Netty 4.0 源码分析（四）：ByteBuf

Netty是基于流的消息传递机制。Netty框架中，所有消息的传输都依赖于ByteBuf接口，ByteBuf是Netty NIO框架中的缓冲区。ByteBuf接口可以理解为一般的Byte数组，不过Netty对Byte进行了封装，增加了一些实用的方法。

ChannelBuf接口
package io.netty.buffer;
public interface ChannelBuf {
ChannelBufType type();
boolean isPooled();
}

ByteBuf接口
package io.netty.buffer;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.GatheringByteChannel;
import java.nio.channels.ScatteringByteChannel;
import java.nio.charset.Charset;
import java.nio.charset.UnsupportedCharsetException;
public interface ByteBuf extends ChannelBuf, Comparable<ByteBuf> {
int capacity();
ByteBuf capacity(int newCapacity);
int maxCapacity();
ByteOrder order();
ByteBuf order(ByteOrder endianness);
boolean isDirect();
int readerIndex();
ByteBuf readerIndex(int readerIndex);
int writerIndex();
ByteBuf writerIndex(int writerIndex);
ByteBuf setIndex(int readerIndex, int writerIndex);
int readableBytes();
int writableBytes();
boolean readable();
boolean writable();
ByteBuf clear();
ByteBuf markReaderIndex();
ByteBuf resetReaderIndex();
ByteBuf markWriterIndex();
ByteBuf resetWriterIndex();
ByteBuf discardReadBytes();
ByteBuf ensureWritableBytes(int minWritableBytes);
int ensureWritableBytes(int minWritableBytes, boolean force);
boolean getBoolean(int index);
byte getByte(int index);
short getUnsignedByte(int index);
short getShort(int index);
int getUnsignedShort(int index);
int getMedium(int index);
int getUnsignedMedium(int index);
int getInt(int index);
long getUnsignedInt(int index);
long getLong(int index);
char getChar(int index);
float getFloat(int index);
double getDouble(int index);
ByteBuf getBytes(int index, ByteBuf dst);
ByteBuf getBytes(int index, ByteBuf dst, int length);
ByteBuf getBytes(int index, ByteBuf dst, int dstIndex, int length);
ByteBuf getBytes(int index, byte[] dst);
ByteBuf getBytes(int index, byte[] dst, int dstIndex, int length);
ByteBuf getBytes(int index, ByteBuffer dst);
ByteBuf getBytes(int index, OutputStream out, int length) throws IOException;
int getBytes(int index, GatheringByteChannel out, int length) throws IOException;
ByteBuf setBoolean(int index, boolean value);
ByteBuf setByte(int index, int value);
ByteBuf setShort(int index, int value);
ByteBuf setMedium(int index, int value);
ByteBuf setInt(int index, int value);
ByteBuf setLong(int index, long value);
ByteBuf setChar(int index, int value);
ByteBuf setFloat(int index, float value);
ByteBuf setDouble(int index, double value);
ByteBuf setBytes(int index, ByteBuf src);
ByteBuf setBytes(int index, ByteBuf src, int length);
ByteBuf setBytes(int index, ByteBuf src, int srcIndex, int length);
ByteBuf setBytes(int index, byte[] src);
ByteBuf setBytes(int index, byte[] src, int srcIndex, int length);
ByteBuf setBytes(int index, ByteBuffer src);
int setBytes(int index, InputStream in, int length) throws IOException;
int setBytes(int index, ScatteringByteChannel in, int length) throws IOException;
ByteBuf setZero(int index, int length);
boolean readBoolean();
byte readByte();
short readUnsignedByte();
short readShort();
int readUnsignedShort();
int readMedium();
int readUnsignedMedium();
int readInt();
long readUnsignedInt();
long readLong();
char readChar();
float readFloat();
double readDouble();
ByteBuf readBytes(int length);
ByteBuf readSlice(int length);
ByteBuf readBytes(ByteBuf dst);
ByteBuf readBytes(ByteBuf dst, int length);
ByteBuf readBytes(ByteBuf dst, int dstIndex, int length);
ByteBuf readBytes(byte[] dst);
ByteBuf readBytes(byte[] dst, int dstIndex, int length);
ByteBuf readBytes(ByteBuffer dst);
ByteBuf readBytes(OutputStream out, int length) throws IOException;
int readBytes(GatheringByteChannel out, int length) throws IOException;
ByteBuf skipBytes(int length);
ByteBuf writeBoolean(boolean value);
ByteBuf writeByte(int value);
ByteBuf writeShort(int value);
ByteBuf writeMedium(int value);
ByteBuf writeInt(int value);
ByteBuf writeLong(long value);
ByteBuf writeChar(int value);
ByteBuf writeFloat(float value);
ByteBuf writeDouble(double value);
ByteBuf writeBytes(ByteBuf src);
ByteBuf writeBytes(ByteBuf src, int length);
ByteBuf writeBytes(ByteBuf src, int srcIndex, int length);
ByteBuf writeBytes(byte[] src);
ByteBuf writeBytes(byte[] src, int srcIndex, int length);
ByteBuf writeBytes(ByteBuffer src);
int writeBytes(InputStream in, int length) throws IOException;
int writeBytes(ScatteringByteChannel in, int length) throws IOException;
ByteBuf writeZero(int length);
int indexOf(int fromIndex, int toIndex, byte value);
int indexOf(int fromIndex, int toIndex, ByteBufIndexFinder indexFinder);
int bytesBefore(byte value);
int bytesBefore(ByteBufIndexFinder indexFinder);
int bytesBefore(int length, byte value);
int bytesBefore(int length, ByteBufIndexFinder indexFinder);
int bytesBefore(int index, int length, byte value);
int bytesBefore(int index, int length, ByteBufIndexFinder indexFinder);
ByteBuf copy();
ByteBuf copy(int index, int length);
ByteBuf slice();
ByteBuf slice(int index, int length);
ByteBuf duplicate();
boolean hasNioBuffer();
ByteBuffer nioBuffer();
ByteBuffer nioBuffer(int index, int length);
boolean hasNioBuffers();
ByteBuffer[] nioBuffers();
ByteBuffer[] nioBuffers(int offset, int length);
boolean hasArray();
byte[] array();
int arrayOffset();
String toString(Charset charset);
String toString(int index, int length, Charset charset);
@Override
int hashCode();
@Override
boolean equals(Object obj);
@Override
int compareTo(ByteBuf buffer);
@Override
String toString();
Unsafe unsafe();
interface Unsafe {
ByteBuffer nioBuffer();
ByteBuffer[] nioBuffers();
ByteBuf newBuffer(int initialCapacity);
void discardSomeReadBytes();
void acquire();
void release();
}
}



ByteBuf Index

ByteBuf通过两个指针来协助I/O的读写操作，读操作的readIndex和写操作的writeIndex
readerIndex和writerIndex都是一开始都是0，随着数据的写入writerIndex会增加，读取数据会使readerIndex增加，但是他不会超过writerIndx，在读取之后，0-readerIndex的就被视为discard的.调用discardReadBytes方法,可以释放这部分空间,他的作用类似ByeBuffer的compact方法;
读和写的时候Index是分开的，因此也就没必要再每次读完以后调用flip方法，另外还有indexOf、bytesBefore等一些方便的方法；

ByteBuf的几个重要方法
discardReadBytes()
丢弃已读的内容。其执行过程如下：
调用discardReadBytes()之前：



调用 discardReadBytes()方法后



clear()
丢弃所有的数据，并将readerIndex和writerIndex重置为0。

调用clear()之前



调用clear()之后