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(poslong 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(retrytry {
                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