1、原生 ByteBuffer、ByteBuf 内部结构设计
首先我们来回顾一下 java.nio.ByteBuffe r的内部结构设计。
ByteBuffer 内部持有一个 byte[] bt, 再加上 position、limit、capacity、mark 四个属性。
- position
当前可用操作的位置，调用 get() 方法，返回 bt[postion] 处的值，如果是调用 put 方法,将数据放入 bt[position] 的位置。 - limit
可读或可写的最大索引。 - capacity
最大容量，ByteBuffer 对象一旦创建，不能修改 capacity 属性。 - mark
用来标记当前位置，以便可用回退到该位置，默认为 undefind。
下面主要举例说明一下 ByteBuffer 的存储。
ByteBuffer bf = ByteBuffer.alloct(10);
bf.put( (byte)"h" );
bf.put( (byte)"e" );
bf.put( (byte)"l" );
bf.put( (byte)"l" );
bf.put( (byte)"o" );将产生下图的结构：
那我们怎么从这个 ByteBuffer 中读取数据呢？如果调用 get 方法将会读出位置为5的数据，无法读取到 hello, 需要将 ByteBuffer 翻转，从写模式切换到读模式。
ByteBuffer 提供了一个方法 flip 方法，翻转缓存区，其如下如下：
bf.limit = bf.postion;
bf.positioin = 0;关于 ByteBuffer 更多知识，建议下载一本电子书（《java NIO 中文版》）
为了简化缓存区的读、写的操作， Netty ByteBuf 引入了两个位置属性，readIndex,writeIndex，来取代position。这样就不需要在读、写之间切换了。其结构设计如下：
- discardable bytes
已读的内容。 - readable bytes
暂未读的内容。 - writable bytes
区域是可以写区域。
discardable bytes 区域是可以使用 discardReadBytes() 方法，将 readables bytes 丢弃，然后将 readable bytes 移动到 byte 数组的开始处，这也相当于 ByteBuffer的compact()方法（压缩），但这会产生缓存区内部元素的移动，会降低性能。
由于引入了 writerIndex, limit 属性不需要了。
上述简单介绍了ByteBuffer、ByteBuf 数据的内部存储，这对我们正确使用缓存区至关重要。
2、ByteBuf类图与常用API学习
2.1 ByteBuf 类图
现阶段主要描述一下主要类的作用。
- AbstractReferenceCountedByteBuf
不难理解，ByteBuf，一个最最重要的方面就是内存的管理，涉及到创建、重复利用、回收等方方面面，而AbstractReferenceCountedByteBuf 就是使用引用计数来管理内存的回收。 - CompositeByteBuf ByteBuf
组合ByteBuf，比如一个协议分为协议头，协议主体，这两部分都是一个单独的 ByteBuf, 在处理这两个ByteBuf 时，然后需要将这个数据在网络上传输。我们的常规做法，是需要创建一个更大的 ByteBuf, 然后将 head,body 的内容写入新创建的ByteBuf, CompositeByteBuf 就是用来解决这个，提供一个类似数据库视图这样的概念。 - PooledByteBuf
池化的 ByteBuf, ByteBuf 对象池. - UnpooledDirectByteBuf
非池化的直接内存（不占用 JVM 的堆内存，也叫堆外内存)。 - UnpooledHeapByteBuf
非池化的堆内存（占用JVM堆内存）。
2.2 ByteBuf 主要 API
顺序读写API(read、write) 相关API，会移动对应的读写指针，而随机访问API(get、put)相关API并不会改变读写指针。
2.2.1 顺序读API
/**
* Gets a boolean at the current {@code readerIndex} and increases
* the {@code readerIndex} by {@code 1} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 1}
*/
public abstract boolean readBoolean();
/**
* Gets a byte at the current {@code readerIndex} and increases
* the {@code readerIndex} by {@code 1} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 1}
*/
public abstract byte readByte();
/**
* Gets an unsigned byte at the current {@code readerIndex} and increases
* the {@code readerIndex} by {@code 1} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 1}
*/
public abstract short readUnsignedByte();
/**
* Gets a 16-bit short integer at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 2} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 2}
*/
public abstract short readShort();
/**
* Gets an unsigned 16-bit short integer at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 2} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 2}
*/
public abstract int readUnsignedShort();
/**
* Gets a 24-bit medium integer at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 3} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 3}
*/
public abstract int readMedium();
/**
* Gets an unsigned 24-bit medium integer at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 3} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 3}
*/
public abstract int readUnsignedMedium();
/**
* Gets a 32-bit integer at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 4} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 4}
*/
public abstract int readInt();
/**
* Gets an unsigned 32-bit integer at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 4} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 4}
*/
public abstract long readUnsignedInt();
/**
* Gets a 64-bit integer at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 8} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 8}
*/
public abstract long readLong();
/**
* Gets a 2-byte UTF-16 character at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 2} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 2}
*/
public abstract char readChar();
/**
* Gets a 32-bit floating point number at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 4} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 4}
*/
public abstract float readFloat();
/**
* Gets a 64-bit floating point number at the current {@code readerIndex}
* and increases the {@code readerIndex} by {@code 8} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.readableBytes} is less than {@code 8}
*/
public abstract double readDouble();
/**
* Transfers this buffer's data to a newly created buffer starting at
* the current {@code readerIndex} and increases the {@code readerIndex}
* by the number of the transferred bytes (= {@code length}).
* The returned buffer's {@code readerIndex} and {@code writerIndex} are
* {@code 0} and {@code length} respectively.
*
* @param length the number of bytes to transfer
*
* @return the newly created buffer which contains the transferred bytes
*
* @throws IndexOutOfBoundsException
* if {@code length} is greater than {@code this.readableBytes}
*/
public abstract ByteBuf readBytes(int length);
/**
* Returns a new slice of this buffer's sub-region starting at the current
* {@code readerIndex} and increases the {@code readerIndex} by the size
* of the new slice (= {@code length}).
*
* @param length the size of the new slice
*
* @return the newly created slice
*
* @throws IndexOutOfBoundsException
* if {@code length} is greater than {@code this.readableBytes}
*/
public abstract ByteBuf readSlice(int length);
/**
* Transfers this buffer's data to the specified destination starting at
* the current {@code readerIndex} until the destination becomes
* non-writable, and increases the {@code readerIndex} by the number of the
* transferred bytes. This method is basically same with
* {@link #readBytes(ByteBuf, int, int)}, except that this method
* increases the {@code writerIndex} of the destination by the number of
* the transferred bytes while {@link #readBytes(ByteBuf, int, int)}
* does not.
*
* @throws IndexOutOfBoundsException
* if {@code dst.writableBytes} is greater than
* {@code this.readableBytes}
*/
public abstract ByteBuf readBytes(ByteBuf dst);
/**
* Transfers this buffer's data to the specified destination starting at
* the current {@code readerIndex} and increases the {@code readerIndex}
* by the number of the transferred bytes (= {@code length}). This method
* is basically same with {@link #readBytes(ByteBuf, int, int)},
* except that this method increases the {@code writerIndex} of the
* destination by the number of the transferred bytes (= {@code length})
* while {@link #readBytes(ByteBuf, int, int)} does not.
*
* @throws IndexOutOfBoundsException
* if {@code length} is greater than {@code this.readableBytes} or
* if {@code length} is greater than {@code dst.writableBytes}
*/
public abstract ByteBuf readBytes(ByteBuf dst, int length);
/**
* Transfers this buffer's data to the specified destination starting at
* the current {@code readerIndex} and increases the {@code readerIndex}
* by the number of the transferred bytes (= {@code length}).
*
* @param dstIndex the first index of the destination
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code dstIndex} is less than {@code 0},
* if {@code length} is greater than {@code this.readableBytes}, or
* if {@code dstIndex + length} is greater than
* {@code dst.capacity}
*/
public abstract ByteBuf readBytes(ByteBuf dst, int dstIndex, int length);
/**
* Transfers this buffer's data to the specified destination starting at
* the current {@code readerIndex} and increases the {@code readerIndex}
* by the number of the transferred bytes (= {@code dst.length}).
*
* @throws IndexOutOfBoundsException
* if {@code dst.length} is greater than {@code this.readableBytes}
*/
public abstract ByteBuf readBytes(byte[] dst);
/**
* Transfers this buffer's data to the specified destination starting at
* the current {@code readerIndex} and increases the {@code readerIndex}
* by the number of the transferred bytes (= {@code length}).
*
* @param dstIndex the first index of the destination
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code dstIndex} is less than {@code 0},
* if {@code length} is greater than {@code this.readableBytes}, or
* if {@code dstIndex + length} is greater than {@code dst.length}
*/
public abstract ByteBuf readBytes(byte[] dst, int dstIndex, int length);
/**
* Transfers this buffer's data to the specified destination starting at
* the current {@code readerIndex} until the destination's position
* reaches its limit, and increases the {@code readerIndex} by the
* number of the transferred bytes.
*
* @throws IndexOutOfBoundsException
* if {@code dst.remaining()} is greater than
* {@code this.readableBytes}
*/
public abstract ByteBuf readBytes(ByteBuffer dst);
/**
* Transfers this buffer's data to the specified stream starting at the
* current {@code readerIndex}.
*
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if {@code length} is greater than {@code this.readableBytes}
* @throws IOException
* if the specified stream threw an exception during I/O
*/
public abstract ByteBuf readBytes(OutputStream out, int length) throws IOException;
/**
* Transfers this buffer's data to the specified stream starting at the
* current {@code readerIndex}.
*
* @param length the maximum number of bytes to transfer
*
* @return the actual number of bytes written out to the specified channel
*
* @throws IndexOutOfBoundsException
* if {@code length} is greater than {@code this.readableBytes}
* @throws IOException
* if the specified channel threw an exception during I/O
*/
public abstract int readBytes(GatheringByteChannel out, int length) throws IOException;对于上面的方法，我挑如下三个进行重点说明一下，其他的类似：
- public abstract ByteBuf readBytes(byte[] dst)
从 ByteBuf 缓存区中读取 dst.length 个字节到 dst 目标数组中，返回当前的 ByteBuf, 这是级联设计风格，如果 ByteBuf 可剩余可读字节小于 dst.length，则抛出异常。 - public abstract ByteBuf readBytes(int length)
从 ByteBuf 缓存区读取 length 个字节到新创建的 ByteBuf, 注意返回的 ByteBuf 是先创建的，这不是级联设计风格。 - public abstract ByteBuf readBytes(ByteBuf dst)
从 ByteBuf 缓存区读取 ds t可写字节长度到 dst 中，返回的 ByteBuf 是级联风格设计，如果 dst 的可剩余写长度大于操作的缓存区时，将抛出异常。 - public abstract ByteBuf readSlice(int length)
从 ButeBuf 缓存区读取 length 个字节到 ByteBuf 中，这里的 ByteBuf 是原 ByteBuf 的视图，独立的 readIndex, writerIndex。
2.2.2 顺序写API
/**
* Sets the specified boolean at the current {@code writerIndex}
* and increases the {@code writerIndex} by {@code 1} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.writableBytes} is less than {@code 1}
*/
public abstract ByteBuf writeBoolean(boolean value);
/**
* Sets the specified byte at the current {@code writerIndex}
* and increases the {@code writerIndex} by {@code 1} in this buffer.
* The 24 high-order bits of the specified value are ignored.
*
* @throws IndexOutOfBoundsException
* if {@code this.writableBytes} is less than {@code 1}
*/
public abstract ByteBuf writeByte(int value);
/**
* Sets the specified 16-bit short integer at the current
* {@code writerIndex} and increases the {@code writerIndex} by {@code 2}
* in this buffer. The 16 high-order bits of the specified value are ignored.
*
* @throws IndexOutOfBoundsException
* if {@code this.writableBytes} is less than {@code 2}
*/
public abstract ByteBuf writeShort(int value);
/**
* Sets the specified 24-bit medium integer at the current
* {@code writerIndex} and increases the {@code writerIndex} by {@code 3}
* in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.writableBytes} is less than {@code 3}
*/
public abstract ByteBuf writeMedium(int value);
/**
* Sets the specified 32-bit integer at the current {@code writerIndex}
* and increases the {@code writerIndex} by {@code 4} in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.writableBytes} is less than {@code 4}
*/
public abstract ByteBuf writeInt(int value);
/**
* Sets the specified 64-bit long integer at the current
* {@code writerIndex} and increases the {@code writerIndex} by {@code 8}
* in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.writableBytes} is less than {@code 8}
*/
public abstract ByteBuf writeLong(long value);
/**
* Sets the specified 2-byte UTF-16 character at the current
* {@code writerIndex} and increases the {@code writerIndex} by {@code 2}
* in this buffer. The 16 high-order bits of the specified value are ignored.
*
* @throws IndexOutOfBoundsException
* if {@code this.writableBytes} is less than {@code 2}
*/
public abstract ByteBuf writeChar(int value);
/**
* Sets the specified 32-bit floating point number at the current
* {@code writerIndex} and increases the {@code writerIndex} by {@code 4}
* in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.writableBytes} is less than {@code 4}
*/
public abstract ByteBuf writeFloat(float value);
/**
* Sets the specified 64-bit floating point number at the current
* {@code writerIndex} and increases the {@code writerIndex} by {@code 8}
* in this buffer.
*
* @throws IndexOutOfBoundsException
* if {@code this.writableBytes} is less than {@code 8}
*/
public abstract ByteBuf writeDouble(double value);
/**
* Transfers the specified source buffer's data to this buffer starting at
* the current {@code writerIndex} until the source buffer becomes
* unreadable, and increases the {@code writerIndex} by the number of
* the transferred bytes. This method is basically same with
* {@link #writeBytes(ByteBuf, int, int)}, except that this method
* increases the {@code readerIndex} of the source buffer by the number of
* the transferred bytes while {@link #writeBytes(ByteBuf, int, int)}
* does not.
*
* @throws IndexOutOfBoundsException
* if {@code src.readableBytes} is greater than
* {@code this.writableBytes}
*/
public abstract ByteBuf writeBytes(ByteBuf src);
/**
* Transfers the specified source buffer's data to this buffer starting at
* the current {@code writerIndex} and increases the {@code writerIndex}
* by the number of the transferred bytes (= {@code length}). This method
* is basically same with {@link #writeBytes(ByteBuf, int, int)},
* except that this method increases the {@code readerIndex} of the source
* buffer by the number of the transferred bytes (= {@code length}) while
* {@link #writeBytes(ByteBuf, int, int)} does not.
*
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if {@code length} is greater than {@code this.writableBytes} or
* if {@code length} is greater then {@code src.readableBytes}
*/
public abstract ByteBuf writeBytes(ByteBuf src, int length);
/**
* Transfers the specified source buffer's data to this buffer starting at
* the current {@code writerIndex} and increases the {@code writerIndex}
* by the number of the transferred bytes (= {@code length}).
*
* @param srcIndex the first index of the source
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code srcIndex} is less than {@code 0},
* if {@code srcIndex + length} is greater than
* {@code src.capacity}, or
* if {@code length} is greater than {@code this.writableBytes}
*/
public abstract ByteBuf writeBytes(ByteBuf src, int srcIndex, int length);
/**
* Transfers the specified source array's data to this buffer starting at
* the current {@code writerIndex} and increases the {@code writerIndex}
* by the number of the transferred bytes (= {@code src.length}).
*
* @throws IndexOutOfBoundsException
* if {@code src.length} is greater than {@code this.writableBytes}
*/
public abstract ByteBuf writeBytes(byte[] src);
/**
* Transfers the specified source array's data to this buffer starting at
* the current {@code writerIndex} and increases the {@code writerIndex}
* by the number of the transferred bytes (= {@code length}).
*
* @param srcIndex the first index of the source
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code srcIndex} is less than {@code 0},
* if {@code srcIndex + length} is greater than
* {@code src.length}, or
* if {@code length} is greater than {@code this.writableBytes}
*/
public abstract ByteBuf writeBytes(byte[] src, int srcIndex, int length);
/**
* Transfers the specified source buffer's data to this buffer starting at
* the current {@code writerIndex} until the source buffer's position
* reaches its limit, and increases the {@code writerIndex} by the
* number of the transferred bytes.
*
* @throws IndexOutOfBoundsException
* if {@code src.remaining()} is greater than
* {@code this.writableBytes}
*/
public abstract ByteBuf writeBytes(ByteBuffer src);
/**
* Transfers the content of the specified stream to this buffer
* starting at the current {@code writerIndex} and increases the
* {@code writerIndex} by the number of the transferred bytes.
*
* @param length the number of bytes to transfer
*
* @return the actual number of bytes read in from the specified stream
*
* @throws IndexOutOfBoundsException
* if {@code length} is greater than {@code this.writableBytes}
* @throws IOException
* if the specified stream threw an exception during I/O
*/
public abstract int writeBytes(InputStream in, int length) throws IOException;
/**
* Transfers the content of the specified channel to this buffer
* starting at the current {@code writerIndex} and increases the
* {@code writerIndex} by the number of the transferred bytes.
*
* @param length the maximum number of bytes to transfer
*
* @return the actual number of bytes read in from the specified channel
*
* @throws IndexOutOfBoundsException
* if {@code length} is greater than {@code this.writableBytes}
* @throws IOException
* if the specified channel threw an exception during I/O
*/
public abstract int writeBytes(ScatteringByteChannel in, int length) throws IOException;
/**
* Fills this buffer with <tt>NUL (0x00)</tt> starting at the current
* {@code writerIndex} and increases the {@code writerIndex} by the
* specified {@code length}.
*
* @param length the number of <tt>NUL</tt>s to write to the buffer
*
* @throws IndexOutOfBoundsException
* if {@code length} is greater than {@code this.writableBytes}
*/
public abstract ByteBuf writeZero(int length);重点强调一下如下方法，其他的类似。
- public abstract ByteBuf writeBytes(byte[] src)
将字节数组 src 全部写入到缓存区中，如果 src.length 大于 ByteBuf 可写区域的话，会抛出异常。
2.2.3 随机读API
/**
* Gets a boolean at the specified absolute (@code index) in this buffer.
* This method does not modify the {@code readerIndex} or {@code writerIndex}
* of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 1} is greater than {@code this.capacity}
*/
public abstract boolean getBoolean(int index);
/**
* Gets a byte at the specified absolute {@code index} in this buffer.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 1} is greater than {@code this.capacity}
*/
public abstract byte getByte(int index);
/**
* Gets an unsigned byte at the specified absolute {@code index} in this
* buffer. This method does not modify {@code readerIndex} or
* {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 1} is greater than {@code this.capacity}
*/
public abstract short getUnsignedByte(int index);
/**
* Gets a 16-bit short integer at the specified absolute {@code index} in
* this buffer. This method does not modify {@code readerIndex} or
* {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 2} is greater than {@code this.capacity}
*/
public abstract short getShort(int index);
/**
* Gets an unsigned 16-bit short integer at the specified absolute
* {@code index} in this buffer. This method does not modify
* {@code readerIndex} or {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 2} is greater than {@code this.capacity}
*/
public abstract int getUnsignedShort(int index);
/**
* Gets a 24-bit medium integer at the specified absolute {@code index} in
* this buffer. This method does not modify {@code readerIndex} or
* {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 3} is greater than {@code this.capacity}
*/
public abstract int getMedium(int index);
/**
* Gets an unsigned 24-bit medium integer at the specified absolute
* {@code index} in this buffer. This method does not modify
* {@code readerIndex} or {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 3} is greater than {@code this.capacity}
*/
public abstract int getUnsignedMedium(int index);
/**
* Gets a 32-bit integer at the specified absolute {@code index} in
* this buffer. This method does not modify {@code readerIndex} or
* {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 4} is greater than {@code this.capacity}
*/
public abstract int getInt(int index);
/**
* Gets an unsigned 32-bit integer at the specified absolute {@code index}
* in this buffer. This method does not modify {@code readerIndex} or
* {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 4} is greater than {@code this.capacity}
*/
public abstract long getUnsignedInt(int index);
/**
* Gets a 64-bit long integer at the specified absolute {@code index} in
* this buffer. This method does not modify {@code readerIndex} or
* {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 8} is greater than {@code this.capacity}
*/
public abstract long getLong(int index);
/**
* Gets a 2-byte UTF-16 character at the specified absolute
* {@code index} in this buffer. This method does not modify
* {@code readerIndex} or {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 2} is greater than {@code this.capacity}
*/
public abstract char getChar(int index);
/**
* Gets a 32-bit floating point number at the specified absolute
* {@code index} in this buffer. This method does not modify
* {@code readerIndex} or {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 4} is greater than {@code this.capacity}
*/
public abstract float getFloat(int index);
/**
* Gets a 64-bit floating point number at the specified absolute
* {@code index} in this buffer. This method does not modify
* {@code readerIndex} or {@code writerIndex} of this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 8} is greater than {@code this.capacity}
*/
public abstract double getDouble(int index);
/**
* Transfers this buffer's data to the specified destination starting at
* the specified absolute {@code index} until the destination becomes
* non-writable. This method is basically same with
* {@link #getBytes(int, ByteBuf, int, int)}, except that this
* method increases the {@code writerIndex} of the destination by the
* number of the transferred bytes while
* {@link #getBytes(int, ByteBuf, int, int)} does not.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* the source buffer (i.e. {@code this}).
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + dst.writableBytes} is greater than
* {@code this.capacity}
*/
public abstract ByteBuf getBytes(int index, ByteBuf dst);
/**
* Transfers this buffer's data to the specified destination starting at
* the specified absolute {@code index}. This method is basically same
* with {@link #getBytes(int, ByteBuf, int, int)}, except that this
* method increases the {@code writerIndex} of the destination by the
* number of the transferred bytes while
* {@link #getBytes(int, ByteBuf, int, int)} does not.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* the source buffer (i.e. {@code this}).
*
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0},
* if {@code index + length} is greater than
* {@code this.capacity}, or
* if {@code length} is greater than {@code dst.writableBytes}
*/
public abstract ByteBuf getBytes(int index, ByteBuf dst, int length);
/**
* Transfers this buffer's data to the specified destination starting at
* the specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex}
* of both the source (i.e. {@code this}) and the destination.
*
* @param dstIndex the first index of the destination
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0},
* if the specified {@code dstIndex} is less than {@code 0},
* if {@code index + length} is greater than
* {@code this.capacity}, or
* if {@code dstIndex + length} is greater than
* {@code dst.capacity}
*/
public abstract ByteBuf getBytes(int index, ByteBuf dst, int dstIndex, int length);
/**
* Transfers this buffer's data to the specified destination starting at
* the specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + dst.length} is greater than
* {@code this.capacity}
*/
public abstract ByteBuf getBytes(int index, byte[] dst);
/**
* Transfers this buffer's data to the specified destination starting at
* the specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex}
* of this buffer.
*
* @param dstIndex the first index of the destination
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0},
* if the specified {@code dstIndex} is less than {@code 0},
* if {@code index + length} is greater than
* {@code this.capacity}, or
* if {@code dstIndex + length} is greater than
* {@code dst.length}
*/
public abstract ByteBuf getBytes(int index, byte[] dst, int dstIndex, int length);
/**
* Transfers this buffer's data to the specified destination starting at
* the specified absolute {@code index} until the destination's position
* reaches its limit.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer while the destination's {@code position} will be increased.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + dst.remaining()} is greater than
* {@code this.capacity}
*/
public abstract ByteBuf getBytes(int index, ByteBuffer dst);
/**
* Transfers this buffer's data to the specified stream starting at the
* specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + length} is greater than
* {@code this.capacity}
* @throws IOException
* if the specified stream threw an exception during I/O
*/
public abstract ByteBuf getBytes(int index, OutputStream out, int length) throws IOException;
/**
* Transfers this buffer's data to the specified channel starting at the
* specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @param length the maximum number of bytes to transfer
*
* @return the actual number of bytes written out to the specified channel
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + length} is greater than
* {@code this.capacity}
* @throws IOException
* if the specified channel threw an exception during I/O
*/
public abstract int getBytes(int index, GatheringByteChannel out, int length) throws IOException;重点解读如下API，其他类似。
- public abstract ByteBuf getBytes(int index, byte[] dst, int dstIndex, int length)
从源 ByteBuf 的 index 处开始读 length 个字节到目标 byte 中，从 dstIndex 开始。
2.2.4 随机写API
/**
* Sets the specified boolean at the specified absolute {@code index} in this
* buffer.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 1} is greater than {@code this.capacity}
*/
public abstract ByteBuf setBoolean(int index, boolean value);
/**
* Sets the specified byte at the specified absolute {@code index} in this
* buffer. The 24 high-order bits of the specified value are ignored.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 1} is greater than {@code this.capacity}
*/
public abstract ByteBuf setByte(int index, int value);
/**
* Sets the specified 16-bit short integer at the specified absolute
* {@code index} in this buffer. The 16 high-order bits of the specified
* value are ignored.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 2} is greater than {@code this.capacity}
*/
public abstract ByteBuf setShort(int index, int value);
/**
* Sets the specified 24-bit medium integer at the specified absolute
* {@code index} in this buffer. Please note that the most significant
* byte is ignored in the specified value.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 3} is greater than {@code this.capacity}
*/
public abstract ByteBuf setMedium(int index, int value);
/**
* Sets the specified 32-bit integer at the specified absolute
* {@code index} in this buffer.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 4} is greater than {@code this.capacity}
*/
public abstract ByteBuf setInt(int index, int value);
/**
* Sets the specified 64-bit long integer at the specified absolute
* {@code index} in this buffer.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 8} is greater than {@code this.capacity}
*/
public abstract ByteBuf setLong(int index, long value);
/**
* Sets the specified 2-byte UTF-16 character at the specified absolute
* {@code index} in this buffer.
* The 16 high-order bits of the specified value are ignored.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 2} is greater than {@code this.capacity}
*/
public abstract ByteBuf setChar(int index, int value);
/**
* Sets the specified 32-bit floating-point number at the specified
* absolute {@code index} in this buffer.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 4} is greater than {@code this.capacity}
*/
public abstract ByteBuf setFloat(int index, float value);
/**
* Sets the specified 64-bit floating-point number at the specified
* absolute {@code index} in this buffer.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* {@code index + 8} is greater than {@code this.capacity}
*/
public abstract ByteBuf setDouble(int index, double value);
/**
* Transfers the specified source buffer's data to this buffer starting at
* the specified absolute {@code index} until the source buffer becomes
* unreadable. This method is basically same with
* {@link #setBytes(int, ByteBuf, int, int)}, except that this
* method increases the {@code readerIndex} of the source buffer by
* the number of the transferred bytes while
* {@link #setBytes(int, ByteBuf, int, int)} does not.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* the source buffer (i.e. {@code this}).
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + src.readableBytes} is greater than
* {@code this.capacity}
*/
public abstract ByteBuf setBytes(int index, ByteBuf src);
/**
* Transfers the specified source buffer's data to this buffer starting at
* the specified absolute {@code index}. This method is basically same
* with {@link #setBytes(int, ByteBuf, int, int)}, except that this
* method increases the {@code readerIndex} of the source buffer by
* the number of the transferred bytes while
* {@link #setBytes(int, ByteBuf, int, int)} does not.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* the source buffer (i.e. {@code this}).
*
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0},
* if {@code index + length} is greater than
* {@code this.capacity}, or
* if {@code length} is greater than {@code src.readableBytes}
*/
public abstract ByteBuf setBytes(int index, ByteBuf src, int length);
/**
* Transfers the specified source buffer's data to this buffer starting at
* the specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex}
* of both the source (i.e. {@code this}) and the destination.
*
* @param srcIndex the first index of the source
* @param length the number of bytes to transfer
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0},
* if the specified {@code srcIndex} is less than {@code 0},
* if {@code index + length} is greater than
* {@code this.capacity}, or
* if {@code srcIndex + length} is greater than
* {@code src.capacity}
*/
public abstract ByteBuf setBytes(int index, ByteBuf src, int srcIndex, int length);
/**
* Transfers the specified source array's data to this buffer starting at
* the specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + src.length} is greater than
* {@code this.capacity}
*/
public abstract ByteBuf setBytes(int index, byte[] src);
/**
* Transfers the specified source array's data to this buffer starting at
* the specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0},
* if the specified {@code srcIndex} is less than {@code 0},
* if {@code index + length} is greater than
* {@code this.capacity}, or
* if {@code srcIndex + length} is greater than {@code src.length}
*/
public abstract ByteBuf setBytes(int index, byte[] src, int srcIndex, int length);
/**
* Transfers the specified source buffer's data to this buffer starting at
* the specified absolute {@code index} until the source buffer's position
* reaches its limit.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + src.remaining()} is greater than
* {@code this.capacity}
*/
public abstract ByteBuf setBytes(int index, ByteBuffer src);
/**
* Transfers the content of the specified source stream to this buffer
* starting at the specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @param length the number of bytes to transfer
*
* @return the actual number of bytes read in from the specified channel.
* {@code -1} if the specified channel is closed.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + length} is greater than {@code this.capacity}
* @throws IOException
* if the specified stream threw an exception during I/O
*/
public abstract int setBytes(int index, InputStream in, int length) throws IOException;
/**
* Transfers the content of the specified source channel to this buffer
* starting at the specified absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @param length the maximum number of bytes to transfer
*
* @return the actual number of bytes read in from the specified channel.
* {@code -1} if the specified channel is closed.
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + length} is greater than {@code this.capacity}
* @throws IOException
* if the specified channel threw an exception during I/O
*/
public abstract int setBytes(int index, ScatteringByteChannel in, int length) throws IOException;
/**
* Fills this buffer with <tt>NUL (0x00)</tt> starting at the specified
* absolute {@code index}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*
* @param length the number of <tt>NUL</tt>s to write to the buffer
*
* @throws IndexOutOfBoundsException
* if the specified {@code index} is less than {@code 0} or
* if {@code index + length} is greater than {@code this.capacity}
*/
public abstract ByteBuf setZero(int index, int length);
1.2.2.5 ByteBuf复制相关API
/**
* Returns a copy of this buffer's readable bytes. Modifying the content
* of the returned buffer or this buffer does not affect each other at all.
* This method is identical to {@code buf.copy(buf.readerIndex(), buf.readableBytes())}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*/
public abstract ByteBuf copy();
/**
* Returns a copy of this buffer's sub-region. Modifying the content of
* the returned buffer or this buffer does not affect each other at all.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*/
public abstract ByteBuf copy(int index, int length);
/**
* Returns a slice of this buffer's readable bytes. Modifying the content
* of the returned buffer or this buffer affects each other's content
* while they maintain separate indexes and marks. This method is
* identical to {@code buf.slice(buf.readerIndex(), buf.readableBytes())}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*/
public abstract ByteBuf slice();
/**
* Returns a slice of this buffer's sub-region. Modifying the content of
* the returned buffer or this buffer affects each other's content while
* they maintain separate indexes and marks.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*/
public abstract ByteBuf slice(int index, int length);
/**
* Returns a buffer which shares the whole region of this buffer.
* Modifying the content of the returned buffer or this buffer affects
* each other's content while they maintain separate indexes and marks.
* This method is identical to {@code buf.slice(0, buf.capacity())}.
* This method does not modify {@code readerIndex} or {@code writerIndex} of
* this buffer.
*/
public abstract ByteBuf duplicate();需要说明的是，slice、duplicate 方法，返回的 ByteBuf 与原共享数据结构，只是单独维护 readIndex,writerIndex。copy 方法会创建不同的 byte[] 数组，相互之间不会影响。
2.2.7 ByteBuf mark,reset,clear 的 API
/**
* Sets the {@code readerIndex} and {@code writerIndex} of this buffer to
* {@code 0}.
* This method is identical to {@link #setIndex(int, int) setIndex(0, 0)}.
* <p>
* Please note that the behavior of this method is different
* from that of NIO buffer, which sets the {@code limit} to
* the {@code capacity} of the buffer.
*/
public abstract ByteBuf clear();
/**
* Marks the current {@code readerIndex} in this buffer. You can
* reposition the current {@code readerIndex} to the marked
* {@code readerIndex} by calling {@link #resetReaderIndex()}.
* The initial value of the marked {@code readerIndex} is {@code 0}.
*/
public abstract ByteBuf markReaderIndex();
/**
* Repositions the current {@code readerIndex} to the marked
* {@code readerIndex} in this buffer.
*
* @throws IndexOutOfBoundsException
* if the current {@code writerIndex} is less than the marked
* {@code readerIndex}
*/
public abstract ByteBuf resetReaderIndex();
/**
* Marks the current {@code writerIndex} in this buffer. You can
* reposition the current {@code writerIndex} to the marked
* {@code writerIndex} by calling {@link #resetWriterIndex()}.
* The initial value of the marked {@code writerIndex} is {@code 0}.
*/
public abstract ByteBuf markWriterIndex();
/**
* Repositions the current {@code writerIndex} to the marked
* {@code writerIndex} in this buffer.
*
* @throws IndexOutOfBoundsException
* if the current {@code readerIndex} is greater than the marked
* {@code writerIndex}
*/
public abstract ByteBuf resetWriterIndex();
/**
* Discards the bytes between the 0th index and {@code readerIndex}.
* It moves the bytes between {@code readerIndex} and {@code writerIndex}
* to the 0th index, and sets {@code readerIndex} and {@code writerIndex}
* to {@code 0} and {@code oldWriterIndex - oldReaderIndex} respectively.
* <p>
* Please refer to the class documentation for more detailed explanation.
*/
public abstract ByteBuf discardReadBytes();
/**
* Similar to {@link ByteBuf#discardReadBytes()} except that this method might discard
* some, all, or none of read bytes depending on its internal implementation to reduce
* overall memory bandwidth consumption at the cost of potentially additional memory
* consumption.
*/
public abstract ByteBuf discardSomeReadBytes();2.2.8 ByteBuf 与 java nio 原生 ByteBuffer 转换
/**
* Returns the maximum number of NIO {@link ByteBuffer}s that consist this buffer. Note that {@link #nioBuffers()}
* or {@link #nioBuffers(int, int)} might return a less number of {@link ByteBuffer}s.
*
* @return {@code -1} if this buffer has no underlying {@link ByteBuffer}.
* the number of the underlying {@link ByteBuffer}s if this buffer has at least one underlying
* {@link ByteBuffer}. Note that this method does not return {@code 0} to avoid confusion.
*
* @see #nioBuffer()
* @see #nioBuffer(int, int)
* @see #nioBuffers()
* @see #nioBuffers(int, int)
*/
public abstract int nioBufferCount();
/**
* Exposes this buffer's readable bytes as an NIO {@link ByteBuffer}. The returned buffer
* shares the content with this buffer, while changing the position and limit of the returned
* NIO buffer does not affect the indexes and marks of this buffer. This method is identical
* to {@code buf.nioBuffer(buf.readerIndex(), buf.readableBytes())}. This method does not
* modify {@code readerIndex} or {@code writerIndex} of this buffer. Please note that the
* returned NIO buffer will not see the changes of this buffer if this buffer is a dynamic
* buffer and it adjusted its capacity.
*
* @throws UnsupportedOperationException
* if this buffer cannot create a {@link ByteBuffer} that shares the content with itself
*
* @see #nioBufferCount()
* @see #nioBuffers()
* @see #nioBuffers(int, int)
*/
public abstract ByteBuffer nioBuffer();
/**
* Exposes this buffer's sub-region as an NIO {@link ByteBuffer}. The returned buffer
* shares the content with this buffer, while changing the position and limit of the returned
* NIO buffer does not affect the indexes and marks of this buffer. This method does not
* modify {@code readerIndex} or {@code writerIndex} of this buffer. Please note that the
* returned NIO buffer will not see the changes of this buffer if this buffer is a dynamic
* buffer and it adjusted its capacity.
*
* @throws UnsupportedOperationException
* if this buffer cannot create a {@link ByteBuffer} that shares the content with itself
*
* @see #nioBufferCount()
* @see #nioBuffers()
* @see #nioBuffers(int, int)
*/
public abstract ByteBuffer nioBuffer(int index, int length);
/**
* Internal use only: Exposes the internal NIO buffer.
*/
public abstract ByteBuffer internalNioBuffer(int index, int length);
/**
* Exposes this buffer's readable bytes as an NIO {@link ByteBuffer}'s. The returned buffer
* shares the content with this buffer, while changing the position and limit of the returned
* NIO buffer does not affect the indexes and marks of this buffer. This method does not
* modify {@code readerIndex} or {@code writerIndex} of this buffer. Please note that the
* returned NIO buffer will not see the changes of this buffer if this buffer is a dynamic
* buffer and it adjusted its capacity.
*
*
* @throws UnsupportedOperationException
* if this buffer cannot create a {@link ByteBuffer} that shares the content with itself
*
* @see #nioBufferCount()
* @see #nioBuffer()
* @see #nioBuffer(int, int)
*/
public abstract ByteBuffer[] nioBuffers();
/**
* Exposes this buffer's bytes as an NIO {@link ByteBuffer}'s for the specified index and length
* The returned buffer shares the content with this buffer, while changing the position and limit
* of the returned NIO buffer does not affect the indexes and marks of this buffer. This method does
* not modify {@code readerIndex} or {@code writerIndex} of this buffer. Please note that the
* returned NIO buffer will not see the changes of this buffer if this buffer is a dynamic
* buffer and it adjusted its capacity.
*
* @throws UnsupportedOperationException
* if this buffer cannot create a {@link ByteBuffer} that shares the content with itself
*
* @see #nioBufferCount()
* @see #nioBuffer()
* @see #nioBuffer(int, int)
*/
public abstract ByteBuffer[] nioBuffers(int index, int length);看了这么多的分类的API,貌似没有看到创建相关的API,比如java.nio.ByteBuffer的 allocateDirect(int capacity) 等方法。原因是Netty 对 ByteBuf 的创建单独抽象成一个体系 ByteBufAllocator，包括了直接内存、堆内存，池化的 ByteBuf，这些将在源码分析ByteBuf 相关类时重点去学习。
来源：https://blog.csdn.net/prestigeding/article/details/53977445
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