使用Java多线程将大文件分解，并下载大文件

主要思路是将大文件分解成若干个小文件进行下载.

本功能由两个类实现:DownLoadManager和DownloadThreadTask实现. 其中DownLoadManager类主要负责下载任务的初始化和调度, DownloadThreadTask主要负责处理下载任务.

用到的主要工具类有

1. org.apache.http.impl.client.CloseableHttpClient 模拟httpClient客户端发送http请求，可以控制到请求文件的字节位置。

2. BufferedInputStream都熟悉，用它接受请求来的流信息缓存。

3. RandomAccessFile文件随机类，可以向文件写入指定位置的流信息。

代码如下:

DownLoadManager

package util;

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import org.apache.commons.lang.exception.ExceptionUtils;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
 * 文件下载管理类
 */
public class DownLoadManager{
    private static final Logger LOGGER = LoggerFactory.getLogger(DownLoadManager.class);
    /**
     * 每个线程下载的字节数
     */
    private long unitSize = 1000 * 1024;
    private ExecutorService taskExecutor = Executors.newFixedThreadPool(10);
    
    private CloseableHttpClient httpClient;

    public DownLoadManager() {
        PoolingHttpClientConnectionManager cm = new PoolingHttpClientConnectionManager();
        cm.setMaxTotal(100);
        httpClient = HttpClients.custom().setConnectionManager(cm).build();
    }
    public static void main(String[] args) throws IOException {
        new DownLoadManager().doDownload();
    }
    /**
     * 启动多个线程下载文件
     */
    public void doDownload() throws IOException {
        //要下载的url
        String remoteFileUrl = "http://cn-sdjn-cu-v-04.acgvideo.com/vg4/e/60/21029304-1.mp4?expires=1504513500&platform=pc&ssig=4-pYfuQHUKppz3XGrw6Hnw&oi=1032322900&nfa=T7l/1XWXSxfil4KoioiGpQ==&dynamic=1&hfa=2073962963&hfb=Yjk5ZmZjM2M1YzY4ZjAwYTMzMTIzYmIyNWY4ODJkNWI=";
        String localPath = "E://temp//";
        String fileName = new URL(remoteFileUrl).getFile();
        System.out.println("远程文件名称：" + fileName);
        fileName = fileName.substring(fileName.lastIndexOf("/") + 1, fileName.length()).replace("%20", " ");

        System.out.println("本地文件名称：" + fileName);
        long fileSize = this.getRemoteFileSize(remoteFileUrl);
        this.createFile(localPath + System.currentTimeMillis() + fileName, fileSize);
        Long threadCount = (fileSize / unitSize) + (fileSize % unitSize != 0 ? 1 : 0);
        long offset = 0;
        
        CountDownLatch end = new CountDownLatch(threadCount.intValue());
        if (fileSize <= unitSize) {// 如果远程文件尺寸小于等于unitSize
            DownloadThreadTask downloadThread = new DownloadThreadTask(remoteFileUrl, localPath + fileName, offset, fileSize, end, httpClient);
            taskExecutor.execute(downloadThread);
        } else {// 如果远程文件尺寸大于unitSize
            for (int i = 1; i < threadCount; i++) {
                DownloadThreadTask downloadThread = new DownloadThreadTask(remoteFileUrl, localPath + fileName, offset, unitSize, end, httpClient);
                taskExecutor.execute(downloadThread);
                offset = offset + unitSize;
            }
            if (fileSize % unitSize != 0) {// 如果不能整除，则需要再创建一个线程下载剩余字节
                DownloadThreadTask downloadThread = new DownloadThreadTask(remoteFileUrl, localPath + fileName, offset, fileSize - unitSize * (threadCount - 1), end, httpClient);
                taskExecutor.execute(downloadThread);
            }
        }
        try {
            end.await();
        } catch (InterruptedException e) {
            LOGGER.error("DownLoadManager exception msg:{}", ExceptionUtils.getFullStackTrace(e));
            e.printStackTrace();
        }
        taskExecutor.shutdown();
        LOGGER.debug("下载完成！{} ", localPath + fileName);
    }

    /**
     * 获取远程文件尺寸
     */
    private long getRemoteFileSize(String remoteFileUrl) throws IOException {
        long fileSize = 0;
        HttpURLConnection httpConnection = (HttpURLConnection) new URL(remoteFileUrl).openConnection();
        //使用HEAD方法
        httpConnection.setRequestMethod("HEAD");
        int responseCode = httpConnection.getResponseCode();
        if (responseCode >= 400) {
            LOGGER.debug("Web服务器响应错误!");
            return 0;
        }
        String sHeader;
        for (int i = 1;; i++) {
            sHeader = httpConnection.getHeaderFieldKey(i);
            if (sHeader != null && sHeader.equals("Content-Length")) {
                System.out.println("文件大小ContentLength:" + httpConnection.getContentLength());
                fileSize = Long.parseLong(httpConnection.getHeaderField(sHeader));
                break;
            }
        }
        return fileSize;
    }

    /**
     * 创建指定大小的文件
     */
    private void createFile(String fileName, long fileSize) throws IOException {
        File newFile = new File(fileName);
        RandomAccessFile raf = new RandomAccessFile(newFile, "rw");
        raf.setLength(fileSize);
        raf.close();
    }
}

DownloadThreadTask

package util;

import java.io.BufferedInputStream;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.util.concurrent.CountDownLatch;
import org.apache.commons.lang.exception.ExceptionUtils;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.protocol.BasicHttpContext;
import org.apache.http.protocol.HttpContext;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * 负责文件下载的类
 */
public class DownloadThreadTask implements Runnable {

    private static final Logger LOGGER = LoggerFactory.getLogger(DownloadThreadTask.class);

    /**
     * 待下载的文件
     */
    private String url = null;

    /**
     * 本地文件名
     */
    private String fileName = null;

    /**
     * 偏移量
     */
    private long offset = 0;

    /**
     * 分配给本线程的下载字节数
     */
    private long length = 0;
    
    private CountDownLatch end;
    private CloseableHttpClient httpClient;
    private HttpContext context;

    /**
     * @param url
     *            下载文件地址
     * @param fileName
     *            另存文件名
     * @param offset
     *            本线程下载偏移量
     * @param length
     *            本线程下载长度
     */
    public DownloadThreadTask(String url, String file, long offset, long length, CountDownLatch end, CloseableHttpClient httpClient) {
        this.url = url;
        this.fileName = file;
        this.offset = offset;
        this.length = length;
        this.end = end;
        this.httpClient = httpClient;
        this.context = new BasicHttpContext();
        LOGGER.debug("偏移量=" + offset + ";字节数=" + length);
    }
    
    public void run() {
        try {
            HttpGet httpGet = new HttpGet(this.url);
            httpGet.addHeader("Range", "bytes=" + this.offset + "-" + (this.offset + this.length - 1));
            httpGet.addHeader("Referer", "http://api.bilibili.com");
            CloseableHttpResponse response = httpClient.execute(httpGet, context);
            BufferedInputStream bis = new BufferedInputStream(response.getEntity().getContent());
            byte[] buff = new byte[1024];
            int bytesRead;
            File newFile = new File(fileName);
            RandomAccessFile raf = new RandomAccessFile(newFile, "rw");
            while ((bytesRead = bis.read(buff, 0, buff.length)) != -1) {
                raf.seek(this.offset);
                raf.write(buff, 0, bytesRead);
                this.offset = this.offset + bytesRead;
            }
            raf.close();
            bis.close();
        } catch (ClientProtocolException e) {
            LOGGER.error("DownloadThread exception msg:{}", ExceptionUtils.getFullStackTrace(e));
        } catch (IOException e) {
            LOGGER.error("DownloadThread exception msg:{}", ExceptionUtils.getFullStackTrace(e));
        } finally {
            end.countDown();
            LOGGER.info(end.getCount() + " is go on!");
            System.out.println(end.getCount() + " is go on!");
        }
    }
}