多线程下载文件，比单线程要快，当然，线程不是越多越好，这和获取的源文件还有和网速有关。
原理：在请求服务器的某个文件时，我们能得到这个文件的大小长度信息，我们就可以下载此长度的某一个片段，来达到多线程下载的目的！每条线程分别下载他们自己的片段！
下载流程（代码片段）
1. 根据访问的URL路径调用openConnection()获得HttpURLConnection对象，接着调用getContentLengthLong()方法获得文件的 字节大小，然后通过RandomAccessFile对象调用setLength()设置本地文件的长度。（这个文件是null数据文件，通过多线程进行对RandomAccessFile对象的本地文件随机位置写入数据）
URL url = new URL(str_url);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
long fileLength = conn.getContentLengthLong(); // 得到需要下载的文件大小
RandomAccessFile file = new RandomAccessFile(storagePath, "rwd");
file.setLength(fileLength); // 关键方法 ： 设置本地文件长度
file.close();
conn.disconnect();2. 根据获得的文件长度，计算每条线程下载的起始位置与结束位置，因为不一定平均分，所以最后一条线程下载剩余的字节
long oneThreadReadByteLength = fileLength / threadNumber;
for (int i = 0; i < threadNumber; i++) {
long startPosition = i * oneThreadReadByteLength;
long endPosition = i == threadNumber - 1 ? fileLength : (i + 1) * oneThreadReadByteLength - 1;
}3. 每条线程请求的范围参数设置，请求头参数 ： Range:bytes=0-length
URL url = new URL(str_url);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestProperty("Range", "bytes=" + startPosition + "-" + endPosition); // 关键方法: 每条线程请求的字节范围4. 每条线程存储数据到文件的起始位置设置（RandomAccessFile的seek()方法），以及响应码206判断
if (conn.getResponseCode() == HttpURLConnection.HTTP_PARTIAL) { // 关键响应码 ：206，请求成功 + 请求数据字节范围成功
RandomAccessFile file = new RandomAccessFile(storagePath, "rwd");
file.seek(startPosition); // 关键方法 ：每条线程起始写入文件的位置
InputStream in = conn.getInputStream();
byte[] buf = new byte[8192];
int len;
while ((len = in.read(buf)) > 0) {
file.write(buf, 0, len);
}
}完整代码
main.java
public class Main {
public static void main(String[] args) throws Exception {
String path = "https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1588346633185&di=8f8b2b357c8461d232fcce9e0c476f3a&imgtype=0&src=http%3A%2F%2Fi0.hdslb.com%2Fbfs%2Farticle%2Fe79d2c22d40a801e7b02183dee9db3e5c71514af.jpg";
MultiThreadDownload mtd = new MultiThreadDownload(path, "G:\\LeiMus.jpg", 3);
mtd.download();
}
}MultiThreadDownload.java
package com.bin.demo;
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 MultiThreadDownload {
private String str_url;
private String storagePath;
private int threadNumber;
private static long downloadByteCount;
MultiThreadDownload(String str_url, String storagePath, int threadNumber) {
this.str_url = str_url;
this.storagePath = storagePath;
this.threadNumber = threadNumber;
}
public void download() throws IOException, InterruptedException {
long startTime = System.currentTimeMillis();
System.out.println("Download......");
/*
* 首先设置本地文件的大小
* 当然这是个null数据的文件
* 这样才能通过RandomAccessFile的数组下标机制达到随机位置写入
*/
URL url = new URL(str_url);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setConnectTimeout(10000);
conn.setRequestMethod("GET");
long fileLength = conn.getContentLengthLong(); // 得到需要下载的文件大小
conn.disconnect();
RandomAccessFile file = new RandomAccessFile(storagePath, "rwd");
file.setLength(fileLength); // 关键方法 ： 设置本地文件长度
file.close();
/*
* 计算每条线程下载的字节数，以及每条线程起始下载位置与结束的下载位置，
* 因为不一定平均分，所以最后一条线程下载剩余的字节
* 然后创建线程任务并启动
* Main线程等待每条线程结束(join()方法)
*/
long oneThreadReadByteLength = fileLength / threadNumber;
for (int i = 0; i < threadNumber; i++) {
long startPosition = i * oneThreadReadByteLength;
long endPosition = i == threadNumber - 1 ? fileLength : (i + 1) * oneThreadReadByteLength - 1;
Thread t = new Thread(new Task(startPosition, endPosition));
t.start();
t.join();
}
/*
* 检查文件是否下载完整，不完整则删除
*/
if (downloadByteCount == fileLength) {
System.out.println("ALL Thread Download OK.");
System.out.println("time = " + ((System.currentTimeMillis() - startTime) / 1000) + " S");
} else {
System.out.println("Download Error.");
new File(storagePath).delete();
}
}
class Task implements Runnable {
private long startPosition;
private long endPosition;
Task(long startPosition, long endPosition) {
this.startPosition = startPosition;
this.endPosition = endPosition;
}
@Override
public void run() {
try {
URL url = new URL(str_url);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setConnectTimeout(10000);
conn.setRequestMethod("GET");
conn.setRequestProperty("Range", "bytes=" + startPosition + "-" + endPosition); // 关键方法: 每条线程请求的字节范围
if (conn.getResponseCode() == HttpURLConnection.HTTP_PARTIAL) { // 关键响应码 ：206，请求成功 + 请求数据字节范围成功
RandomAccessFile file = new RandomAccessFile(storagePath, "rwd");
file.seek(startPosition); // 关键方法 ：每条线程起始写入文件的位置
InputStream in = conn.getInputStream();
byte[] buf = new byte[8192];
int len;
while ((len = in.read(buf)) > 0) {
file.write(buf, 0, len);
downloadByteCount += len;
}
// 关闭网络连接及本地流
in.close();
file.close();
conn.disconnect();
System.out.println(Thread.currentThread().getName() + ": download OK");
}
} catch (IOException e) {
System.out.println(Thread.currentThread().getName() + "_Error : " + e);
}
}
}
}
输出：
Download......
Thread-0: download OK
Thread-1: download OK
Thread-2: download OK
ALL Thread Download OK.
time = 1 S
我下载的是百度图片，另外多线程下载的结果时间和网速和文件大小有关。
附加我的结果图片：
