Java 实现文件下载

使用HttpClient下载

阻塞方式下载

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import java.io.IOException;
import java.net.URI;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.net.http.HttpClient;
import java.nio.file.Paths;
import java.nio.file.Path;

public class Main {
    public static void main(String[] args) {
        downloadAFile("https://pc-package.wpscdn.cn/wps/download/W.P.S.20.2735.exe", "W.P.S.20.2735.exe");
    }

    public static void downloadAFile(String url, String fileName) {
        HttpClient client = HttpClient.newHttpClient();
        //创建一个HttpRequest请求
        HttpRequest request = HttpRequest.newBuilder(URI.create(url)).build();
        Path filePath = Paths.get(fileName);
        try {
            //发送Http请求
            HttpResponse<Path> response = client.send(request, HttpResponse.BodyHandlers.ofFile(filePath));
            //判断是否成功下载
            if (response.statusCode() == 200) {
                System.out.printf("文件%s已经成功保存!", fileName);
            }
        } catch (IOException | InterruptedException e) {
            throw new RuntimeException(e);
        }
    }

}

在上述代码中，实现了一个名为downloadAFile()函数。使用 HttpClient.send() 方法发送Http请求并将响应体写入文件，并获得HttpResponse<Path>类型的response对象，使用该对象获得请求的状态码statusCode。

异步方式下载

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import java.io.IOException;
import java.net.URI;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.net.http.HttpClient;
import java.nio.file.Paths;
import java.nio.file.Path;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        downloadAFile("https://pc-package.wpscdn.cn/wps/download/W.P.S.20.2735.exe", "W.P.S.20.2735.exe");
        System.out.println("下载已经开始，按回车键结束...");
        Scanner scanner = new Scanner(System.in);
        scanner.nextLine();
    }

    public static void downloadAFile(String url, String fileName) {
        HttpClient client = HttpClient.newHttpClient();
        //创建一个HttpRequest请求
        HttpRequest request = HttpRequest.newBuilder(URI.create(url)).build();
        Path filePath = Paths.get(fileName);
        //异步下载
        client.sendAsync(request, HttpResponse.BodyHandlers.ofFile(filePath)).thenAccept(Main::downloadCallBack).exceptionally(e -> {
            System.out.println("下载失败！" + e.getMessage());
            return null;
        });
    }

    private static void downloadCallBack(HttpResponse<Path> pathHttpResponse) {
        if (pathHttpResponse.statusCode() == 200) {
            System.out.println("下载成功！文件已经保存至：" + pathHttpResponse.body());
        } else {
            System.out.printf("下载失败！状态码为%d。\n", pathHttpResponse.statusCode());
        }
    }
}

这里使用client.sendAsync()异步下载文件，thenAccept()来指定下载完成时的回调函数，exceptionally()来处理异常。注意，由于这里是异步下载的，不能使用try{}catch{}来处理异常，异常不能被捕获。

使用OKHttp下载

导入OkHttp依赖

在pom.xml的<dependencies></dependencies>中添加以下代码：

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<!-- https://mvnrepository.com/artifact/com.squareup.okhttp3/okhttp -->
        <dependency>
            <groupId>com.squareup.okhttp3</groupId>
            <artifactId>okhttp</artifactId>
            <version>3.14.9</version>
        </dependency>

OKHttp获取文本文件

Download.java

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package org.example;
import okhttp3.*;

import java.io.IOException;

public class Download{
    public final OkHttpClient client = new OkHttpClient();
    public void run() throws IOException {
        Request request = new Request.Builder().url("http://www.baidu.com").build();
        try(Response response = client.newCall(request).execute()){
            if (response.body() != null) {
                System.out.println(response.body().string());
            }
        }catch (Exception e){
            System.out.println(e.getMessage());
        }
    }
}

Main.java

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package org.example;

import java.io.IOException;

public class Main {
    public static void main(String[] args) {
        Download download = new Download();
        try{
            download.run();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

OKHttp异步请求

Download.java

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package org.example;

import okhttp3.*;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Objects;

public class Download {
    private final String url;
    private final String fileName;
    private final String filePath;
    private final OkHttpClient client = new OkHttpClient();

    public Download(String url, String fileName, String filePath) {
        this.url = url;
        this.fileName = fileName;
        this.filePath = filePath;
    }

    public void run() {
        Request request = new Request.Builder().url(url).build();
        Call call = client.newCall(request);
        call.enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {
                e.printStackTrace();
            }

            @Override
            //响应时的回调函数
            public void onResponse(Call call, Response response) throws IOException {
                if (!response.isSuccessful()) {
                    throw new IOException("Unexpected code " + response);
                }

                try (InputStream inputStream = Objects.requireNonNull(response.body()).byteStream();
                     //从响应体获取字节流
                     FileOutputStream outputStream = new FileOutputStream(new File(filePath, fileName))) {
                    //创建文件输出流
                    byte[] buffer = new byte[2048]; //创建缓存区，大小为2048KB
                    int len;     //用于还剩下多少字节没有被读取
                    while ((len = inputStream.read(buffer)) != -1) {
                        //还没有被读取完
                        outputStream.write(buffer, 0, len);  //将缓冲区数据写入文件
                    }

                    System.out.printf("文件%s已下载至%s%n", fileName, filePath);

                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        });
    }
}

Main.java

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package org.example;

public class Main {
    public static void main(String[] args) {
        String url = "https://down-tencent.huorong.cn/sysdiag-all-x64-6.0.4.0-2024.11.16.1.exe";
        String filename = "sysdiag-all-x64-6.0.4.0-2024.11.16.1.exe";
        Download download = new Download(url, filename, "D:/");
        download.run();
        
    }
}

上述代码中，使用Request.Builder().url(url).build()创建一个Request对象，使用Call类的call对象发起Http Get请求，使用enqueue()方法处理响应和错误。 在处理响应时，首先将response.body()响应体通过byteStream()方法转化为字节流，然后于FileInputStream一起使用写入文件。

OKHttp同步请求

无需使用call.enqueue()方法，最后使用断言assertThat(response.code(), equalTo(200));判断状态码是否等于200即可。

自定义请求头

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    Request request = new Request.Builder().url(url).addHeader("Content-Length","application/x-msdownload").build();

允许重定向

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    private final OkHttpClient client = new OkHttpClient().newBuilder().followRedirects(true).build();

超时设置

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    private final OkHttpClient client = new OkHttpClient().newBuilder().readTimeout(3, TimeUnit.SECONDS).build();

取消请求

可以使用Call.cancel()终止请求。