完整代碼：https://github.com/iyuanyb/Downloader

多線(xiàn)程下載及斷點(diǎn)續(xù)傳的實(shí)現(xiàn)是使用 HTTP/1.1 引入的 Range 請(qǐng)求參數(shù)，可以訪問(wèn)Web資源的指定區(qū)間的內(nèi)容。雖然實(shí)現(xiàn)了多線(xiàn)程及斷點(diǎn)續(xù)傳，但還有很多不完善的地方。

包含四個(gè)類(lèi)：

Downloader: 主類(lèi)，負(fù)責(zé)分配任務(wù)給各個(gè)子線(xiàn)程，及檢測(cè)進(jìn)度DownloadFile: 表示要下載的哪個(gè)文件，為了能寫(xiě)輸入到文件的指定位置，使用 RandomAccessFile 類(lèi)操作文件，多個(gè)線(xiàn)程寫(xiě)同一個(gè)文件需要保證線(xiàn)程安全，這里直接調(diào)用 getChannel 方法，獲取一個(gè)文件通道，F(xiàn)ileChannel是線(xiàn)程安全的。DownloadTask: 實(shí)際執(zhí)行下載的線(xiàn)程，獲取 [lowerBound, upperBound] 區(qū)間的數(shù)據(jù)，當(dāng)下載過(guò)程中出現(xiàn)異常時(shí)要通知其他線(xiàn)程（使用 AtomicBoolean），結(jié)束下載Logger: 實(shí)時(shí)記錄下載進(jìn)度，以便續(xù)傳時(shí)知道從哪開(kāi)始。感覺(jué)這里做的比較差，為了能實(shí)時(shí)寫(xiě)出日志及方便地使用Properties類(lèi)的load/store方法格式化輸入輸出，每次都是打開(kāi)后再關(guān)閉。

演示：

隨便找一個(gè)文件下載：

強(qiáng)行結(jié)束程序并重新運(yùn)行：

日志文件：

斷點(diǎn)續(xù)傳的關(guān)鍵是記錄各個(gè)線(xiàn)程的下載進(jìn)度，這里細(xì)節(jié)比較多，花了很久。只需要記錄每個(gè)線(xiàn)程請(qǐng)求的Range區(qū)間極客，每次成功寫(xiě)數(shù)據(jù)到文件時(shí)，就更新一次下載區(qū)間。下面是下載完成后的日志內(nèi)容。

代碼:

Downloader.java

package downloader; import java.io.*;import java.net.*;import java.nio.file.Files;import java.nio.file.Path;import java.util.concurrent.atomic.AtomicBoolean; public class Downloader { private static final int DEFAULT_THREAD_COUNT = 4; // 默認(rèn)線(xiàn)程數(shù)量 private AtomicBoolean canceled; // 取消狀態(tài)，如果有一個(gè)子線(xiàn)程出現(xiàn)異常，則取消整個(gè)下載任務(wù) private DownloadFile file; // 下載的文件對(duì)象 private String storageLocation; private final int threadCount; // 線(xiàn)程數(shù)量 private long fileSize; // 文件大小 private final String url; private long beginTime; // 開(kāi)始時(shí)間 private Logger logger; public Downloader(String url) { this(url, DEFAULT_THREAD_COUNT); } public Downloader(String url, int threadCount) { this.url = url; this.threadCount = threadCount; this.canceled = new AtomicBoolean(false); this.storageLocation = url.substring(url.lastIndexOf(’/’)+1); this.logger = new Logger(storageLocation + '.log', url, threadCount); } public void start() { boolean reStart = Files.exists(Path.of(storageLocation + '.log')); if (reStart) { logger = new Logger(storageLocation + '.log'); System.out.printf('* 繼續(xù)上次下載進(jìn)度[已下載：%.2fMB]：%sn', logger.getWroteSize() / 1014.0 / 1024, url); } else { System.out.println('* 開(kāi)始下載：' + url); } if (-1 == (this.fileSize = getFileSize())) return; System.out.printf('* 文件大小：%.2fMBn', fileSize / 1024.0 / 1024); this.beginTime = System.currentTimeMillis(); try { this.file = new DownloadFile(storageLocation, fileSize, logger); if (reStart) {file.setWroteSize(logger.getWroteSize()); } // 分配線(xiàn)程下載 dispatcher(reStart); // 循環(huán)打印進(jìn)度 printDownloadProgress(); } catch (IOException e) { System.err.println('x 創(chuàng)建文件失敗[' + e.getMessage() + ']'); } } /** * 分配器，決定每個(gè)線(xiàn)程下載哪個(gè)區(qū)間的數(shù)據(jù) */ private void dispatcher(boolean reStart) { long blockSize = fileSize / threadCount; // 每個(gè)線(xiàn)程要下載的數(shù)據(jù)量 long lowerBound = 0, upperBound = 0; long[][] bounds = null; int threadID = 0; if (reStart) { bounds = logger.getBounds(); } for (int i = 0; i < threadCount; i++) { if (reStart) {threadID = (int)(bounds[i][0]);lowerBound = bounds[i][1];upperBound = bounds[i][2]; } else {threadID = i;lowerBound = i * blockSize;// fileSize-1 !!!!! fu.ck，找了一下午的錯(cuò)upperBound = (i == threadCount - 1) ? fileSize-1 : lowerBound + blockSize; } new DownloadTask(url, lowerBound, upperBound, file, canceled, threadID).start(); } } /** * 循環(huán)打印進(jìn)度，直到下載完畢，或任務(wù)被取消 */ private void printDownloadProgress() { long downloadedSize = file.getWroteSize(); int i = 0; long lastSize = 0; // 三秒前的下載量 while (!canceled.get() && downloadedSize < fileSize) { if (i++ % 4 == 3) { // 每3秒打印一次System.out.printf('下載進(jìn)度：%.2f%%, 已下載：%.2fMB，當(dāng)前速度：%.2fMB/sn', downloadedSize / (double)fileSize * 100 , downloadedSize / 1024.0 / 1024, (downloadedSize - lastSize) / 1024.0 / 1024 / 3);lastSize = downloadedSize;i = 0; } try {Thread.sleep(1000); } catch (InterruptedException ignore) {} downloadedSize = file.getWroteSize(); } file.close(); if (canceled.get()) { try {Files.delete(Path.of(storageLocation)); } catch (IOException ignore) { } System.err.println('x 下載失敗，任務(wù)已取消'); } else { System.out.println('* 下載成功，本次用時(shí)'+ (System.currentTimeMillis() - beginTime) / 1000 +'秒'); } } /** * @return 要下載的文件的尺寸 */ private long getFileSize() { if (fileSize != 0) { return fileSize; } HttpURLConnection conn = null; try { conn = (HttpURLConnection)new URL(url).openConnection(); conn.setConnectTimeout(3000); conn.setRequestMethod('HEAD'); conn.connect(); System.out.println('* 連接服務(wù)器成功'); } catch (MalformedURLException e) { throw new RuntimeException('URL錯(cuò)誤'); } catch (IOException e) { System.err.println('x 連接服務(wù)器失敗['+ e.getMessage() +']'); return -1; } return conn.getContentLengthLong(); } public static void main(String[] args) throws IOException { new Downloader('http://js.xiazaicc.com//down2/ucliulanqi_downcc.zip').start(); }}

DownloadTask.java

package downloader; import java.io.*;import java.net.HttpURLConnection;import java.net.URL;import java.nio.ByteBuffer;import java.nio.channels.Channels;import java.nio.channels.ReadableByteChannel;import java.util.concurrent.atomic.AtomicBoolean; class DownloadTask extends Thread { private final String url; private long lowerBound; // 下載的文件區(qū)間 private long upperBound; private AtomicBoolean canceled; private DownloadFile downloadFile; private int threadId; DownloadTask(String url, long lowerBound, long upperBound, DownloadFile downloadFile, AtomicBoolean canceled, int threadID) { this.url = url; this.lowerBound = lowerBound; this.upperBound = upperBound; this.canceled = canceled; this.downloadFile = downloadFile; this.threadId = threadID; } @Override public void run() { ReadableByteChannel input = null; try { ByteBuffer buffer = ByteBuffer.allocate(1024 * 1024 * 2); // 2MB input = connect(); System.out.println('* [線(xiàn)程' + threadId + ']連接成功，開(kāi)始下載...'); int len; while (!canceled.get() && lowerBound <= upperBound) {buffer.clear();len = input.read(buffer);downloadFile.write(lowerBound, buffer, threadId, upperBound);lowerBound += len; } if (!canceled.get()) {System.out.println('* [線(xiàn)程' + threadId + ']下載完成' + ': ' + lowerBound + '-' + upperBound); } } catch (IOException e) { canceled.set(true); System.err.println('x [線(xiàn)程' + threadId + ']遇到錯(cuò)誤[' + e.getMessage() + ']，結(jié)束下載'); } finally { if (input != null) {try { input.close();} catch (IOException e) { e.printStackTrace();} } } } /** * 連接WEB服務(wù)器，并返回一個(gè)數(shù)據(jù)通道 * @return 返回通道 * @throws IOException 網(wǎng)絡(luò)連接錯(cuò)誤 */ private ReadableByteChannel connect() throws IOException { HttpURLConnection conn = (HttpURLConnection)new URL(url).openConnection(); conn.setConnectTimeout(3000); conn.setRequestMethod('GET'); conn.setRequestProperty('Range', 'bytes=' + lowerBound + '-' + upperBound);// System.out.println('thread_'+ threadId +': ' + lowerBound + '-' + upperBound); conn.connect(); int statusCode = conn.getResponseCode(); if (HttpURLConnection.HTTP_PARTIAL != statusCode) { conn.disconnect(); throw new IOException('狀態(tài)碼錯(cuò)誤：' + statusCode); } return Channels.newChannel(conn.getInputStream()); }}

DownloadFile.java

package downloader; import java.io.IOException;import java.io.RandomAccessFile;import java.nio.ByteBuffer;import java.nio.channels.FileChannel;import java.util.concurrent.atomic.AtomicLong; class DownloadFile { private final RandomAccessFile file; private final FileChannel channel; // 線(xiàn)程安全類(lèi) private AtomicLong wroteSize; // 已寫(xiě)入的長(zhǎng)度 private Logger logger; DownloadFile(String fileName, long fileSize, Logger logger) throws IOException { this.wroteSize = new AtomicLong(0); this.logger = logger; this.file = new RandomAccessFile(fileName, 'rw'); file.setLength(fileSize); channel = file.getChannel(); } /** * 寫(xiě)數(shù)據(jù) * @param offset 寫(xiě)偏移量 * @param buffer 數(shù)據(jù) * @throws IOException 寫(xiě)數(shù)據(jù)出現(xiàn)異常 */ void write(long offset, ByteBuffer buffer, int threadID, long upperBound) throws IOException { buffer.flip(); int length = buffer.limit(); while (buffer.hasRemaining()) { channel.write(buffer, offset); } wroteSize.addAndGet(length); logger.updateLog(threadID, length, offset + length, upperBound); // 更新日志 } /** * @return 已經(jīng)下載的數(shù)據(jù)量，為了知道何時(shí)結(jié)束整個(gè)任務(wù)，以及統(tǒng)計(jì)信息 */ long getWroteSize() { return wroteSize.get(); } // 繼續(xù)下載時(shí)調(diào)用 void setWroteSize(long wroteSize) { this.wroteSize.set(wroteSize); } void close() { try { file.close(); } catch (IOException e) { e.printStackTrace(); } }}

Logger.java

package downloader; import java.io.*;import java.util.Properties; class Logger { private String logFileName; // 下載的文件的名字 private Properties log; /** * 重新開(kāi)始下載時(shí)，使用該構(gòu)造函數(shù) * @param logFileName */ Logger(String logFileName) { this.logFileName = logFileName; log = new Properties(); FileInputStream fin = null; try { log.load(new FileInputStream(logFileName)); } catch (IOException ignore) { } finally { try {fin.close(); } catch (Exception ignore) {} } } Logger(String logFileName, String url, int threadCount) { this.logFileName = logFileName; this.log = new Properties(); log.put('url', url); log.put('wroteSize', '0'); log.put('threadCount', String.valueOf(threadCount)); for (int i = 0; i < threadCount; i++) { log.put('thread_' + i, '0-0'); } } synchronized void updateLog(int threadID, long length, long lowerBound, long upperBound) { log.put('thread_'+threadID, lowerBound + '-' + upperBound); log.put('wroteSize', String.valueOf(length + Long.parseLong(log.getProperty('wroteSize')))); FileOutputStream file = null; try { file = new FileOutputStream(logFileName); // 每次寫(xiě)時(shí)都清空文件 log.store(file, null); } catch (IOException e) { e.printStackTrace(); } finally { if (file != null) {try { file.close();} catch (IOException e) { e.printStackTrace();} } } } /** * 獲取區(qū)間信息 * ret[i][0] = threadID, ret[i][1] = lowerBoundID, ret[i][2] = upperBoundID * @return */ long[][] getBounds() { long[][] bounds = new long[Integer.parseInt(log.get('threadCount').toString())][3]; int[] index = {0}; log.forEach((k, v) -> { String key = k.toString(); if (key.startsWith('thread_')) {String[] interval = v.toString().split('-');bounds[index[0]][0] = Long.parseLong(key.substring(key.indexOf('_') + 1));bounds[index[0]][1] = Long.parseLong(interval[0]);bounds[index[0]++][2] = Long.parseLong(interval[1]); } }); return bounds; } long getWroteSize() { return Long.parseLong(log.getProperty('wroteSize')); }}

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