Java使用HttpClient实现文件下载

下面是使用HttpClient实现文件下载的完整攻略，我将详细讲解该过程并提供两个示例说明。

简介

HttpClient是Apache软件基金会下的一个开源HTTP客户端库，它支持Http/Https协议，并具有稳定、高效、易用的特点。本文将介绍如何使用HttpClient来实现文件下载。

下载依赖

我们需要在项目中引入HttpClient的依赖，该依赖在Maven仓库中的名称为“httpclient"，需要注意的是，如果是使用HttpClient版本大于4.3.x，还需要引入另一个依赖“httpcore”，如下所示：

<dependency>
    <groupId>org.apache.httpcomponents</groupId>    
    <artifactId>httpclient</artifactId>    
    <version>4.5.6</version>    
</dependency>
<dependency>
    <groupId>org.apache.httpcomponents</groupId>    
    <artifactId>httpcore</artifactId>    
    <version>4.4.11</version>    
</dependency>

实现文件下载

使用HttpClient实现文件下载的主要步骤如下：

1. 创建HttpClient实例。

2. 创建HttpGet实例，并设置下载文件的URL地址。

3. 执行HttpGet请求，获取文件内容，并将内容写入到文件中。

4. 关闭HttpClient连接。

具体实现方式如下所示：

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
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;
public class FileDownloader {
    private static final String FILE_URL = "https://example.com/file.zip";
    private static final String FILE_NAME = "file.zip";
    public static void main(String[] args) {
        ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1);
        executorService.scheduleAtFixedRate(() -> {
            try {
                downloadFile(FILE_URL, FILE_NAME);
            } catch (IOException e) {
                System.err.println("Download failed!" + e.getMessage());
            }
        }, 0, 1, TimeUnit.DAYS);
    }
    private static void downloadFile(String fileUrl, String fileName) throws IOException {
        CloseableHttpClient httpclient = HttpClients.createDefault();
        HttpGet httpget = new HttpGet(fileUrl);
        HttpResponse response = httpclient.execute(httpget);
        HttpEntity entity = response.getEntity();
        if (entity != null) {
            InputStream inputStream = entity.getContent();
            FileOutputStream outputStream = new FileOutputStream(new File(fileName));
            int bytesRead = 0;
            byte[] buffer = new byte[1024];
            while ((bytesRead = inputStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, bytesRead);
            }
            outputStream.close();
            EntityUtils.consume(entity);
        }
        httpclient.close();
    }
}

在上述代码中，我们使用了HttpGet请求来下载文件；在实际应用中，可以将文件下载包装成一个服务，按需进行调用。

示例一

我们以下载网络上的一张图片为例，实现文件的下载。

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
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;
public class ImageDownloader {
    private static final String IMAGE_URL = "https://example.com/image.jpg";
    private static final String IMAGE_NAME = "image.jpg";
    public static void main(String[] args) {
        CloseableHttpClient httpclient = HttpClients.createDefault();
        HttpGet httpget = new HttpGet(IMAGE_URL);
        try {
            HttpResponse response = httpclient.execute(httpget);
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                InputStream inputStream = entity.getContent();
                FileOutputStream fileOutputStream = new FileOutputStream(new File(IMAGE_NAME));
                int bytesRead = 0;
                byte[] buffer = new byte[1024];
                while ((bytesRead = inputStream.read(buffer)) != -1) {
                    fileOutputStream.write(buffer, 0, bytesRead);
                }
                fileOutputStream.close();
                EntityUtils.consume(entity);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                httpclient.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

在上述代码中，我们使用了HttpGet请求下载了一张图片，该图片的URL地址为：https://example.com/image.jpg，将下载的图片保存到了本地的文件中，文件名为：image.jpg。

示例二

我们以使用HttpClient下载一个CSV文件为例。

import java.io.*;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
public class CsvFileDownloader {
    public static void main(String[] args) throws IOException {
        CloseableHttpClient httpclient = HttpClients.createDefault();
        HttpGet httpget = new HttpGet("https://example.com/data.csv");
        HttpResponse response = httpclient.execute(httpget);
        HttpEntity entity = response.getEntity();
        if (entity != null) {
            InputStream inputStream = entity.getContent();
            writeInputStreamToFile(inputStream, "data.csv");
        }
        httpclient.close();
    }
    private static void writeInputStreamToFile(InputStream inputStream, String fileName) throws IOException {
        OutputStream outputStream = new FileOutputStream(new File(fileName));
        int bytesRead = 0;
        byte[] buffer = new byte[1024];
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.close();
    }
}

在上述代码中，我们使用了HttpGet请求下载了一个CSV文件，该文件的URL地址为：https://example.com/data.csv，将下载的CSV文件保存到了本地的文件中，文件名为：data.csv。

总结：以上是使用Java及HttpClient实现文件下载的完整攻略及两个示例说明。在实际项目中，HttpClient可以支持证书、代理、cookies、连接池等功能，使得实现更加强大和灵活。