Java NIO: Buffer
目录

classic Java I/O(java.io) backwards:

  • do not scale well when moving large amounts of data
  • lack of some common I/O features such as
  • file locking
  • nonblocking I/O
  • readiness selection
  • memory mapping

Java NIO(java.nio), JSR 51, JDK1.4 2002

  • Base: Buffers
  • Channels
  • Selectors

Buffer

抽象类java.nio.Buffer代表了一段数据,主要5个数据成员:

  • capacity: 元素最大容量,一但初使化就不能改变
  • limit: 有效元素尾端
  • position: 下一个待读写的位置索引,get()set()自动修改
  • mark: 标记的索引,mark()使得mark=position,reset()使得position=mark

0 <= mark <= position <= limit <= capacity

Buffer不是线程安全的类。

几个常用操作:

  • get(), put(E): position++;
  • flip(): limit=position,position=0,mark=-1;
  • compact(): 将[position, limit)区间内的元素拷贝到起始位置[0, limit-position),经常和flip()组合使用
  • rewind(): position=0,mark=-1;
  • remaining(): 返回limit-position;
  • clear(): limit=capacity, position=0,mark=-1;

clear() -> channel读入 -> flip() flip() -> channel写出 -> clear()

eight buffer classes

These are all abstract classes:

  • IntBuffer
  • DoubleBuffer
  • ShortBuffer
  • LongBuffer
  • FloatBuffer
  • ByteBuffer
  • CharBuffer
  • MappedByteBuffer: like mmap()

But these are contain factory method(allocate(int capacity )) to create new instance. Or use wrap(T[]) to create new instance with backing array.

Nondirect buffers have backing arrays. It can be used by hasArray() and array().

Duplicating Buffers

  • duplicate(): 创建Buffer,共享(0, capacity)部份数组
  • slice(): 创建Buffer,共享(position, limit)部份数组

Byte Buffers

ByteBuffer byte order is BIG_ENDIAN. Only ByteBuffer can be Direct Buffers. Direct Buffer创建昂贵(jni),但是对于I/O操作十分高效。

使用ByteBuffer.allocateDirect()创建Direct Buffer。

Tips

  • 在Java中,字符串使用Unicode表示,因此每个Unicode字符占16bits。 所以Java中char类型是2个字节:
  • byte 1B
  • char 2B
  • short 2B
  • int 4B
  • float 4B
  • long 8B
  • double 8B
  • boolean Java规范未规定,JVM规范规定boolean映射成int,boolean数组映射成byte数组。

Reference

  • Java NIO book

发表评论