ChannelBuffer介绍介绍

A random and sequential accessible sequence of zero or more bytes (octets). This interface provides an abstract view for one or more primitive byte arrays (byte[] ) and  NIO buffers .

Creation of a buffer

It is recommended to create a new buffer using the helper methods in  ChannelBuffers   rather than calling an individual implementation's constructor.

Random Access Indexing

Just like an ordinary primitive byte array,  ChannelBuffer   uses  zero-based indexing . It means the index of the first byte is always  0   and the index of the last byte is always  capacity - 1 . For example, to iterate all bytes of a buffer, you can do the following, regardless of its internal implementation:

 ChannelBuffer buffer = ...;
 for (int i = 0; i < buffer.capacity(); i ++) {
     byte b = array.getByte(i);
     System.out.println((char) b);
 }
 

Sequential Access Indexing

ChannelBuffer   provides two pointer variables to support sequential read and write operations -  readerIndex   for a read operation and  writerIndex   for a write operation respectively. The following diagram shows how a buffer is segmented into three areas by the two pointers:

      +-------------------+------------------+------------------+
      | discardable bytes |  readable bytes  |  writable bytes  |
      |                   |     (CONTENT)    |                  |
      +-------------------+------------------+------------------+
      |                   |                  |                  |
      0      <=      readerIndex   <=   writerIndex    <=    capacity
 

Readable bytes (the actual content)

This segment is where the actual data is stored. Any operation whose name starts with  read   or  skip   will get or skip the data at the current  readerIndex   and increase it by the number of read bytes. If the argument of the read operation is also a  ChannelBuffer   and no destination index is specified, the specified buffer's  readerIndex   is increased together.

If there's not enough content left,  IndexOutOfBoundsException   is raised. The default value of newly allocated, wrapped or copied buffer's  readerIndex   is  0 .

 // Iterates the readable bytes of a buffer.
 ChannelBuffer buffer = ...;
 while (buffer.readable()) {
     System.out.println(buffer.readByte());
 }
 

Writable bytes

This segment is a undefined space which needs to be filled. Any operation whose name ends with  write   will write the data at the current  writerIndex   and increase it by the number of written bytes. If the argument of the write operation is also a  ChannelBuffer , and no source index is specified, the specified buffer's  readerIndex   is increased together.

If there's not enough writable bytes left,  IndexOutOfBoundsException   is raised. The default value of newly allocated buffer's  writerIndex   is  0 . The default value of wrapped or copied buffer's  writerIndex   is the  capacity   of the buffer.

 // Fills the writable bytes of a buffer with random integers.
 ChannelBuffer buffer = ...;
 while (buffer.writableBytes() >= 4) {
     buffer.writeInt(random.nextInt());
 }
 

Discardable bytes

This segment contains the bytes which were read already by a read operation. Initially, the size of this segment is  0 , but its size increases up to the  writerIndex   as read operations are executed. The read bytes can be discarded by callingdiscardReadBytes()   to reclaim unused area as depicted by the following diagram:

  BEFORE discardReadBytes()

      +-------------------+------------------+------------------+
      | discardable bytes |  readable bytes  |  writable bytes  |
      +-------------------+------------------+------------------+
      |                   |                  |                  |
      0      <=      readerIndex   <=   writerIndex    <=    capacity


  AFTER discardReadBytes()

      +------------------+--------------------------------------+
      |  readable bytes  |    writable bytes (got more space)   |
      +------------------+--------------------------------------+
      |                  |                                      |
 readerIndex (0) <= writerIndex (decreased)        <=        capacity
 

Please note that there is no guarantee about the content of writable bytes after calling  discardReadBytes() . The writable bytes will not be moved in most cases and could even be filled with completely different data depending on the underlying buffer implementation.

Clearing the buffer indexes

 You can set both  readerIndex   and  writerIndex   to  0   by calling  clear() . It does not clear the buffer content (e.g. filling with  0 ) but just clears the two pointers. Please also note that the semantic of this operation is different fromByteBuffer.clear() .

  BEFORE clear()

      +-------------------+------------------+------------------+
      | discardable bytes |  readable bytes  |  writable bytes  |
      +-------------------+------------------+------------------+
      |                   |                  |                  |
      0      <=      readerIndex   <=   writerIndex    <=    capacity


  AFTER clear()

      +---------------------------------------------------------+
      |             writable bytes (got more space)             |
      +---------------------------------------------------------+
      |                                                         |
      0 = readerIndex = writerIndex            <=            capacity
 

Search operations

Various  indexOf()   methods help you locate an index of a value which meets a certain criteria. Complicated dynamic sequential search can be done with  ChannelBufferIndexFinder   as well as simple static single byte search.

Mark and reset

There are two marker indexes in every buffer. One is for storing  readerIndex   and the other is for storing  writerIndex . You can always reposition one of the two indexes by calling a reset method. It works in a similar fashion to the mark and reset methods in  InputStream   except that there's no  readlimit .

Derived buffers

You can create a view of an existing buffer by calling either  duplicate() ,  slice()   or  slice(int, int) . A derived buffer will have an independent  readerIndex ,  writerIndex   and marker indexes, while it shares other internal data representation, just like a NIO buffer does.

In case a completely fresh copy of an existing buffer is required, please call  copy()   method instead.

Conversion to existing JDK types

NIO Buffers

Various  toByteBuffer()   and  toByteBuffers()   methods convert a  ChannelBuffer   into one or more NIO buffers. These methods avoid buffer allocation and memory copy whenever possible, but there's no guarantee that memory copy will not be involved or that an explicit memory copy will be involved.

Strings

Various  toString(String)   methods convert a  ChannelBuffer   into a  String . Please note that  toString()   is not a conversion method.

I/O Streams

Please refer to  ChannelBufferInputStream   and  ChannelBufferOutputStream .