Netty--ChannelBuffer

 

org.jboss.netty.buffer

Interface ChannelBuffer

• All Superinterfaces:
  Comparable< ChannelBuffer>
  All Known Subinterfaces:
  WrappedChannelBuffer
  All Known Implementing Classes:
  AbstractChannelBuffer,  BigEndianHeapChannelBuffer,  ByteBufferBackedChannelBuffer,  CompositeChannelBuffer,  DuplicatedChannelBuffer,  DynamicChannelBuffer,  HeapChannelBuffer, LittleEndianHeapChannelBuffer,  ReadOnlyChannelBuffer,  SlicedChannelBuffer,  TruncatedChannelBuffer

  ---

  
  

  public interface ChannelBuffer
  extends Comparable<ChannelBuffer>

  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 = buffer.getByte(i);
       System.out.println((char) b);
   }
   

  Sequential Access Indexing

  ChannelBuffer 由3部分组成，discardable 已经读了的，readable 还可读 ，writable还可写的

  readerIndex是读指针，writerIndex写指针

  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 剩余多少可以写的

  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 已经读了多少 

  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 calling  discardReadBytes() to reclaim unused area as depicted by the following diagram:

    BEFORE discardReadBytes()
  
        +-------------------+------------------+------------------+
        | discardable bytes |  readable bytes  |  writable bytes  |
        +-------------------+------------------+------------------+
        |                   |                  |                  |
        0      <=      readerIndex   <=   writerIndex    <=    capacity
  
  
    AFTER discardReadBytes() 释放已经读了部分
  
        +------------------+--------------------------------------+
        |  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 from  Buffer.clear().

    BEFORE clear()
  
        +-------------------+------------------+------------------+
        | discardable bytes |  readable bytes  |  writable bytes  |
        +-------------------+------------------+------------------+
        |                   |                  |                  |
        0      <=      readerIndex   <=   writerIndex    <=    capacity
  
  
    AFTER clear() 把readerIndex和writerIndex =0
  
        +---------------------------------------------------------+
        |             writable bytes (got more space)             |
        +---------------------------------------------------------+
        |                                                         |
        0 = readerIndex = writerIndex            <=            capacity
   

  Search operations

  Various  indexOf(int, int, byte) 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.

  If you are decoding variable length data such as NUL-terminated string, you will find bytesBefore(byte) also useful.

  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

  Byte array

  If a  ChannelBuffer is backed by a byte array (i.e.  byte[]), you can access it directly via the  array() method. To determine if a buffer is backed by a byte array,  hasArray() should be used.

  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.

  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.

Mark和Reset，可以和InputStream一样使用，但是有两个指针；

markReaderIndex() // 标记
markWriterIndex() // 
resetReaderIndex() // 回到标记
resetWriterIndex()