- 多线程下载
原理：服务器CPU分配给每条线程的时间片相同，服务器带宽平均分配给每条线程，所以客户端开启的线程越多，就能抢占到更多的服务器资源，所以使用多线程下载的话，速度会更快。
JavaSE实现带断点续传的多线程下载步骤：
1、发送http请求至下载地址，获取要下载的资源文件的大小
2、根据资源文件的大小，创建一个长度一样的临时文件，用来抢占磁盘空间
3、计算每个线程要下载的数据大小和开始位置、结束位置，余数都由最后一个线程完成下载，所以最后一个线程的结束位置要写死
4、再次发送请求，请求要下载的数据区间的数据（判断是否有记录进度的临时文件，有的话就继续上次位置接着下载，没有就从原本开始位置下载）
5、将下载请求到的数据，存储到临时文件中（新建一个记录下载进度的临时文件）
6、等所有线程都下载完毕了，就要将之前的记录进度的临时文件删除掉
1 package com.ahu.multithreaddownload; 2 3 import java.io.BufferedReader; 4 import java.io.File; 5 import java.io.FileInputStream; 6 import java.io.InputStream; 7 import java.io.InputStreamReader; 8 import java.io.RandomAccessFile; 9 import java.net.HttpURLConnection; 10 import java.net.URL; 11 12 /** 13 * 带断点续传的多线程下载 14 * 15 * @author ahu_lichang 16 * 17 */ 18 public class MultiThreadDownload { 19 // 线程数 20 static int ThreadCount = 4; 21 static int finishedThread = 0; 22 23 static String path = "http://172.23.13.179:8080/report.ppt"; 24 25 public static void main(String[] args) { 26 try { 27 // 第一次请求服务器，是为了获取资源文件的大小，从而创建一个相同大小的临时文件，而不是下载资源！ 28 URL url = new URL(path); 29 HttpURLConnection connection = (HttpURLConnection) url 30 .openConnection(); 31 connection.setConnectTimeout(5000); 32 connection.setReadTimeout(5000); 33 connection.setRequestMethod("GET"); 34 //connection.connect(); 35 if (connection.getResponseCode() == 200) { 36 // 获取要下载文件的大小 37 int length = connection.getContentLength(); 38 // 生成临时文件 39 RandomAccessFile raf = new RandomAccessFile(getFileName(path), 40 "rwd"); 41 raf.setLength(length); 42 raf.close(); 43 // 计算每个线程下载的资源大小（有可能存在余数，余数都放在最后一个线程里） 44 int size = length / ThreadCount; 45 // 计算每个线程下载的开始位置和结束位置 46 for (int i = 0; i < ThreadCount; i++) { 47 int startIndex = i * size; 48 int endIndex = (i + 1) * size - 1; 49 // 如果是最后一个线程，必须将结束位置写死 50 if (i == ThreadCount - 1) { 51 endIndex = length - 1; 52 } 53 // 开启线程下载资源 54 new DownloadThread(startIndex, endIndex, i).start(); 55 } 56 } 57 } catch (Exception e) { 58 e.printStackTrace(); 59 } 60 61 } 62 63 /** 64 * 获取服务器中资源的名称 65 * 66 * @param path 67 * @return 68 */ 69 public static String getFileName(String path) { 70 int index = path.lastIndexOf("/"); 71 return path.substring(index + 1); 72 } 73 74 } 75 /** 76 * 下载线程 77 * @author ahu_lichang 78 * 79 */ 80 class DownloadThread extends Thread { 81 int startIndex; 82 int endIndex; 83 int threadId; 84 85 public DownloadThread(int startIndex, int endIndex, int threadId) { 86 super(); 87 this.startIndex = startIndex; 88 this.endIndex = endIndex; 89 this.threadId = threadId; 90 } 91 92 public void run() { 93 try { 94 File progressFile = new File(threadId + ".txt"); 95 //如果存在进度临时文件，就接着上次的后面进行下载 96 if (progressFile.exists()) { 97 FileInputStream fis = new FileInputStream(progressFile); 98 BufferedReader br = new BufferedReader(new InputStreamReader( 99 fis));100 // 得到上一次下载的总进度，然后与原本的开始位置相加，得到新的开始位置101 startIndex += Integer.parseInt(br.readLine());102 fis.close();103 }104 System.out.println("线程" + threadId + "下载区间" + startIndex + "---"105 + endIndex);106 // 再次请求服务器，下载资源文件107 URL url = new URL(MultiThreadDownload.path);108 HttpURLConnection connection = (HttpURLConnection) url109 .openConnection();110 connection.setConnectTimeout(5000);111 connection.setReadTimeout(5000);112 connection.setRequestMethod("GET");113 // 设置本次所请求的数据区间114 connection.setRequestProperty("Range", "bytes=" + startIndex + "-"115 + endIndex);116 //connection.connect();117 // 请求部分数据的响应码是206118 if (connection.getResponseCode() == 206) {119 // 拿到1/3源文件的数据120 InputStream is = connection.getInputStream();121 byte[] b = new byte[1024];122 int len = 0;123 int total = 0;// 记录下载到临时文件中数据的大小124 // 把拿到的数据写入到临时文件中125 RandomAccessFile raf = new RandomAccessFile(126 MultiThreadDownload127 .getFileName(MultiThreadDownload.path),128 "rwd");129 // 把文件的写入位置移动至startIndex。这样才不会覆盖写入130 raf.seek(startIndex);131 while ((len = is.read(b)) != -1) {132 raf.write(b, 0, len);133 total += len;134 // 生成用来记录下载进度的临时文件135 RandomAccessFile progressRaf = new RandomAccessFile(136 progressFile, "rwd");137 progressRaf.write((total + "").getBytes());138 progressRaf.close();139 }140 System.out.println("线程" + threadId + "下载完毕！！！");141 raf.close();142 143 // 全部下载完成后，将临时存放进度的文件删除144 MultiThreadDownload.finishedThread++;145 // 注意线程安全问题146 synchronized (MultiThreadDownload.path) {147 if (MultiThreadDownload.finishedThread == MultiThreadDownload.ThreadCount) {148 for (int i = 0; i < MultiThreadDownload.ThreadCount; i++) {149 File f = new File(i + ".txt");150 f.delete();151 }152 MultiThreadDownload.finishedThread = 0;153 }154 }155 }156 } catch (Exception e) {157 e.printStackTrace();158 }159 }160 }
- Android上实现带断点续传的多线程下载
布局文件：
1 213 18 19 24 29 30
MainActivity:
1 package com.ahu.lichang.multithreaddownload; 2 3 import android.app.Activity; 4 import android.os.Bundle; 5 import android.os.Environment; 6 import android.os.Handler; 7 import android.view.View; 8 import android.widget.ProgressBar; 9 import android.widget.TextView; 10 11 import java.io.BufferedReader; 12 import java.io.File; 13 import java.io.FileInputStream; 14 import java.io.InputStream; 15 import java.io.InputStreamReader; 16 import java.io.RandomAccessFile; 17 import java.net.HttpURLConnection; 18 import java.net.URL; 19 20 public class MainActivity extends Activity { 21 static int ThreadCount = 3; 22 static int finishedThread = 0; 23 24 int currentProgress; 25 String fileName = "QQPlayer.exe"; 26 //确定下载地址 27 String path = "http://172.23.13.179:8080/" + fileName; 28 private ProgressBar pb; 29 TextView tv; 30 31 Handler handler = new Handler(){ 32 public void handleMessage(android.os.Message msg) { 33 //把变量改成long，在long下运算 34 tv.setText((long)pb.getProgress() * 100 / pb.getMax() + "%"); 35 } 36 }; 37 @Override 38 protected void onCreate(Bundle savedInstanceState) { 39 super.onCreate(savedInstanceState); 40 setContentView(R.layout.activity_main); 41 pb = (ProgressBar) findViewById(R.id.pb); 42 tv = (TextView) findViewById(R.id.tv); 43 } 44 public void download(View view){ 45 Thread t = new Thread(){ 46 @Override 47 public void run() { 48 //发送get请求，请求这个地址的资源 49 try { 50 URL url = new URL(path); 51 HttpURLConnection conn = (HttpURLConnection) url.openConnection(); 52 conn.setRequestMethod("GET"); 53 conn.setConnectTimeout(5000); 54 conn.setReadTimeout(5000); 55 56 if(conn.getResponseCode() == 200){ 57 //拿到所请求资源文件的长度 58 int length = conn.getContentLength(); 59 60 //设置进度条的最大值就是原文件的总长度 61 pb.setMax(length); 62 63 File file = new File(Environment.getExternalStorageDirectory(), fileName); 64 //生成临时文件 65 RandomAccessFile raf = new RandomAccessFile(file, "rwd"); 66 //设置临时文件的大小 67 raf.setLength(length); 68 raf.close(); 69 //计算出每个线程应该下载多少字节 70 int size = length / ThreadCount; 71 for (int i = 0; i < ThreadCount; i++) { 72 //计算线程下载的开始位置和结束位置 73 int startIndex = i * size; 74 int endIndex = (i + 1) * size - 1; 75 //如果是最后一个线程，那么结束位置写死 76 if(i == ThreadCount - 1){ 77 endIndex = length - 1; 78 } 79 new DownLoadThread(startIndex, endIndex, i).start(); 80 } 81 } 82 } catch (Exception e) { 83 e.printStackTrace(); 84 } 85 } 86 }; 87 t.start(); 88 } 89 class DownLoadThread extends Thread{ 90 int startIndex; 91 int endIndex; 92 int threadId; 93 94 public DownLoadThread(int startIndex, int endIndex, int threadId) { 95 super(); 96 this.startIndex = startIndex; 97 this.endIndex = endIndex; 98 this.threadId = threadId; 99 }100 101 @Override102 public void run() {103 //再次发送http请求，下载原文件104 try {105 File progressFile = new File(Environment.getExternalStorageDirectory(), threadId + ".txt");106 //判断进度临时文件是否存在107 if(progressFile.exists()){108 FileInputStream fis = new FileInputStream(progressFile);109 BufferedReader br = new BufferedReader(new InputStreamReader(fis));110 //从进度临时文件中读取出上一次下载的总进度，然后与原本的开始位置相加，得到新的开始位置111 int lastProgress = Integer.parseInt(br.readLine());112 startIndex += lastProgress;113 114 //把上次下载的进度显示至进度条115 currentProgress += lastProgress;116 pb.setProgress(currentProgress);117 //发送消息，让主线程刷新文本进度118 handler.sendEmptyMessage(1);119 120 fis.close();121 }122 System.out.println("线程" + threadId + "的下载区间是：" + startIndex + "---" + endIndex);123 HttpURLConnection conn;124 URL url = new URL(path);125 conn = (HttpURLConnection) url.openConnection();126 conn.setRequestMethod("GET");127 conn.setConnectTimeout(5000);128 conn.setReadTimeout(5000);129 //设置本次http请求所请求的数据的区间130 conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);131 132 //请求部分数据，相应码是206133 if(conn.getResponseCode() == 206){134 //流里此时只有1/3原文件的数据135 InputStream is = conn.getInputStream();136 byte[] b = new byte[1024];137 int len = 0;138 int total = 0;139 //拿到临时文件的输出流140 File file = new File(Environment.getExternalStorageDirectory(), fileName);141 RandomAccessFile raf = new RandomAccessFile(file, "rwd");142 //把文件的写入位置移动至startIndex143 raf.seek(startIndex);144 while((len = is.read(b)) != -1){145 //每次读取流里数据之后，同步把数据写入临时文件146 raf.write(b, 0, len);147 total += len;148 System.out.println("线程" + threadId + "下载了" + total);149 150 //每次读取流里数据之后，把本次读取的数据的长度显示至进度条151 currentProgress += len;152 pb.setProgress(currentProgress);153 //发送消息，让主线程刷新文本进度154 handler.sendEmptyMessage(1);155 156 //生成一个专门用来记录下载进度的临时文件157 RandomAccessFile progressRaf = new RandomAccessFile(progressFile, "rwd");158 //每次读取流里数据之后，同步把当前线程下载的总进度写入进度临时文件中159 progressRaf.write((total + "").getBytes());160 progressRaf.close();161 }162 System.out.println("线程" + threadId + "下载完毕");163 raf.close();164 165 finishedThread++;166 synchronized (path) {167 if(finishedThread == ThreadCount){168 for (int i = 0; i < ThreadCount; i++) {169 File f = new File(Environment.getExternalStorageDirectory(), i + ".txt");170 f.delete();171 }172 finishedThread = 0;173 }174 }175 }176 } catch (Exception e) {177 e.printStackTrace();178 }179 }180 }181 } 12 3
