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## org.jboss.netty.handler.codec.frame
Class LengthFieldPrepender

    java.lang.Object
      org.jboss.netty.handler.codec.oneone.OneToOneEncoder
          org.jboss.netty.handler.codec.frame.LengthFieldPrepender

All Implemented Interfaces:
  ChannelDownstreamHandler, ChannelHandler

    @ChannelHandler.Sharable
    public class LengthFieldPrepender

extends OneToOneEncoder

An encoder that prepends the length of the message. The length value is prepended as a binary form. It is encoded in either big endian or little endian depending on the default `ByteOrder` of the current `ChannelBufferFactory`.

For example, ``LengthFieldPrepender`(2)` will encode the following 12-bytes string:

     +----------------+
     | "HELLO, WORLD" |
     +----------------+

into the following:

     +--------+----------------+
     + 0x000C | "HELLO, WORLD" |
     +--------+----------------+

If you turned on the `lengthIncludesLengthFieldLength` flag in the constructor, the encoded data would look like the following (12 (original data) + 2 (prepended data) = 14 (0xE)):

     +--------+----------------+
     + 0x000E | "HELLO, WORLD" |
     +--------+----------------+

Version:
  $Rev: 2121 $, $Date: 2010-02-02 09:38:07 +0900 (Tue, 02 Feb 2010) $
Author:
  The Netty Project, Trustin Lee

Nested Class Summary

Nested classes/interfaces inherited from interface org.jboss.netty.channel.ChannelHandler
`ChannelHandler.Sharable`

Constructor Summary
`LengthFieldPrepender(int lengthFieldLength)` Creates a new instance.
`LengthFieldPrepender(int lengthFieldLength, boolean lengthIncludesLengthFieldLength)` Creates a new instance.

Method Summary
`protected Object` | `encode(ChannelHandlerContext ctx, Channel channel, Object msg)` Transforms the specified message into another message and return the transformed message.

Methods inherited from class org.jboss.netty.handler.codec.oneone.OneToOneEncoder
`handleDownstream`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### LengthFieldPrepender

    public LengthFieldPrepender(int lengthFieldLength)

  Creates a new instance.

  Parameters:
    `lengthFieldLength` - the length of the prepended length field. Only 1, 2, 3, 4, and 8 are allowed.
  Throws:
    `IllegalArgumentException` - if `lengthFieldLength` is not 1, 2, 3, 4, or 8

### LengthFieldPrepender

    public LengthFieldPrepender(int lengthFieldLength,
                                boolean lengthIncludesLengthFieldLength)

  Creates a new instance.

  Parameters:
    `lengthFieldLength` - the length of the prepended length field. Only 1, 2, 3, 4, and 8 are allowed.
    `lengthIncludesLengthFieldLength` - if `true`, the length of the prepended length field is added to the value of the prepended length field.
  Throws:
    `IllegalArgumentException` - if `lengthFieldLength` is not 1, 2, 3, 4, or 8

Method Detail

### encode

    protected Object encode(ChannelHandlerContext ctx,
                            Channel channel,
                            Object msg)
                     throws Exception

  Description copied from class: `OneToOneEncoder`
  Transforms the specified message into another message and return the transformed message. Note that you can not return `null`, unlike you can in `OneToOneDecoder.decode(ChannelHandlerContext, Channel, Object)`; you must return something, at least `ChannelBuffers.EMPTY_BUFFER`.

  Specified by:
    `encode` in class `OneToOneEncoder`
  Throws:
    `Exception`

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