java - 限制输入流

Question

我是 Java 的新手。我编写了一个通过 Internet 下载特定文件的 Swing 应用程序。

我想做的是限制 InputStream 每秒读取 n(比如说 10240 字节，这可能会改变)字节。

读取整个文件后，应将文件保存到本地目录中，例如 C:\Downloads\

我猜我应该创建一个扩展到 InputStream 的类并覆盖它的方法，但我不确定如何以及使用哪些方法来实现我的目标。

我使用 this HTTP 下载和 this 的示例FTP 示例。

HTTP 示例；

package net.codejava.networking;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

/**
 * A utility that downloads a file from a URL.
 * @author www.codejava.net
 *
 */
public class HttpDownloadUtility {
    private static final int BUFFER_SIZE = 4096;

    /**
     * Downloads a file from a URL
     * @param fileURL HTTP URL of the file to be downloaded
     * @param saveDir path of the directory to save the file
     * @throws IOException
     */
    public static void downloadFile(String fileURL, String saveDir)
            throws IOException {
        URL url = new URL(fileURL);
        HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
        int responseCode = httpConn.getResponseCode();

        // always check HTTP response code first
        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());
            }

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

            // opens input stream from the HTTP connection
            InputStream inputStream = httpConn.getInputStream();
            String saveFilePath = saveDir + File.separator + fileName;

            // opens an output stream to save into file
            FileOutputStream outputStream = new FileOutputStream(saveFilePath);

            int bytesRead = -1;
            byte[] buffer = new byte[BUFFER_SIZE];
            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();
    }
}

FTP 示例；

import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;

/**
 * A program demonstrates how to upload files from local computer to a remote
 * FTP server using Apache Commons Net API.
 * @author www.codejava.net
 */
public class FTPDownloadFileDemo {

    public static void main(String[] args) {
        String server = "www.myserver.com";
        int port = 21;
        String user = "user";
        String pass = "pass";

        FTPClient ftpClient = new FTPClient();
        try {

            ftpClient.connect(server, port);
            ftpClient.login(user, pass);
            ftpClient.enterLocalPassiveMode();
            ftpClient.setFileType(FTP.BINARY_FILE_TYPE);

            // APPROACH #1: using retrieveFile(String, OutputStream)
            String remoteFile1 = "/test/video.mp4";
            File downloadFile1 = new File("D:/Downloads/video.mp4");
            OutputStream outputStream1 = new BufferedOutputStream(new FileOutputStream(downloadFile1));
            boolean success = ftpClient.retrieveFile(remoteFile1, outputStream1);
            outputStream1.close();

            if (success) {
                System.out.println("File #1 has been downloaded successfully.");
            }

            // APPROACH #2: using InputStream retrieveFileStream(String)
            String remoteFile2 = "/test/song.mp3";
            File downloadFile2 = new File("D:/Downloads/song.mp3");
            OutputStream outputStream2 = new BufferedOutputStream(new FileOutputStream(downloadFile2));
            InputStream inputStream = ftpClient.retrieveFileStream(remoteFile2);
            byte[] bytesArray = new byte[4096];
            int bytesRead = -1;
            while ((bytesRead = inputStream.read(bytesArray)) != -1) {
                outputStream2.write(bytesArray, 0, bytesRead);
            }

            success = ftpClient.completePendingCommand();
            if (success) {
                System.out.println("File #2 has been downloaded successfully.");
            }
            outputStream2.close();
            inputStream.close();

        } catch (IOException ex) {
            System.out.println("Error: " + ex.getMessage());
            ex.printStackTrace();
        } finally {
            try {
                if (ftpClient.isConnected()) {
                    ftpClient.logout();
                    ftpClient.disconnect();
                }
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
    }
}

我想要的是限制这 2 个输入流的下载速率，以便在从远程位置获取这些文件时下载速度受到限制。

如果您能给我一个带有简短解释的示例，我将不胜感激。

Answer 1

这有点原始，但它应该可以满足您的要求(虽然没有测试)。

int bytesReadSinceSleep = 0;
long lastSleepTime = System.currentTimeMillis();

while ((bytesRead = inputStream.read(buffer)) != -1) {
    outputStream.write(buffer, 0, bytesRead);

    // increment bytes read this second
    bytesReadSinceSleep += bytesRead;

    // if we've passed the throttling point, sleep for the remainder of the second
    if (bytesReadSinceSleep >= MAX_BYTES_PER_SECOND) {

        // calculate time elapsed since last sleep
        long timeElapsed = System.currentTimeMillis() - lastSleepTime;

        // sleep for the remainder of 1 second (if there is a remainder)
        Thread.sleep(Math.max(1000 - timeElapsed, 0));

        // reset byte count
        bytesReadSinceSleep = 0;

        // reset sleep time
        lastSleepTime = System.currentTimeMillis();
    }
}

如果 BUFFER_SIZE 不是 MAX_BYTES_PER_SECOND 的一个因素，并且您对节流率很在意，您可能需要使用 read() 接受偏移量和限制参数以准确读取正确数量的重载。

---

为了更简洁的解决方案，您可以使用 Guava 的 RateLimiter:

RateLimiter limiter = RateLimiter.create(MAX_BYTES_PER_SECOND);
while ((bytesRead = inputStream.read(buffer)) != -1) {
    outputStream.write(buffer, 0, bytesRead);
    limiter.acquire(bytesRead);
}