Java NIO详解(一): FileChannel

本文主要是介绍Java NIO详解(一): FileChannel，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

概述

FileChannel是用来读，写，映射, 操作文件的channel(概念参见我的另一篇Java NIO概述).

A channel for reading, writing, mapping, and manipulating a file.

获取FileChannel对象有三种方式:

FileInputStream.getChannel();
FileOutputStream.getChannel();
RandomAccessFile.getChannel();

FileChannel特点

不能注册到selector
只能阻塞的运行(就是channel的read方法是阻塞的)
FileChannel对象是线程安全的

上代码

public static void testFileNio(){FileInputStream is = null;FileChannel inChannel = null;FileOutputStream fos = null;FileChannel outChannel = null;try {is = new FileInputStream("f:/text.txt");inChannel = is.getChannel();System.out.println("file size --->"+inChannel.size());ByteBuffer buffer = ByteBuffer.allocate(2);System.out.println("buffer init position --->"+buffer.position()+"---- buffer init remaining --->"+buffer.remaining());//这里标记了后面才可以调用buffer.reset(), 而且只能调用一次,//不然会抛出java.nio.InvalidMarkException//buffer.mark();int flag = 0;int i = 1;while(flag != -1){System.out.println("this is "+i+" time");//返回读取的字节数, 当读到末尾时, 可能返回0 或者 -1flag = inChannel.read(buffer);if(flag == -1 || flag == 0){break;}System.out.println("buffer after read , position-->"+buffer.position()+"---remaining-->"+buffer.remaining());//写数据fos = new FileOutputStream(new File("f:/text2.txt"),true);outChannel = fos.getChannel();if(buffer.remaining() > "this is the end".getBytes().length ){buffer.put("this is the end".getBytes());}System.out.println("buffer after append--position-->"+buffer.position()+"---remaining-->"+buffer.remaining()+"--- flag --->"+flag);//开始写, remaining也变成了当前的position,然后将position置为0buffer.flip();System.out.println("buffer after flip  position-->"+buffer.position()+"---remaining-->"+buffer.remaining());while(buffer.hasRemaining()){outChannel.write(buffer);}System.out.println("buffer after write,  position-->"+buffer.position()+"---remaining-->"+buffer.remaining());//和buffer.reset()的区别, buffer.reset()是回到buffer.mark()的位置//此处不能调用buffer.flip(),;//clear方法会将position置0, limit置为capacity, 也就是remaining是capacity的值         buffer.clear();System.out.println("buffer clear position --->"+buffer.position()+"---- buffer  clear remaining --->"+buffer.remaining());i++;}} catch (IOException e) {e.printStackTrace();}finally{//本来想省略以下代码的, 但是还是忍不住唠叨几句//1.空指针判断, 这个应该没啥//2. 为什么不是所有的close操作都写在一个catch块了,而是写的这么冗长// 原因是如果一个close操作抛出异常, 其他close操作依然可以执行; 如果写在一个catch块，第一个close抛出异常的话,  其他close操作都将无法进行if(is != null){try {is.close();} catch (IOException e) {e.printStackTrace();}}if(inChannel != null){try{inChannel.close();}catch (IOException e){e.printStackTrace();}}if(fos != null){try{fos.close();}catch (IOException e){e.printStackTrace();}}if(outChannel != null){try{outChannel.close();}catch(IOException e){e.printStackTrace();}}}
}

//输出结果如下
file size --->10
buffer init position --->0---- buffer init remaining --->2
this is 1 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip  position-->0---remaining-->2
buffer after write,  position-->2---remaining-->0
buffer clear position --->0---- buffer  clear remaining --->2
this is 2 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip  position-->0---remaining-->2
buffer after write,  position-->2---remaining-->0
buffer clear position --->0---- buffer  clear remaining --->2
this is 3 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip  position-->0---remaining-->2
buffer after write,  position-->2---remaining-->0
buffer clear position --->0---- buffer  clear remaining --->2
this is 4 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip  position-->0---remaining-->2
buffer after write,  position-->2---remaining-->0
buffer clear position --->0---- buffer  clear remaining --->2
this is 5 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip  position-->0---remaining-->2
buffer after write,  position-->2---remaining-->0
buffer clear position --->0---- buffer  clear remaining --->2
this is 6 time

问题

既然FileChannel只能阻塞的运行, 不能注册到selector，那么与FileInputStream，FileOutputStream, RandomAccessFile有何区别呢？

答案：其实在Java SE API文档里已经给出答案了,如下:

In addition to the familiar read, write, and close operations of byte channels, this class defines the following file-specific
operations:

Bytes may be read or written at an absolute position in a file in a
way that does not affect the channel’s current position.

A region of a file may be mapped directly into memory; for large files
this is often much more efficient than invoking the usual read or
write methods.

Updates made to a file may be forced out to the underlying storage
device, ensuring that data are not lost in the event of a system
crash.

Bytes can be transferred from a file to some other channel, and vice
versa, in a way that can be optimized by many operating systems into a
very fast transfer directly to or from the filesystem cache.

A region of a file may be locked against access by other programs.