Java通过URL下载文件到本地

在Java中，通过URL下载文件到本地非常常见。 下面将介绍如何使用Java实现从URL下载文件到本地的操作。

一、建立URL连接并打开连接

public static void downloadFileFromURL(String fileURL, String saveDir) throws IOException {
    URL url = new URL(fileURL);
    HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
    httpConn.setRequestMethod("GET");
    int responseCode = httpConn.getResponseCode();
    if (responseCode == HttpURLConnection.HTTP_OK) {
        //连接成功
    } else {
        //连接失败
    }
}

在第一步中，我们通过URL对象建立与远程服务器的连接，并向该服务器发起一个HTTP GET请求，并且我们通过getResopnseCode()方法获取服务器的响应状态码。

二、从连接中获取输入流并从输入流中读取数据

public static void downloadFileFromURL(String fileURL, String saveDir) throws IOException {
    URL url = new URL(fileURL);
    HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
    httpConn.setRequestMethod("GET");
    int responseCode = httpConn.getResponseCode();
    if (responseCode == HttpURLConnection.HTTP_OK) {
        // 获取文件名
        String fileName = "";
        String disposition = httpConn.getHeaderField("Content-Disposition");
        String contentType = httpConn.getContentType();
        int contentLength = httpConn.getContentLength();
        if (disposition != null) {
            int index = disposition.indexOf("filename=");
            if (index > 0) {
                fileName = disposition.substring(index + 10, disposition.length() - 1);
            }
        } else {
            fileName = fileURL.substring(fileURL.lastIndexOf("/") + 1, fileURL.length());
        }

        // 输出文件信息
        System.out.println("Content-Type = " + contentType);
        System.out.println("Content-Disposition = " + disposition);
        System.out.println("Content-Length = " + contentLength);
        System.out.println("fileName = " + fileName);

        // 开始下载文件
        InputStream inputStream = httpConn.getInputStream();
        String saveFilePath = saveDir + File.separator + fileName;
        FileOutputStream outputStream = new FileOutputStream(saveFilePath);
        int bytesRead = -1;
        byte[] buffer = new byte[4096];
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.close();
        inputStream.close();
        System.out.println("文件已经下载到 " + saveFilePath + ".");
    } else {
        System.out.println("服务器返回响应码 " + responseCode);
    }
}

在第二步中，我们通过输入流从连接中读取远程服务器传输的数据。

三、将输入流写入本地文件

public static void downloadFileFromURL(String fileURL, String saveDir) throws IOException {
    URL url = new URL(fileURL);
    HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
    httpConn.setRequestMethod("GET");
    int responseCode = httpConn.getResponseCode();
    if (responseCode == HttpURLConnection.HTTP_OK) {
        // 获取文件名
        String fileName = "";
        String disposition = httpConn.getHeaderField("Content-Disposition");
        String contentType = httpConn.getContentType();
        int contentLength = httpConn.getContentLength();
        if (disposition != null) {
            int index = disposition.indexOf("filename=");
            if (index > 0) {
                fileName = disposition.substring(index + 10, disposition.length() - 1);
            }
        } else {
            fileName = fileURL.substring(fileURL.lastIndexOf("/") + 1, fileURL.length());
        }

        // 输出文件信息
        System.out.println("Content-Type = " + contentType);
        System.out.println("Content-Disposition = " + disposition);
        System.out.println("Content-Length = " + contentLength);
        System.out.println("fileName = " + fileName);

        // 开始下载文件
        InputStream inputStream = httpConn.getInputStream();
        String saveFilePath = saveDir + File.separator + fileName;
        FileOutputStream outputStream = new FileOutputStream(saveFilePath);
        int bytesRead = -1;
        byte[] buffer = new byte[4096];
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.close();
        inputStream.close();
        System.out.println("文件已经下载到 " + saveFilePath + ".");
    } else {
        System.out.println("服务器返回响应码 " + responseCode);
    }
}

在第三步中，我们创建一个FileStream对象并将输入流中的数据写入该对象。 最后，将FileStream中的数据写入本地文件。

四、异常处理

在对下载文件进行异常处理时，我们需要考虑多种情况：

• 连接失败
• 文件名提取失败
• 无法获取响应状态码。

为了更好地处理这些异常情况，我们需要将下载文件的实现代码封装成一个方法，并抛出异常。

五、完整代码示例

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;

public class DownloadFileUtil {

    public static void downloadFileFromURL(String fileURL, String saveDir) throws IOException {

        // 建立URL连接并打开连接
        URL url = new URL(fileURL);
        HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
        httpConn.setRequestMethod("GET");

        // 从连接中获取输入流并从输入流中读取数据
        int responseCode = httpConn.getResponseCode();
        if (responseCode == HttpURLConnection.HTTP_OK) {
            String fileName = "";
            String disposition = httpConn.getHeaderField("Content-Disposition");
            String contentType = httpConn.getContentType();
            int contentLength = httpConn.getContentLength();

            if (disposition != null) {
                // extracts file name from header field
                int index = disposition.indexOf("filename=");
                if (index > 0) {
                    fileName = disposition.substring(index + 10, disposition.length() - 1);
                }
            } else {
                // extracts file name from URL
                fileName = fileURL.substring(fileURL.lastIndexOf("/") + 1, fileURL.length());
            }

            // 输出文件信息
            System.out.println("Content-Type = " + contentType);
            System.out.println("Content-Disposition = " + disposition);
            System.out.println("Content-Length = " + contentLength);
            System.out.println("fileName = " + fileName);

            // 将输入流写入本地文件
            InputStream inputStream = httpConn.getInputStream();
            String saveFilePath = saveDir + File.separator + fileName;
            FileOutputStream outputStream = new FileOutputStream(saveFilePath);
            int bytesRead = -1;
            byte[] buffer = new byte[4096];
            while ((bytesRead = inputStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, bytesRead);
            }
            outputStream.close();
            inputStream.close();
            System.out.println("文件已经下载到 " + saveFilePath + ".");
        } else {
            System.out.println("服务器返回响应码 " + responseCode);
        }
    }
}

六、总结

在Java中实现从URL下载文件到本地非常常见。到现在，您应该已经掌握了如何使用Java对文件进行下载，并成功下载文件到本地。尽管这个过程看起来很简单，但是由于许多服务器强制使用HTTPS，所以我们需要使用Java的加密通信。