Java 多线程下载技术实现

多线程下载

多线程下载技术，简单的说就是把要下载的文件分成几块，由不同的线程来负责每一块数据的下载任务。

技术要点

RandomAccessFile：

Java中用来实现随机访问文件的类

http Range请求头

具体思路

1、文件分块。 文件分块大小（blockSize）= （文件大小 +线程数 - 1 ）/ 线程数 ;

2、确定每一个线程所要下载的 文件的起始和结束位置。

现假设为每个线程分别编号：0，1， 2，3；则

第一个线程负责的下载位置是： 0*blockSize - (0+1)*blockSize -1，

第二个线程负责的下载位置是： 1*blockSize - (1+1)*blockSize -1，

以此类推第i个线程负责的下载位置是：i*blockSize - (i+1)*blockSize -1；

即线程（编号为id）下载开始位置 start = id*block;

即线程（编号为id）下载结束位置 end = (id+1)*block -1;

3、设置http 请求头， conn.setRequestProperty(“Range”, “bytes=” + start + “-” + end);

代码实现

一个简单的Java多线程下载代码如下：

package com.ricky.java.test.download;

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

public class Downloader {
private URL url;    // 目标地址
private File file;  // 本地文件
private static final int THREAD_AMOUNT = 8;                 // 线程数
private static final String DOWNLOAD_DIR_PATH = "D:/Download";      // 下载目录
private int threadLen;                                      // 每个线程下载多少

public Downloader(String address, String filename) throws IOException {     // 通过构造函数传入下载地址
url = new URL(address);
File dir = new File(DOWNLOAD_DIR_PATH);
if(!dir.exists()){
dir.mkdirs();
}
file = new File(dir, filename);
}

public void download() throws IOException {
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setConnectTimeout(5000);

int totalLen = conn.getContentLength();                             // 获取文件长度
threadLen = (totalLen + THREAD_AMOUNT - 1) / THREAD_AMOUNT;         // 计算每个线程要下载的长度

System.out.println("totalLen="+totalLen+",threadLen:"+threadLen);

RandomAccessFile raf = new RandomAccessFile(file, "rws");           // 在本地创建一个和服务端大小相同的文件
raf.setLength(totalLen);                                            // 设置文件的大小
raf.close();

for (int i = 0; i < THREAD_AMOUNT; i++)                             // 开启3条线程, 每个线程下载一部分数据到本地文件中
new DownloadThread(i).start();
}

private class DownloadThread extends Thread {
private int id;
public DownloadThread(int id) {
this.id = id;
}
public void run() {
int start = id * threadLen;                     // 起始位置
int end = id * threadLen + threadLen - 1;       // 结束位置
System.out.println("线程" + id + ": " + start + "-" + end);

try {
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setConnectTimeout(5000);
conn.setRequestProperty("Range", "bytes=" + start + "-" + end);     // 设置当前线程下载的范围

InputStream in = conn.getInputStream();
RandomAccessFile raf = new RandomAccessFile(file, "rws");
raf.seek(start);            // 设置保存数据的位置

byte[] buffer = new byte[1024];
int len;
while ((len = in.read(buffer)) != -1)
raf.write(buffer, 0, len);
raf.close();

System.out.println("线程" + id + "下载完毕");
} catch (IOException e) {
e.printStackTrace();
}
}
}

public static void main(String[] args) throws IOException {

String address = "http://dldir1.qq.com/qqfile/qq/QQ7.9/16621/QQ7.9.exe";
new Downloader(address, "QQ7.9.exe").download();

//      String address = "http://api.t.dianping.com/n/api.xml?cityId=2";
//      new Downloader(address, "2.xml").download();
}
}

封装多线程下载

文件下载是一个常用的模块，我们可以对其封装一下，方便以后调用。涉及到的开发技术如下：

JDK 1.7

Eclipse Juno

Maven 3

HttpClient 4.3.6

工程目录结构如下所示：



com.ricky.common.java.download.FileDownloader

package com.ricky.common.java.download;

import org.apache.log4j.Logger;

import com.ricky.common.java.download.config.FileDownloaderConfiguration;

/**
* Java 文件多线程下载
* @author Ricky Fung
*
*/
public class FileDownloader {

protected Logger mLogger = Logger.getLogger("devLog");

private volatile static FileDownloader fileDownloader;

private FileDownloaderEngine downloaderEngine;

private FileDownloaderConfiguration configuration;

public static FileDownloader getInstance(){

if(fileDownloader==null){
synchronized (FileDownloader.class) {
if(fileDownloader==null){
fileDownloader = new FileDownloader();
}
}
}

return fileDownloader;
}

protected FileDownloader(){
}

public synchronized void init(FileDownloaderConfiguration configuration){
if (configuration == null) {
throw new IllegalArgumentException("FileDownloader configuration can not be initialized with null");
}
if (this.configuration == null) {
mLogger.info("init FileDownloader");
downloaderEngine = new FileDownloaderEngine(configuration);
this.configuration = configuration;
}else{
mLogger.warn("Try to initialize FileDownloader which had already been initialized before.");
}

}

public boolean download(String url, String filename){

return downloaderEngine.download(url, filename);
}

public boolean isInited() {
return configuration != null;
}

public void destroy() {
if(downloaderEngine!=null){
downloaderEngine.close();
downloaderEngine = null;
}
}
}

com.ricky.common.java.download.config.FileDownloaderConfiguration

package com.ricky.common.java.download.config;

import java.io.File;

public class FileDownloaderConfiguration {
private final int connectTimeout;
private final int socketTimeout;
private final int maxRetryCount;
private final int coreThreadNum;
private final long requestBytesSize;
private final File downloadDestinationDir;

private FileDownloaderConfiguration(Builder builder) {
this.connectTimeout = builder.connectTimeout;
this.socketTimeout = builder.socketTimeout;
this.maxRetryCount = builder.maxRetryCount;
this.coreThreadNum = builder.coreThreadNum;
this.requestBytesSize = builder.requestBytesSize;
this.downloadDestinationDir = builder.downloadDestinationDir;
}

public int getConnectTimeout() {
return connectTimeout;
}
public int getSocketTimeout() {
return socketTimeout;
}
public int getMaxRetryCount() {
return maxRetryCount;
}
public int getCoreThreadNum() {
return coreThreadNum;
}
public long getRequestBytesSize() {
return requestBytesSize;
}
public File getDownloadDestinationDir() {
return downloadDestinationDir;
}

public static FileDownloaderConfiguration.Builder custom() {
return new Builder();
}

public static class Builder {
private int connectTimeout;
private int socketTimeout;
private int maxRetryCount;
private int coreThreadNum;
private long requestBytesSize;
private File downloadDestinationDir;

public Builder connectTimeout(int connectTimeout) {
this.connectTimeout = connectTimeout;
return this;
}
public Builder socketTimeout(int socketTimeout) {
this.socketTimeout = socketTimeout;
return this;
}
public Builder coreThreadNum(int coreThreadNum) {
this.coreThreadNum = coreThreadNum;
return this;
}
public Builder maxRetryCount(int maxRetryCount) {
this.maxRetryCount = maxRetryCount;
return this;
}
public Builder requestBytesSize(long requestBytesSize) {
this.requestBytesSize = requestBytesSize;
return this;
}
public Builder downloadDestinationDir(File downloadDestinationDir) {
this.downloadDestinationDir = downloadDestinationDir;
return this;
}

public FileDownloaderConfiguration build() {

initDefaultValue(this);

return new FileDownloaderConfiguration(this);
}

private void initDefaultValue(Builder builder) {

if(builder.connectTimeout<1){
builder.connectTimeout = 6*1000;
}

if(builder.socketTimeout<1){
builder.socketTimeout = 6*1000;
}
if(builder.maxRetryCount<1){
builder.maxRetryCount = 1;
}
if(builder.coreThreadNum<1){
builder.coreThreadNum = 3;
}
if(builder.requestBytesSize<1){
builder.requestBytesSize = 1024*128;
}
if(builder.downloadDestinationDir==null){
builder.downloadDestinationDir = new File("./");
}
}
}
}

com.ricky.common.java.download.FileDownloaderEngine

package com.ricky.common.java.download;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.util.BitSet;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import org.apache.log4j.Logger;

import com.ricky.common.java.download.config.FileDownloaderConfiguration;
import com.ricky.common.java.download.job.DownloadWorker;
import com.ricky.common.java.download.job.Worker.DownloadListener;

public class FileDownloaderEngine {

protected Logger mLogger = Logger.getLogger("devLog");

private FileDownloaderConfiguration configuration;

private ExecutorService pool;

private HttpRequestImpl httpRequestImpl;

private File downloadDestinationDir;

private int coreThreadNum;

public FileDownloaderEngine(FileDownloaderConfiguration configuration){

this.configuration = configuration;

this.coreThreadNum = configuration.getCoreThreadNum();
this.httpRequestImpl = new HttpRequestImpl(this.configuration);
this.pool = Executors.newFixedThreadPool(this.configuration.getCoreThreadNum());

this.downloadDestinationDir = this.configuration.getDownloadDestinationDir();
if(!this.downloadDestinationDir.exists()){
this.downloadDestinationDir.mkdirs();
}
}

public boolean download(String url, String filename){

long start_time = System.currentTimeMillis();
mLogger.info("开始下载,url:"+url+",filename:"+filename);

long total_file_len = httpRequestImpl.getFileSize(url);                     // 获取文件长度

if(total_file_len<1){
mLogger.warn("获取文件大小失败,url:"+url+",filename:"+filename);
return false;
}

final BitSet downloadIndicatorBitSet = new BitSet(coreThreadNum);   //标记每个线程下载是否成功

File file = null;
try {

file = new File(downloadDestinationDir, filename);

RandomAccessFile raf = new RandomAccessFile(file, "rws");           // 在本地创建一个和服务端大小相同的文件
raf.setLength(total_file_len);                                          // 设置文件的大小
raf.close();

mLogger.info("create new file:"+file);

} catch (FileNotFoundException e) {
mLogger.error("create new file error", e);
} catch (IOException e) {
mLogger.error("create new file error", e);
}

if(file==null || !file.exists()){
mLogger.warn("创建文件失败,url:"+url+",filename:"+filename);
return false;
}

long thread_download_len = (total_file_len + coreThreadNum - 1) / coreThreadNum;            // 计算每个线程要下载的长度

mLogger.info("filename:"+filename+",total_file_len="+total_file_len+",coreThreadNum:"+coreThreadNum+",thread_download_len:"+thread_download_len);

CountDownLatch latch = new CountDownLatch(coreThreadNum);//两个工人的协作

for (int i = 0; i < coreThreadNum; i++){

DownloadWorker worker = new DownloadWorker(i, url, thread_download_len, file, httpRequestImpl, latch);
worker.addListener(new DownloadListener() {

@Override
public void notify(int thread_id, String url, long start, long end,
boolean result, String msg) {

mLogger.info("thread_id:"+thread_id+" download result:"+result+",url->"+url);

modifyState(downloadIndicatorBitSet, thread_id);
}
});

pool.execute(worker);
}

try {
latch.await();
} catch (InterruptedException e) {
mLogger.error("CountDownLatch Interrupt", e);
}

mLogger.info("下载结束,url:"+url+",耗时:"+((System.currentTimeMillis()-start_time)/1000)+"(s)");

return downloadIndicatorBitSet.cardinality()==coreThreadNum;
}

private synchronized void modifyState(BitSet bitSet, int index){
bitSet.set(index);
}

/**释放资源*/
public void close(){

if(httpRequestImpl!=null){
httpRequestImpl.close();
httpRequestImpl = null;
}
if(pool!=null){
pool.shutdown();
pool = null;
}

}

}

com.ricky.common.java.download.job.DownloadWorker

package com.ricky.common.java.download.job;

import java.io.File;
import java.util.concurrent.CountDownLatch;

import org.apache.log4j.Logger;

import com.ricky.common.java.download.HttpRequestImpl;
import com.ricky.common.java.download.RetryFailedException;

public class DownloadWorker extends Worker {

protected Logger mLogger = Logger.getLogger("devLog");

private int id;
private String url;
private File file;
private long thread_download_len;

private CountDownLatch latch;

private HttpRequestImpl httpRequestImpl;

public DownloadWorker(int id, String url, long thread_download_len, File file, HttpRequestImpl httpRequestImpl, CountDownLatch latch) {
this.id = id;
this.url = url;
this.thread_download_len = thread_download_len;
this.file = file;
this.httpRequestImpl = httpRequestImpl;
this.latch = latch;
}

@Override
public void run() {

long start = id * thread_download_len;                      // 起始位置
long end = id * thread_download_len + thread_download_len - 1;      // 结束位置

mLogger.info("线程:" + id +" 开始下载 url:"+url+ ",range:" + start + "-" + end);

boolean result = false;
try {
httpRequestImpl.downloadPartFile(id, url, file, start, end);
result = true;
mLogger.info("线程:" + id + " 下载 "+url+ " range[" + start + "-" + end+"] 成功");

} catch (RetryFailedException e) {
mLogger.error("线程:" + id +" 重试出错", e);
}catch (Exception e) {
mLogger.error("线程:" + id +" 下载出错", e);
}

if(listener!=null){
mLogger.info("notify FileDownloaderEngine download result");
listener.notify(id, url, start, end, result, "");
}

latch.countDown();
}

}

com.ricky.common.java.download.job.Worker

package com.ricky.common.java.download.job;

public abstract class Worker implements Runnable {

protected DownloadListener listener;

public void addListener(DownloadListener listener){
this.listener = listener;
}

public interface DownloadListener{

public void notify(int thread_id, String url, long start, long end, boolean result, String msg);
}
}

com.ricky.common.java.download.HttpRequestImpl

package com.ricky.common.java.download;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;

import org.apache.commons.io.IOUtils;
import org.apache.http.HttpEntity;
import org.apache.http.HttpStatus;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.log4j.Logger;

import com.ricky.common.java.download.config.FileDownloaderConfiguration;
import com.ricky.common.java.http.HttpClientManager;

public class HttpRequestImpl {

protected Logger mLogger = Logger.getLogger("devLog");

private int connectTimeout;
private int socketTimeout;
private int maxRetryCount;
private long requestBytesSize;

private CloseableHttpClient httpclient = HttpClientManager.getHttpClient();

public HttpRequestImpl(FileDownloaderConfiguration configuration){
connectTimeout = configuration.getConnectTimeout();
socketTimeout = configuration.getSocketTimeout();
maxRetryCount = configuration.getMaxRetryCount();
requestBytesSize = configuration.getRequestBytesSize();
}

public void downloadPartFile(int id, String url, File file, long start, long end){

RandomAccessFile raf = null;
try {
raf = new RandomAccessFile(file, "rws");
} catch (FileNotFoundException e) {
mLogger.error("file not found:"+file, e);
throw new IllegalArgumentException(e);
}

int retry = 0;
long pos = start;
while(pos<end){

long end_index = pos + requestBytesSize;
if(end_index>end){
end_index = end;
}

boolean success = false;
try {
success = requestByRange(url, raf, pos, end_index);
} catch (ClientProtocolException e) {
mLogger.error("download error,start:"+pos+",end:"+end_index, e);
}catch (IOException e) {
mLogger.error("download error,start:"+pos+",end:"+end_index, e);
}catch (Exception e) {
mLogger.error("download error,start:"+pos+",end:"+end_index, e);
}

//          mLogger.info("线程:" + id +",download url:"+url+",range:"+ pos + "-" + end_index+",success="+success );

if(success){
pos += requestBytesSize;
retry = 0;
}else{
if(retry < maxRetryCount){
retry++;
mLogger.warn("线程:" + id +",url:"+url+",range:"+pos+","+end_index+" 下载失败,重试"+retry+"次");
}else{
mLogger.warn("线程:" + id +",url:"+url+",range:"+pos+","+end_index+" 下载失败,放弃重试!");
throw new RetryFailedException("超过最大重试次数");
}
}
}

}

private boolean requestByRange(String url, RandomAccessFile raf, long start, long end) throws ClientProtocolException, IOException {

HttpGet httpget = new HttpGet(url);
httpget.setHeader("User-Agent", "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/42.0.2311.152 Safari/537.36");
httpget.setHeader("Range", "bytes=" + start + "-" + end);

RequestConfig requestConfig = RequestConfig.custom()
.setConnectTimeout(connectTimeout)
.setSocketTimeout(socketTimeout)
.build();

httpget.setConfig(requestConfig);

CloseableHttpResponse response = null;
try {
response = httpclient.execute(httpget);

int code = response.getStatusLine().getStatusCode();

if(code==HttpStatus.SC_OK || code== HttpStatus.SC_PARTIAL_CONTENT){

HttpEntity entity = response.getEntity();

if (entity != null) {

InputStream in = entity.getContent();
raf.seek(start);// 设置保存数据的位置

byte[] buffer = new byte[1024];
int len;
while ((len = in.read(buffer)) != -1){
raf.write(buffer, 0, len);
}

return true;
}else{
mLogger.warn("response entity is null,url:"+url);
}
}else{
mLogger.warn("response error, code="+code+",url:"+url);
}
}finally {
IOUtils.closeQuietly(response);
}

return false;
}

public long getFileSize(String url){

int retry = 0;
long filesize = 0;
while(retry<maxRetryCount){
try {
filesize = getContentLength(url);
} catch (Exception e) {
mLogger.error("get File Size error", e);
}

if(filesize>0){
break;
}else{
retry++;
mLogger.warn("get File Size failed,retry:"+retry);
}
}

return filesize;
}

private long getContentLength(String url) throws ClientProtocolException, IOException{

HttpGet httpget = new HttpGet(url);
httpget.setHeader("User-Agent", "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/42.0.2311.152 Safari/537.36");

RequestConfig requestConfig = RequestConfig.custom()
.setConnectTimeout(connectTimeout)
.setSocketTimeout(socketTimeout)
.build();

httpget.setConfig(requestConfig);

CloseableHttpResponse response = null;
try {
response = httpclient.execute(httpget);

int code = response.getStatusLine().getStatusCode();

if(code==HttpStatus.SC_OK){

HttpEntity entity = response.getEntity();

if (entity != null) {
return entity.getContentLength();
}
}else{
mLogger.warn("response code="+code);
}

}finally {
IOUtils.closeQuietly(response);
}

return -1;
}

public void close(){

if(httpclient!=null){
try {
httpclient.close();
} catch (IOException e) {
e.printStackTrace();
}
httpclient = null;
}
}
}

最后是客户端调用代码

package com.ricky.common.java;

import java.io.File;

import com.ricky.common.java.download.FileDownloader;
import com.ricky.common.java.download.config.FileDownloaderConfiguration;

public class FileDownloaderTest {

public static void main(String[] args) {

FileDownloader fileDownloader = FileDownloader.getInstance();
FileDownloaderConfiguration configuration = FileDownloaderConfiguration
.custom()
.coreThreadNum(5)
.downloadDestinationDir(new File("D:/Download"))
.build();
fileDownloader.init(configuration);

String url = "http://dldir1.qq.com/qqfile/qq/QQ7.9/16621/QQ7.9.exe";;
String filename = "QQ7.9.exe";

boolean result = fileDownloader.download(url, filename);

System.out.println("download result:"+result);

fileDownloader.destroy();   //close it when you not need
}
}

源代码

https://github.com/TiFG/FileDownloader