java.nio.ByteOrder;
 
 /**
  * An encoder that prepends the length of the message.  The length value is
@@ -49,6 +51,7 @@ import io.netty.channel.ChannelHandlerContext;
 @Sharable
 public class LengthFieldPrepender extends MessageToByteEncoder<ByteBuf> {
 
+    private final ByteOrder byteOrder;
     private final int lengthFieldLength;
     private final boolean lengthIncludesLengthFieldLength;
     private final int lengthAdjustment;
@@ -114,6 +117,28 @@ public class LengthFieldPrepender extends MessageToByteEncoder<ByteBuf> {
      *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
      */
     public LengthFieldPrepender(int lengthFieldLength, int lengthAdjustment, boolean lengthIncludesLengthFieldLength) {
+        this(ByteOrder.BIG_ENDIAN, lengthFieldLength, lengthAdjustment, lengthIncludesLengthFieldLength);
+    }
+
+    /**
+     * Creates a new instance.
+     *
+     * @param byteOrder         the {@link ByteOrder} of the length field
+     * @param lengthFieldLength the length of the prepended length field.
+     *                          Only 1, 2, 3, 4, and 8 are allowed.
+     * @param lengthAdjustment  the compensation value to add to the value
+     *                          of the length field
+     * @param lengthIncludesLengthFieldLength
+     *                          if {@code true}, the length of the prepended
+     *                          length field is added to the value of the
+     *                          prepended length field.
+     *
+     * @throws IllegalArgumentException
+     *         if {@code lengthFieldLength} is not 1, 2, 3, 4, or 8
+     */
+    public LengthFieldPrepender(
+            ByteOrder byteOrder, int lengthFieldLength,
+            int lengthAdjustment, boolean lengthIncludesLengthFieldLength) {
         if (lengthFieldLength != 1 && lengthFieldLength != 2 &&
             lengthFieldLength != 3 && lengthFieldLength != 4 &&
             lengthFieldLength != 8) {
@@ -121,7 +146,9 @@ public class LengthFieldPrepender extends MessageToByteEncoder<ByteBuf> {
                     "lengthFieldLength must be either 1, 2, 3, 4, or 8: " +
                     lengthFieldLength);
         }
+        ObjectUtil.checkNotNull(byteOrder, "byteOrder");
 
+        this.byteOrder = byteOrder;
         this.lengthFieldLength = lengthFieldLength;
         this.lengthIncludesLengthFieldLength = lengthIncludesLengthFieldLength;
         this.lengthAdjustment = lengthAdjustment;
@@ -173,4 +200,9 @@ public class LengthFieldPrepender extends MessageToByteEncoder<ByteBuf> {
 
         out.writeBytes(msg, msg.readerIndex(), msg.readableBytes());
     }
+
+    @Override
+    protected ByteBuf allocateBuffer(ChannelHandlerContext ctx, ByteBuf msg, boolean preferDirect) throws Exception {
+        return super.allocateBuffer(ctx, msg, preferDirect).order(byteOrder);
+    }
 }

codec/src/test/java/io/netty/handler/codec/frame/LengthFieldPrependerTest.java

@@ -25,6 +25,8 @@ import org.junit.Test;
 
 import static io.netty.buffer.Unpooled.*;
 import static org.hamcrest.core.Is.*;
+import java.nio.ByteOrder;
+
 import static org.junit.Assert.*;
 
 public class LengthFieldPrependerTest {
@@ -73,4 +75,22 @@ public class LengthFieldPrependerTest {
             // Expected
         }
     }
+
+    @Test
+    public void testPrependLengthInLittleEndian() throws Exception {
+        final EmbeddedChannel ch = new EmbeddedChannel(new LengthFieldPrepender(ByteOrder.LITTLE_ENDIAN, 4, 0, false));
+        ch.writeOutbound(msg);
+        ByteBuf buf = (ByteBuf) ch.readOutbound();
+        assertEquals(5, buf.readableBytes());
+        byte[] writtenBytes = new byte[buf.readableBytes()];
+        buf.getBytes(0, writtenBytes);
+        assertEquals(1, writtenBytes[0]);
+        assertEquals(0, writtenBytes[1]);
+        assertEquals(0, writtenBytes[2]);
+        assertEquals(0, writtenBytes[3]);
+        assertEquals('A', writtenBytes[4]);
+        buf.release();
+        assertFalse("The channel must have been completely read", ch.finish());
+    }
+
 }