java如何进行文件下载呢?

java下载文件的实现思路:
    使用java.net.URL和java.nio包
	  及使用Apache Commons IO库
	  
使用方法大全：
 **使用 `java.net.URL` 和 `java.io`**：适用于简单的文件下载。
 **使用 `java.nio`**：提供更现代和高效的文件操作方式。
 **使用 Apache Commons IO**：提供简洁的文件操作方法，适合快速开发。
 **使用 `java.net.HttpURLConnection`**：适用于需要处理复杂 HTTP 请求的场景
 

方法一：使用 `java.net.URL` 和 `java.io`

import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.URL;

public class FileDownloader {
    public static void downloadFile(String fileURL, String saveDir) {
        try {
            URL url = new URL(fileURL);
            BufferedInputStream bis = new BufferedInputStream(url.openStream());
            FileOutputStream fis = new FileOutputStream(saveDir);

            byte[] buffer = new byte[1024];
            int count;
            while ((count = bis.read(buffer, 0, 1024)) != -1) {
                fis.write(buffer, 0, count);
            }

            fis.close();
            bis.close();
            System.out.println("File downloaded successfully");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        String fileURL = "https://example.com/sample.pdf";
        String saveDir = "downloaded_sample.pdf";
        downloadFile(fileURL, saveDir);
    }
}

方法二：使用`java.nio`

`java.nio` 包提供了更现代和高效的文件操作方式

import java.io.IOException;
import java.net.URL;
import java.nio.channels.Channels;
import java.nio.channels.ReadableByteChannel;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class FileDownloaderNIO {
    public static void downloadFile(String fileURL, String saveDir) {
        try (ReadableByteChannel rbc = Channels.newChannel(new URL(fileURL).openStream());
             FileOutputStream fos = new FileOutputStream(saveDir)) {
            fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
            System.out.println("File downloaded successfully");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        String fileURL = "https://example.com/sample.pdf";
        String saveDir = "downloaded_sample_nio.pdf";
        downloadFile(fileURL, saveDir);
    }
}

方法三:使用Apache Commons IO

Apache Commons IO 库提供了更简洁的文件操作方法。

引入依赖

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.11.0</version>
</dependency>


import org.apache.commons.io.FileUtils;
import java.io.File;
import java.io.IOException;
import java.net.URL;

public class FileDownloaderApacheIO {
    public static void downloadFile(String fileURL, String saveDir) {
        try {
            URL url = new URL(fileURL);
            File destination = new File(saveDir);
            FileUtils.copyURLToFile(url, destination);
            System.out.println("File downloaded successfully");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        String fileURL = "https://java265.com/sample.pdf";
        String saveDir = "downloaded_sample_apacheio.pdf";
        downloadFile(fileURL, saveDir);
    }
}

方法四：使用 `java.net.HttpURLConnection`

import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;

public class FileDownloaderHttpURLConnection {
    public static void downloadFile(String fileURL, String saveDir) {
        try {
            URL url = new URL(fileURL);
            HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
            int responseCode = httpConn.getResponseCode();

            // 检查响应码是否为 HTTP_OK
            if (responseCode == HttpURLConnection.HTTP_OK) {
                String fileName = "";
                String disposition = httpConn.getHeaderField("Content-Disposition");
                String contentType = httpConn.getContentType();
                int contentLength = httpConn.getContentLength();

                if (disposition != null) {
                    // 从 Content-Disposition 头中提取文件名
                    int index = disposition.indexOf("filename=");
                    if (index > 0) {
                        fileName = disposition.substring(index + 10, disposition.length() - 1);
                    }
                } else {
                    // 如果没有 Content-Disposition 头，则从 URL 中提取文件名
                    fileName = fileURL.substring(fileURL.lastIndexOf("/") + 1, fileURL.length());
                }

                System.out.println("Content-Type = " + contentType);
                System.out.println("Content-Disposition = " + disposition);
                System.out.println("Content-Length = " + contentLength);
                System.out.println("fileName = " + fileName);

                // 打开输入流
                BufferedInputStream inputStream = new BufferedInputStream(httpConn.getInputStream());
                FileOutputStream outputStream = new FileOutputStream(saveDir);

                byte[] buffer = new byte[4096];
                int bytesRead = -1;
                while ((bytesRead = inputStream.read(buffer)) != -1) {
                    outputStream.write(buffer, 0, bytesRead);
                }

                outputStream.close();
                inputStream.close();

                System.out.println("File downloaded");
            } else {
                System.out.println("No file to download. Server replied HTTP code: " + responseCode);
            }
            httpConn.disconnect();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        String fileURL = "https://example.com/sample.pdf";
        String saveDir = "downloaded_sample_httpurlconnection.pdf";
        downloadFile(fileURL, saveDir);
    }
}