2、java.nio、ByteBuffer的简单使用

//基本概念
limit 对缓存区的读写操作以limit值为上限
position 当前游标位置(也就是当前操作的位置)
capacity 缓存区容量(一般和limit一样)

常用方法

//分配缓存区大小
public static ByteBuffer allocate(int capacity) {
        if (capacity < 0)
            throw createCapacityException(capacity);
        return new HeapByteBuffer(capacity, capacity);
    }

//写入内容
public ByteBuffer put(byte[] src, int offset, int length) {
        checkBounds(offset, length, src.length);
        if (length > remaining())
            throw new BufferOverflowException();
        int end = offset + length;
        for (int i = offset; i < end; i++)
            this.put(src[i]);
        return this;
    }

//读取内容
  public ByteBuffer get(byte[] dst, int offset, int length) {
        checkBounds(offset, length, dst.length);
        if (length > remaining())
            throw new BufferUnderflowException();
        int end = offset + length;
        for (int i = offset; i < end; i++)
            dst[i] = get();
        return this;
    }

//作用转换为读模式，操作上限修改为当前操作的位置,然后游标置0,
//这样就可以从开头读取
public Buffer flip() {
        limit = position;
        position = 0;
        mark = -1;
        return this;
    }

//重置当前读写位置，清除所有内容
public Buffer clear() {
        position = 0;
        limit = capacity;
        mark = -1;
        return this;

//重置当前游标不清除内容,一般配合flip多次读取操作
public Buffer rewind() {
        position = 0;
        mark = -1;
        return this;

//在当前游标位置设置标记
public Buffer mark() {
        mark = position;
        return this;
    }

//将游标的位置设置到上一次mark标记的位置
public Buffer reset() {
        int m = mark;
        if (m < 0)
            throw new InvalidMarkException();
        position = m;
        return this;
    }

public static void main(String[] args) {
    ByteBuffer buff = ByteBuffer.allocate(1024);
    // 写入数据
    buff.put("hello".getBytes());
    System.out.println(
        "当前游标: " + buff.position() + " 当前读写上限: " + buff.limit() + " 当前容量:" + buff.capacity());
    // 设置当前读写上限为当前游标也就是limit=position 这样就不会读的时候分配的1024字节全部读
    buff.flip();
    System.out.println(
        "当前游标: " + buff.position() + " 当前读写上限: " + buff.limit() + " 当前容量:" + buff.capacity());
    // 创建当前缓冲区上限的字节数组
    byte[] ibuff = new byte[buff.limit()];
    // 读入到数组
    buff.get(ibuff);
    // 转换成字符串
    String s = new String(ibuff);
    System.out.println(s);
    // 重置缓存区
    buff.clear();
  }