07.NIO之FileChannel练习

Leefs 2022-05-27 PM 840℃ 0条

> 将数据写入指定文件

```java
import lombok.extern.slf4j.Slf4j;

import java.io.File;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.StandardCharsets;
import java.nio.file.StandardOpenOption;

/**
 * Created by lilinchao
 * Date 2022/5/27
 * Description channel Demo
 */
@Slf4j
public class channelDemo01 {
    public static void main(String[] args) throws IOException {

// 获得一个根据指定文件路径的读写权限文件通道
        FileChannel fileChannel = FileChannel.open(new File("datas/data.txt").toPath(), StandardOpenOption.WRITE,StandardOpenOption.READ);
        //获得一段有指定内容的缓冲区
        ByteBuffer source = ByteBuffer.wrap("helloWorld,Scala,Java".getBytes(StandardCharsets.UTF_8));
        ByteBuffer target = ByteBuffer.allocate(50);
        log.info("fileChannel.position():{}",fileChannel.position());
        //将缓冲区中的内容写入文件通道
        fileChannel.write(source);
        //通道大小
        log.info("fileChannel.position():{}", fileChannel.position());
        //设置读写位置
        fileChannel.position(0);
        //将通道中的内容写到空缓冲区
        fileChannel.read(target);
        //转换缓冲区读写模式
        target.flip();
        log.info("target:{}", new String(target.array(), 0, target.limit()));
        //关闭资源
        fileChannel.close();
    }
}
```

> 数据读写

```java
import lombok.extern.slf4j.Slf4j;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.StandardCharsets;

/**
 * Created by lilinchao
 * Date 2022/5/27
 * Description 数据读写
 */
@Slf4j
public class channelDemo02 {
    public static void main(String[] args) throws IOException {
        //获取输出流
        FileOutputStream outputStream = new FileOutputStream("datas/data.txt");
        //根据输出流获得一个"写"权限的通道
        FileChannel outChannel = outputStream.getChannel();
        //获得一个有指定内容的缓冲区
        ByteBuffer source = ByteBuffer.wrap("HelloWorld".getBytes(StandardCharsets.UTF_8));
        //将缓冲区内容写入到通道
        outChannel.write(source);

//获取输入流
        FileInputStream fileInputStream = new FileInputStream("datas/data.txt");
        //根据输入流获得一个"读"权限的通道
        FileChannel inChannel = fileInputStream.getChannel();
        //获得一个空内容的缓冲区
        ByteBuffer target = ByteBuffer.allocate(50);
        //将通道中的内容读到缓冲区
        inChannel.read(target);
        //转换缓冲区读写模式
        target.flip();
        //读出缓冲区中的内容
        log.info("target:{}", new String(target.array(), 0, target.limit()));

//关闭资源
        outChannel.close();
        inChannel.close();
    }
}
```

> 超过 2g 大小的文件传输

```java
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.channels.FileChannel;

/**
 * Created by lilinchao
 * Date 2022/5/27
 * Description 超过 2g 大小的文件传输
 */
public class TestFileChannelTransferTo {
    public static void main(String[] args) {
        try (
                FileChannel from = new FileInputStream("datas/data.txt").getChannel();
                FileChannel to = new FileOutputStream("datas/to.txt").getChannel()
        ) {
            // 效率高，底层会利用操作系统的零拷贝进行优化
            long size = from.size();
            // left 变量代表还剩余多少字节
            for (long left = size; left > 0; ) {
                System.out.println("position:" + (size - left) + " left:" + left);
                left -= from.transferTo((size - left), left, to);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}
```

> FileChannel Demo 改变子缓冲区内容

```java
import java.io.File;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.IntBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.StandardCharsets;

/**
 * Created by lilinchao
 * Date 2022/5/27
 * Description FileChannel Demo
 */
public class TestFileChannelDemo {
    public static void main(String[] args) throws Exception {
//        testSlice();
//        testFileChannel();
//        testIntBuffer();
//        writeFileChannel();
        testFileChannelTransfer();
    }

/**
     * 切片:改变缓冲区的内容
     */
    public static void testSlice() throws Exception {

//获取通道
        RandomAccessFile accessFile = new RandomAccessFile(new File("datas/data.txt"),"rw");
        FileChannel channel = accessFile.getChannel();

//缓冲区
        ByteBuffer buffer = ByteBuffer.allocate(9);
        //将数据读入缓冲区
        channel.read(buffer);
        buffer.position(3);
        buffer.limit(7);

//用于创建一个新的字节缓冲区，其内容是给定缓冲区内容的共享子序列。
        ByteBuffer slice = buffer.slice();
        //改变子缓冲区内容
        for (int i = 0;i<slice.capacity();i++){
            byte b = slice.get(i);
            b *=2;
            slice.put(i,b);
        }
        buffer.position(0);
        buffer.limit(buffer.capacity());
        while (buffer.remaining() > 0){
            System.out.println(buffer.get());
        }
    }

/**
     * fileChannel读文件
     */
    public static void testFileChannel() throws Exception {

RandomAccessFile accessFile = new RandomAccessFile(new File("datas/data.txt"),"rw");
        FileChannel channel = accessFile.getChannel();
        //缓冲区
        ByteBuffer buffer = ByteBuffer.allocate(1024);
        //将数据读入缓冲区
        //返回的值表示读取到的字节数，如果读到了文件末尾，返回值为 -1
        int read = channel.read(buffer);
        while (read != -1){
            System.out.println("file长度:"+read);
            buffer.flip();
            while (buffer.hasRemaining()){
                System.out.println((char) buffer.get());
            }
            //切换到写模式,position=0,limit变为capacity
            buffer.clear();
            read = channel.read(buffer);
        }
        accessFile.close();
        System.out.println("end");
    }

/**
     * intBuffer
     */
    public static void testIntBuffer(){
        //用于在现有缓冲区旁边分配一个新的int缓冲区
        IntBuffer buffer = IntBuffer.allocate(1024);
        buffer.put(new int[]{1,2,3});
        buffer.flip();
        while (buffer.hasRemaining()){
            System.out.println(buffer.get());
        }
    }
    /**
     * fileChannel写文件
     */
    public static void writeFileChannel() throws Exception {
        RandomAccessFile rw = new RandomAccessFile("datas/data.txt","rw");
        FileChannel channel = rw.getChannel();
        ByteBuffer buffer = ByteBuffer.allocate(1024);
        buffer.put("欢迎来到李林超博客!!!".getBytes(StandardCharsets.UTF_8));
        buffer.flip();
        while (buffer.hasRemaining()){
            channel.write(buffer);
        }
        channel.close();
    }

//fileChannel通道之间传输
    public static void testFileChannelTransfer() throws Exception {
        RandomAccessFile accessFileA = new RandomAccessFile("datas/data.txt","rw");
        FileChannel channelFrom = accessFileA.getChannel();
        RandomAccessFile accessFileB = new RandomAccessFile("datas/dataB.txt","rw");
        FileChannel channelTo = accessFileB.getChannel();
        //from 的内容传输到 channelTo
        channelTo.transferFrom(channelFrom,0,channelFrom.size());
        accessFileA.close();
        accessFileB.close();
    }
}
```

*附参考文章链接：*

*https://blog.csdn.net/qq_38745846/article/details/120786800*

*https://juejin.cn/post/7030802468552474637*

标签: Netty, NIO

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