【知识点】Java Nio实现文件下载

package com.onezero.web.service.impl;

import java.io.*;
import java.net.URL;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.FileChannel;
import java.nio.channels.ReadableByteChannel;

public class fileUtils {

    public static void downFileToTarget(String origFile,String targetFile){
        FileInputStream fis = null;
        FileOutputStream fos = null;

        FileChannel inChannel = null;
        FileChannel outChannel = null;

        try {
            //创建输入流 将源文件读取为流
            fis = new FileInputStream(origFile);
            //创建输出流 将流输入到目标文件
            fos = new FileOutputStream(targetFile);
            //获取通道
            inChannel = fis.getChannel();
            outChannel = fos.getChannel();
            //分配缓冲区 单位kb
            ByteBuffer buffer = ByteBuffer.allocate(1024);

            //将通道中数据存入缓冲区
            while(inChannel.read(buffer)!=-1){
                //Flips this buffer. The limit is set to the current position and then the position is set to zero. If the mark is defined then it is discarded
                //将缓存字节数组的指针设置为数组的开始序列即数组下标0。这样就可以从buffer开头，对该buffer进行遍历读取,读出当前缓冲区中的数据，不是按照缓冲区容量而是内容长度
                buffer.flip();
                //将缓冲区数据写入到输出通道中
                outChannel.write(buffer);
                //写完数据清除缓冲区
                buffer.clear();
            }

        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if(fis!=null){
                try {
                    fis.close();
                }catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(fos != null){
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(inChannel != null){
                try {
                    inChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(outChannel != null){
                try {
                    outChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    public static void downloadByNIO(String url, String filePath, String fileName) {
        FileOutputStream fos = null;

        ReadableByteChannel inChannel = null;
        FileChannel outChannel = null;
        try {
            //获取数据流
            InputStream in = new URL(url).openStream();
            //创建输入流通道
            inChannel = Channels.newChannel(in);

            String targetFile=filePath+fileName;
            fos = new FileOutputStream(targetFile);

            outChannel = fos.getChannel();
            //分配缓冲区 单位kb
            ByteBuffer buffer = ByteBuffer.allocate(1024);

            //将通道中数据存入缓冲区
            while(inChannel.read(buffer)!=-1){
                //Flips this buffer. The limit is set to the current position and then the position is set to zero. If the mark is defined then it is discarded
                //将缓存字节数组的指针设置为数组的开始序列即数组下标0。这样就可以从buffer开头，对该buffer进行遍历读取,读出当前缓冲区中的数据，不是按照缓冲区容量而是内容长度
                buffer.flip();
                //将缓冲区数据写入到输出通道中
                outChannel.write(buffer);
                //写完数据清除缓冲区
                buffer.clear();
            }

        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            if(fos != null){
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(inChannel != null){
                try {
                    inChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(outChannel != null){
                try {
                    outChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    public static void main(String[] args) {
        String origFile = "D:/java-download-001.pdf";
        String targetFile = "D:/1.pdf";
        downFileToTarget(origFile,targetFile);
        String url = "https://www.onezero.cc/wp-content/uploads/2022/02/java-download-001.pdf";
        String filePath = "D:/";
        String fileName = "2.pdf";
        downloadByNIO(url,filePath,fileName);
    }
}