java多线程下载_Java实现多线程下载，支持断点续传
多線程下載及斷點續傳的實現是使用 HTTP/1.1 引入的 Range 請求參數，可以訪問Web資源的指定區間的內容。雖然實現了多線程及斷點續傳，但還有很多不完善的地方。
包含四個類：
Downloader: 主類，負責分配任務給各個子線程，及檢測進度
DownloadFile: 表示要下載的哪個文件，為了能寫輸入到文件的指定位置，使用 RandomAccessFile 類操作文件，多個線程寫同一個文件需要保證線程安全，這里直接調用 getChannel 方法，獲取一個文件通道，FileChannel是線程安全的。
DownloadTask: 實際執行下載的線程，獲取 [lowerBound, upperBound] 區間的數據，當下載過程中出現異常時要通知其他線程(使用 AtomicBoolean)，結束下載
Logger: 實時記錄下載進度，以便續傳時知道從哪開始。感覺這里做的比較差，為了能實時寫出日志及方便地使用Properties類的load/store方法格式化輸入輸出，每次都是打開后再關閉。
演示：
隨便找一個文件下載：
強行結束程序并重新運行：
日志文件：
斷點續傳的關鍵是記錄各個線程的下載進度，這里細節比較多，花了很久。只需要記錄每個線程請求的Range區間極客，每次成功寫數據到文件時，就更新一次下載區間。下面是下載完成后的日志內容。
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; // 默認線程數量
private AtomicBoolean canceled; // 取消狀態，如果有一個子線程出現異常，則取消整個下載任務
private DownloadFile file; // 下載的文件對象
private String storageLocation;
private final int threadCount; // 線程數量
private long fileSize; // 文件大小
private final String url;
private long beginTime; // 開始時間
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("* 繼續上次下載進度[已下載：%.2fMB]：%s\n", logger.getWroteSize() / 1014.0 / 1024, url);
} else {
System.out.println("* 開始下載：" + url);
}
if (-1 == (this.fileSize = getFileSize()))
return;
System.out.printf("* 文件大小：%.2fMB\n", fileSize / 1024.0 / 1024);
this.beginTime = System.currentTimeMillis();
try {
this.file = new DownloadFile(storageLocation, fileSize, logger);
if (reStart) {
file.setWroteSize(logger.getWroteSize());
}
// 分配線程下載
dispatcher(reStart);
// 循環打印進度
printDownloadProgress();
} catch (IOException e) {
System.err.println("x 創建文件失敗[" + e.getMessage() + "]");
}
}
/**
* 分配器，決定每個線程下載哪個區間的數據
*/
private void dispatcher(boolean reStart) {
long blockSize = fileSize / threadCount; // 每個線程要下載的數據量
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
upperBound = (i == threadCount - 1) ? fileSize-1 : lowerBound + blockSize;
}
new DownloadTask(url, lowerBound, upperBound, file, canceled, threadID).start();
}
}
/**
* 循環打印進度，直到下載完畢，或任務被取消
*/
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("下載進度：%.2f%%, 已下載：%.2fMB，當前速度：%.2fMB/s\n",
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，任務已取消");
} else {
System.out.println("* 下載成功，本次用時"+ (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("* 連接服務器成功");
} catch (MalformedURLException e) {
throw new RuntimeException("URL錯誤");
} catch (IOException e) {
System.err.println("x 連接服務器失敗["+ 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("* [線程" + threadId + "]連接成功，開始下載...");
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("* [線程" + threadId + "]下載完成" + ": " + lowerBound + "-" + upperBound);
}
} catch (IOException e) {
canceled.set(true);
System.err.println("x [線程" + threadId + "]遇到錯誤[" + e.getMessage() + "]，結束下載");
} finally {
if (input != null) {
try {
input.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
/**
* 連接WEB服務器，并返回一個數據通道
* @return 返回通道
* @throws IOException 網絡連接錯誤
*/
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("狀態碼錯誤：" + 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; // 線程安全類
private AtomicLong wroteSize; // 已寫入的長度
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();
}
/**
* 寫數據
* @param offset 寫偏移量
* @param buffer 數據
* @throws IOException 寫數據出現異常
*/
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 已經下載的數據量，為了知道何時結束整個任務，以及統計信息
*/
long getWroteSize() {
return wroteSize.get();
}
// 繼續下載時調用
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;
/**
* 重新開始下載時，使用該構造函數
* @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); // 每次寫時都清空文件
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"));
}
}
總結
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