java多线程断点下载原理如图：
代码如下：
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import java.io.bufferedreader;
import java.io.file;
import java.io.fileinputstream;
import java.io.inputstream;
import java.io.inputstreamreader;
import java.io.randomaccessfile;
import java.net.httpurlconnection;
import java.net.url;
public class mutilethreaddownload {
/**
* 线程的数量
*/
private static int threadcount = 3;
/**
* 每个下载区块的大小
*/
private static long blocksize;
/**
* 正在运行的线程的数量
*/
private static int runningthreadcount;
/**
* @param args
* @throws exception
*/
public static void main(string[] args) throws exception {
// 服务器文件的路径
string path = "http://192.168.1.100:8080/ff.exe";
url url = new url(path);
httpurlconnection conn = (httpurlconnection) url.openconnection();
conn.setrequestmethod("get");
conn.setconnecttimeout(5000);
int code = conn.getresponsecode();
if (code == 200) {
long size = conn.getcontentlength();// 得到服务端返回的文件的大小
system.out.println("服务器文件的大小：" + size);
blocksize = size / threadcount;
// 1.首先在本地创建一个大小跟服务器一模一样的空白文件。
file file = new file("temp.exe");
randomaccessfile raf = new randomaccessfile(file, "rw");
raf.setlength(size);
// 2.开启若干个子线程分别去下载对应的资源。
runningthreadcount = threadcount;
for (int i = 1; i <= threadcount; i++) {
long startindex = (i - 1) * blocksize;
long endindex = i * blocksize - 1;
if (i == threadcount) {
// 最后一个线程
endindex = size - 1;
}
system.out.println("开启线程：" + i + "下载的位置：" + startindex + "~"
+ endindex);
new downloadthread(path, i, startindex, endindex).start();
}
}
conn.disconnect();
}
private static class downloadthread extends thread {
private int threadid;
private long startindex;
private long endindex;
private string path;
public downloadthread(string path, int threadid, long startindex,
long endindex) {
this.path = path;
this.threadid = threadid;
this.startindex = startindex;
this.endindex = endindex;
}
@override
public void run() {
try {
// 当前线程下载的总大小
int total = 0;
file positionfile = new file(threadid + ".txt");
url url = new url(path);
httpurlconnection conn = (httpurlconnection) url
.openconnection();
conn.setrequestmethod("get");
// 接着从上一次的位置继续下载数据
if (positionfile.exists() && positionfile.length() > 0) {// 判断是否有记录
fileinputstream fis = new fileinputstream(positionfile);
bufferedreader br = new bufferedreader(
new inputstreamreader(fis));
// 获取当前线程上次下载的总大小是多少
string lasttotalstr = br.readline();
int lasttotal = integer.valueof(lasttotalstr);
system.out.println("上次线程" + threadid + "下载的总大小："
+ lasttotal);
startindex += lasttotal;
total += lasttotal;// 加上上次下载的总大小。
fis.close();
}
conn.setrequestproperty("range", "bytes=" + startindex + "-"
+ endindex);
conn.setconnecttimeout(5000);
int code = conn.getresponsecode();
system.out.println("code=" + code);
inputstream is = conn.getinputstream();
file file = new file("temp.exe");
randomaccessfile raf = new randomaccessfile(file, "rw");
// 指定文件开始写的位置。
raf.seek(startindex);
system.out.println("第" + threadid + "个线程：写文件的开始位置："
+ string.valueof(startindex));
int len = 0;
byte[] buffer = new byte[512];
while ((len = is.read(buffer)) != -1) {
randomaccessfile rf = new randomaccessfile(positionfile,
"rwd");
raf.write(buffer, 0, len);
total += len;
rf.write(string.valueof(total).getbytes());
rf.close();
}
is.close();
raf.close();
} catch (exception e) {
e.printstacktrace();
} finally {
// 只有所有的线程都下载完毕后 才可以删除记录文件。
synchronized (mutilethreaddownload.class) {
system.out.println("线程" + threadid + "下载完毕了");
runningthreadcount--;
if (runningthreadcount < 1) {
system.out.println("所有的线程都工作完毕了。删除临时记录的文件");
for (int i = 1; i <= threadcount; i++) {
file f = new file(i + ".txt");
system.out.println(f.delete());
}
}
}
}
}
}
}
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