Java文件下载实战指南——常用方法及技巧详解-不念博客

1. 简介

Java下载文件是一个常见的任务，通常用于从服务器获取文件，或者在客户端与服务端之间传输文件。

本文将介绍如何使用Java下载文件，包括常用的Java库和下载方法。

2. 使用java.net.URL类下载文件

java.net.URL类是Java标准库中的一个类，可以用来从网络上下载文件。

下面是一个简单的示例：

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

public class FileDownload {
    public static void main(String[] args) {
        String fileUrl = "https://example.com/file.zip";
        String outputFileName = "downloaded_file.zip";
        
        try (BufferedInputStream inputStream = new BufferedInputStream(new URL(fileUrl).openStream());
             FileOutputStream fileOutputStream = new FileOutputStream(outputFileName)) {
            byte[] dataBuffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = inputStream.read(dataBuffer, 0, 1024)) != -1) {
                fileOutputStream.write(dataBuffer, 0, bytesRead);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

3. 使用Apache HttpClient下载文件

Apache HttpClient是一个流行的Java HTTP客户端库，可以用于下载文件。

以下是使用Apache HttpClient下载文件的示例：

import org.apache.http.HttpEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

public class FileDownload {
    public static void main(String[] args) {
        String fileUrl = "https://example.com/file.zip";
        String outputFileName = "downloaded_file.zip";

        try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
            HttpGet httpGet = new HttpGet(fileUrl);
            try (CloseableHttpResponse response = httpClient.execute(httpGet)) {
                HttpEntity entity = response.getEntity();
                if (entity != null) {
                    try (InputStream inputStream = entity.getContent();
                         FileOutputStream fileOutputStream = new FileOutputStream(outputFileName)) {
                        int bytesRead;
                        byte[] buffer = new byte[1024];
                        while ((bytesRead = inputStream.read(buffer)) != -1) {
                            fileOutputStream.write(buffer, 0, bytesRead);
                        }
                    }
                }
                EntityUtils.consume(entity);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

4. 使用Java NIO下载文件

Java NIO（New I/O）提供了一种更高效的I/O处理方式，可以用于下载文件。

以下是使用Java NIO下载文件的示例：

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;
import java.nio.file.StandardOpenOption;

public class FileDownload {
    public static void main(String[] args) {
        String fileUrl = "https://example.com/file.zip";
        String outputFileName = "downloaded_file.zip";

        try (ReadableByteChannel readableByteChannel = Channels.newChannel(new URL(fileUrl).openStream())) {
            Path outputFile = Paths.get(outputFileName);
            Files.createFile(outputFile);
            try (FileChannel fileChannel = FileChannel.open(outputFile, StandardOpenOption.WRITE)) {
                fileChannel.transferFrom(readableByteChannel, 0, Long.MAX_VALUE);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

5. 使用OkHttp下载文件

OkHttp是一个现代、高效的HTTP客户端库，可以用于下载文件。

以下是使用OkHttp下载文件的示例：

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

public class FileDownload {
    public static void main(String[] args) {
        String fileUrl = "https://example.com/file.zip";
        String outputFileName = "downloaded_file.zip";

        OkHttpClient client = new OkHttpClient();
        Request request = new Request.Builder().url(fileUrl).build();

        try (Response response = client.newCall(request).execute()) {
            if (response.isSuccessful()) {
                try (InputStream inputStream = response.body().byteStream();
                     FileOutputStream fileOutputStream = new FileOutputStream(outputFileName)) {
                    byte[] buffer = new byte[1024];
                    int bytesRead;
                    while ((bytesRead = inputStream.read(buffer)) != -1) {
                        fileOutputStream.write(buffer, 0, bytesRead);
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}