原理解析：利用randomaccessfile在本地创建一个随机访问文件，文件大小和服务器要下载的文件大小相同。根据线程的数量（假设有三个线程），服务器的文件三等分，并把我们在本地创建的文件同样三等分，每个线程下载自己负责的部分，到相应的位置即可。
示例图：
示例demo
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import java.io.inputstream;
import java.io.randomaccessfile;
import java.net.httpurlconnection;
import java.net.url;
public class mutildownload {
private static string path = "http://192.168.80.85:8080/test.doc";
private static final int threadcount = 3;
public static void main(string[] args) {
try {
url url = new url(path);
httpurlconnection conn = (httpurlconnection) url.openconnection();
conn.setrequestmethod("get");
conn.setconnecttimeout(5000);
int responsecode = conn.getresponsecode();
if (responsecode == 200) {
int contentlength = conn.getcontentlength();
system.out.println("length" + contentlength);
randomaccessfile rafaccessfile = new randomaccessfile("test.doc", "rw");
rafaccessfile.setlength(contentlength);
int blocksize = contentlength / threadcount;
for (int i = 0; i < threadcount; i++) {
int startindex = i * blocksize; //每个现成下载的开始位置
int endindex = (i + 1) * blocksize - 1;// 每个线程的结束位置
if (i == threadcount - 1) {
//最后一个线程
endindex = contentlength - 1;
}
new downloadthread(startindex, endindex, i).start();
}
}
} catch (exception e) {
}
}
private static class downloadthread extends thread {
private int startindex;
private int endindex;
private int threadid;
public downloadthread(int startindex, int endindex, int threadid) {
this.startindex = startindex;
this.endindex = endindex;
this.threadid = threadid;
}
@override
public void run() {
try {
url url = new url(path);
httpurlconnection conn = (httpurlconnection) url.openconnection();
conn.setrequestmethod("get");
conn.setconnecttimeout(5000);
conn.setrequestproperty("range", "bytes=" + startindex + "-" + endindex); //固定写法，请求部分资源
int responsecode = conn.getresponsecode(); // 206表示请求部分资源
if (responsecode == 206) {
randomaccessfile rafaccessfile = new randomaccessfile("test.doc", "rw");
rafaccessfile.seek(startindex);
inputstream is = conn.getinputstream();
int len = -1;
byte[] buffer = new byte[1024];
while ((len = is.read(buffer)) != -1) {
rafaccessfile.write(buffer, 0, len);
}
rafaccessfile.close();
system.out.println("线程" + threadid + "下载完成");
}
} catch (exception e) {
}
}
}
}
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