Java Http断点续传（下载）

先说使用Java实现断点续传的关键点：

• 本地能够判断当前请求下载的文件是否已经存在，或者部分存在；
• 如果存在且为部分，则本地可以记录下载位置，并从服务器端继续完成下载；

技术上的关键：

• 请求的需要：

  URL url = new URL("urlStr"); HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection//向服务器请求指定区间段的数据，这是实现断点续传的根本。httpcon.setRequestProperty("Range", "bytes=" + startPos+ "-" + endPos);

  
  
• 保存文件的方法：采用IO包中的RandomAccessFile类：

  RandomAccess downfile = new RandomAccessFile("file","rw"); Array[] startPos ;downloadfile.seek(startPos);byte[] buf = new byte[1024]; downloadfile.write(buf, 0, length);

  
  

下面是功能实现的代码以及测试类，在测试类中传入实现类中的两个参数是线程数与文件来源（链接），断点续传的体现可以在下载过程中关闭IDE，在工程文件目录下，可以看到未下载完全的部分文件以及临时文件。重新启动IDE运行程序，文件继续下载，以下程序基本行行注释，在后面我还会说明一些：

一、功能实现类MultiThreadDownLoad

package **********;import java.io.File;import java.io.FileNotFoundException;import java.io.IOException;import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.MalformedURLException;import java.net.URL;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;/* * Encode:UTF-8 *  * Author:GFC *  * 根据输入的url和设定的线程数，来完成断点续传功能。 *  * 每个线程支负责某一小段的数据下载；再通过RandomAccessFile完成数据的整合。 */public class MultiTheradDownLoad {private String urlStr = null;    private String filename = null;    private String tmpfilename = null;    private int threadNum = 0;    private CountDownLatch latch = null;//设置一个计数器，代码内主要用来完成对缓存文件的删除    private long fileLength = 0l;    private long threadLength = 0l;    private long[] startPos;//保留每个线程下载数据的起始位置。    private long[] endPos;//保留每个线程下载数据的截止位置。    private boolean bool = false;    private URL url = null;    //有参构造函数，先构造需要的数据    public MultiTheradDownLoad(String urlStr, int threadNum) {        this.urlStr = urlStr;        this.threadNum = threadNum;        startPos = new long[this.threadNum];        endPos = new long[this.threadNum];        latch = new CountDownLatch(this.threadNum);    }    /*     * 组织断点续传功能的方法     */    public void downloadPart() {        File file = null;        File tmpfile = null;            //设置HTTP网络访问代理    System.setProperty("http.proxySet", "true");     System.setProperty("http.proxyHost", "proxy3.bj.petrochina");   System.setProperty("http.proxyPort", "8080");         //从文件链接中获取文件名，此处没考虑文件名为空的情况，此种情况可能需使用UUID来生成一个唯一数来代表文件名。        filename = urlStr.substring(urlStr.lastIndexOf('/') + 1, urlStr                .contains("?") ? urlStr.lastIndexOf('?') : urlStr.length());        tmpfilename = filename + "_tmp";        try {        //创建url            url = new URL(urlStr);                        //打开下载链接，并且得到一个HttpURLConnection的一个对象httpcon            HttpURLConnection httpcon = (HttpURLConnection) url.openConnection();            httpcon.setRequestMethod("GET");                        //获取请求资源的总长度。            fileLength = httpcon.getContentLengthLong();            //下载文件和临时文件            file = new File(filename);//相对目录            tmpfile = new File(tmpfilename);            //每个线程需下载的资源大小；由于文件大小不确定，为避免数据丢失            threadLength = fileLength%threadNum == 0 ? fileLength/threadNum : fileLength/threadNum+1;            //打印下载信息            System.out.println("fileName: " + filename + " ," + "fileLength= "                    + fileLength + " the threadLength= " + threadLength);                        //各个线程在exec线程池中进行，起始位置--结束位置            if (file.exists() && file.length() == fileLength) {                System.out.println("文件已存在!!");                return;            } else {                setBreakPoint(startPos, endPos, tmpfile);                ExecutorService exec = Executors.newCachedThreadPool();                for (int i = 0; i < threadNum; i++) {                    exec.execute(new DownLoadThread(startPos[i], endPos[i],                            this, i, tmpfile, latch));                }                latch.await();//当你的计数器减为0之前，会在此处一直阻塞。                exec.shutdown();            }        } catch (MalformedURLException e) {            e.printStackTrace();        } catch (IOException e) {            e.printStackTrace();        } catch (InterruptedException e) {            e.printStackTrace();        }                //下载完成后，判断文件是否完整，并删除临时文件        if (file.length() == fileLength) {            if (tmpfile.exists()) {                System.out.println("删除临时文件!!");                tmpfile.delete();            }        }    }    /*     * 断点设置方法，当有临时文件时，直接在临时文件中读取上次下载中断时的断点位置。没有临时文件，即第一次下载时，重新设置断点。     *      * Rantmpfile.seek()跳转到一个位置的目的是为了让各个断点存储的位置尽量分开。     *      * 这是实现断点续传的重要基础。     */    private void setBreakPoint(long[] startPos, long[] endPos, File tmpfile) {        RandomAccessFile rantmpfile = null;        try {            if (tmpfile.exists()) {                System.out.println("继续下载!!");                rantmpfile = new RandomAccessFile(tmpfile, "rw");                for (int i = 0; i < threadNum; i++) {                    rantmpfile.seek(8 * i + 8);                    startPos[i] = rantmpfile.readLong();                    rantmpfile.seek(8 * (i + 1000) + 16);                    endPos[i] = rantmpfile.readLong();                    System.out.println("the Array content in the exit file: ");                    System.out.println("thre thread" + (i + 1) + " startPos:"                            + startPos[i] + ", endPos: " + endPos[i]);                }            } else {                System.out.println("the tmpfile is not available!!");                rantmpfile = new RandomAccessFile(tmpfile, "rw");                                //最后一个线程的截止位置大小为请求资源的大小                for (int i = 0; i < threadNum; i++) {                    startPos[i] = threadLength * i;                    if (i == threadNum - 1) {                        endPos[i] = fileLength;                    } else {                        endPos[i] = threadLength * (i + 1) - 1;                    }                    rantmpfile.seek(8 * i + 8);                    rantmpfile.writeLong(startPos[i]);                    rantmpfile.seek(8 * (i + 1000) + 16);                    rantmpfile.writeLong(endPos[i]);                    System.out.println("the Array content: ");                    System.out.println("thre thread" + (i + 1) + " startPos:"                            + startPos[i] + ", endPos: " + endPos[i]);                }            }        } catch (FileNotFoundException e) {            e.printStackTrace();        } catch (IOException e) {            e.printStackTrace();        } finally {            try {                if (rantmpfile != null) {                    rantmpfile.close();                }            } catch (IOException e) {                e.printStackTrace();            }        }    }        /*     * 实现下载功能的内部类，通过读取断点来设置向服务器请求的数据区间。     */    class DownLoadThread implements Runnable {        private long startPos;        private long endPos;        private MultiTheradDownLoad task = null;        private RandomAccessFile downloadfile = null;        private int id;        private File tmpfile = null;        private RandomAccessFile rantmpfile = null;        private CountDownLatch latch = null;        public DownLoadThread(long startPos, long endPos,                MultiTheradDownLoad task, int id, File tmpfile,                CountDownLatch latch) {            this.startPos = startPos;            this.endPos = endPos;            this.task = task;            this.tmpfile = tmpfile;            try {            //                this.downloadfile = new RandomAccessFile(this.task.filename,"rw");                this.rantmpfile = new RandomAccessFile(this.tmpfile, "rw");            } catch (FileNotFoundException e) {                e.printStackTrace();            }            this.id = id;            this.latch = latch;        }        @Override        public void run() {            HttpURLConnection httpcon = null;            InputStream is = null;            int length = 0;            System.out.println("线程" + id + " 开始下载!!");            while (true) {                try {                    httpcon = (HttpURLConnection) task.url.openConnection();                    httpcon.setRequestMethod("GET");                                        //防止网络阻塞，设置指定的超时时间；单位都是ms。超过指定时间，就会抛出异常                    httpcon.setReadTimeout(20000);//读取数据的超时设置                    httpcon.setConnectTimeout(20000);//连接的超时设置                    if (startPos < endPos) {                                                //向服务器请求指定区间段的数据，这是实现断点续传的根本。                        httpcon.setRequestProperty("Range", "bytes=" + startPos+ "-" + endPos);                        System.out.println("线程 " + id+ " 长度:---- "+ (endPos - startPos));                        downloadfile.seek(startPos);                        if (httpcon.getResponseCode() != HttpURLConnection.HTTP_OK                                && httpcon.getResponseCode() != HttpURLConnection.HTTP_PARTIAL) {                            this.task.bool = true;                            httpcon.disconnect();                            downloadfile.close();                            System.out.println("线程 ---" + id + " 下载完成!!");                            latch.countDown();//计数器自减                            break;                        }                        is = httpcon.getInputStream();//获取服务器返回的资源流                        long count = 0l;                        byte[] buf = new byte[1024];                        while (!this.task.bool && (length = is.read(buf)) != -1) {                            count += length;                            downloadfile.write(buf, 0, length);                                                        //不断更新每个线程下载资源的起始位置，并写入临时文件；为断点续传做准备                            startPos += length;                            rantmpfile.seek(8 * id + 8);                            rantmpfile.writeLong(startPos);                        }                        System.out.println("线程 " + id                                + " 总下载大小: " + count);                                                //关闭流                        is.close();                        httpcon.disconnect();                        downloadfile.close();                        rantmpfile.close();                    }                    latch.countDown();//计数器自减                    System.out.println("线程 " + id + " 下载完成!!");                    break;                } catch (IOException e) {                    e.printStackTrace();                } finally {                    try {                        if (is != null) {                            is.close();                        }                    } catch (IOException e) {                        e.printStackTrace();                    }                }            }        }    }}

其中设置HTTP访问代理在公网上不需要。可以看出来，整个类中分为三大部分：下载功能方法downloadPart();设置断点的私有方法setBreakPoint(long[] startPos, long[] endPos, File tmpfile)；以及实现下载功能的内部类，通过读取断点来设置向服务器请求的数据区间class DownLoadThread,其中重写了线程的run()方法。

二、测试类 

package Download;public class DownLoadTest {public static void main(String[] args) {//http://down.sandai.net/thunder9/Thunder9.1.40.898.exe//http://download.firefox.com.cn/releases-sha2/stub/official/zh-CN/Firefox-latest.exe//http://p0.ifengimg.com/pmop/2017/0830/4AB0880C38002DC837856D274469E3D09BDF43B9_size28_w500_h314.jpeg//http://img.article.pchome.net/00/22/70/23/pic_lib/wm/03.jpgint threadNum = 4;        String filepath = "http://down.sandai.net/thunder9/Thunder9.1.40.898.exe";                MultiTheradDownLoad load = new MultiTheradDownLoad(filepath ,threadNum);            load.downloadPart();        }}