Java NIO系列教程（七） FileChannel

短命女 2023-10-17 12:32 51阅读 0赞

本文转载至：[http://ifeve.com/file-channel/][http_ifeve.com_file-channel]

Java NIO中的FileChannel是一个连接到文件的通道。可以通过文件通道读写文件。

FileChannel无法设置为非阻塞模式，它总是运行在阻塞模式下。

#### 打开FileChannel ####

在使用FileChannel之前，必须先打开它。但是，我们无法直接打开一个FileChannel，需要通过使用一个InputStream、OutputStream或RandomAccessFile来获取一个FileChannel实例。下面是通过RandomAccessFile打开FileChannel的示例：

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#### 从FileChannel读取数据 ####

调用多个read()方法之一从FileChannel中读取数据。如：

首先，分配一个Buffer。从FileChannel中读取的数据将被读到Buffer中。

然后，调用FileChannel.read()方法。该方法将数据从FileChannel读取到Buffer中。read()方法返回的int值表示了有多少字节被读到了Buffer中。如果返回-1，表示到了文件末尾。

#### 向FileChannel写数据 ####

使用FileChannel.write()方法向FileChannel写数据，该方法的参数是一个Buffer。如：

注意FileChannel.write()是在while循环中调用的。因为无法保证write()方法一次能向FileChannel写入多少字节，因此需要重复调用write()方法，直到Buffer中已经没有尚未写入通道的字节。

#### 关闭FileChannel ####

用完FileChannel后必须将其关闭。如：

#### FileChannel的position方法 ####

有时可能需要在FileChannel的某个特定位置进行数据的读/写操作。可以通过调用position()方法获取FileChannel的当前位置。

也可以通过调用position(long pos)方法设置FileChannel的当前位置。

这里有两个例子:

如果将位置设置在文件结束符之后，然后试图从文件通道中读取数据，读方法将返回-1 —— 文件结束标志。

如果将位置设置在文件结束符之后，然后向通道中写数据，文件将撑大到当前位置并写入数据。这可能导致“文件空洞”，磁盘上物理文件中写入的数据间有空隙。

#### FileChannel的size方法 ####

FileChannel实例的size()方法将返回该实例所关联文件的大小。如:

#### FileChannel的truncate方法 ####

可以使用FileChannel.truncate()方法截取一个文件。截取文件时，文件将中指定长度后面的部分将被删除。如：

这个例子截取文件的前1024个字节。

#### FileChannel的force方法 ####

FileChannel.force()方法将通道里尚未写入磁盘的数据强制写到磁盘上。出于性能方面的考虑，操作系统会将数据缓存在内存中，所以无法保证写入到FileChannel里的数据一定会即时写到磁盘上。要保证这一点，需要调用force()方法。

force()方法有一个boolean类型的参数，指明是否同时将文件元数据（权限信息等）写到磁盘上。

下面的例子同时将文件数据和元数据强制写到磁盘上：

[查看源代码][Link 1]

[打印][Link 2] [帮助][Link 3]

java 完整示例代码：

package com.zzg.nio.file;
    
    import java.io.RandomAccessFile;
    import java.nio.ByteBuffer;
    import java.nio.channels.FileChannel;
    
    public class FileChannelUtil {
    	public static void main(String[] args){
    		try{
    			RandomAccessFile file_out = new RandomAccessFile("D:\\in_one.txt","rw");
    			FileChannel out = file_out.getChannel();
    			//读取数据
    			ByteBuffer buffer = ByteBuffer.allocate(32);
    			int size = out.read(buffer);
    			//读写切换
    			buffer.flip();
    			
    			RandomAccessFile file_in = new RandomAccessFile("D:\\in_two.txt","rw");
    			FileChannel in = file_in.getChannel();
    			//写入数据
    			in.write(buffer);
    			
    			buffer.clear();
    		}catch(Exception e){
    			System.out.println("错误信息:"+e.getMessage());
    		}
    	}
    
    }

[http_ifeve.com_file-channel]: http://ifeve.com/file-channel/
[Link 1]: http://ifeve.com/file-channel/#viewSource
[Link 2]: http://ifeve.com/file-channel/#printSource
[Link 3]: http://ifeve.com/file-channel/#about