文件下载采用多线程方式能够充分利用CPU资源，关键点是设置线程的读取开始和结束位置。下面的代码，采用线程池启动10个线程来执行下载。
import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class FileDownLoadTest {
private static final int TCOUNT = 10;
private CountDownLatch latch = new CountDownLatch(TCOUNT);
private long completeLength = 0;
private long fileLength;
/**
* @param args
* @throws Exception
*/
public static void main(String[] args) throws Exception {
new FileDownLoadTest().download("http://localhost:8080/test/IESSAction.docx");
}
public void download(String address) throws Exception{
ExecutorService service = Executors.newFixedThreadPool(TCOUNT);
URL url = new URL(address);
URLConnection cn = url.openConnection();
cn.setRequestProperty("Referer", "http://www.test.com");
fileLength = cn.getContentLength();
long packageLength = fileLength/TCOUNT;
long leftLength = fileLength%TCOUNT;
RandomAccessFile file = new RandomAccessFile("d:\\test.docx","rw");
//计算每个线程请求文件的开始和结束位置
long pos = 0;
long endPos = pos + packageLength;
for(int i=0; i<TCOUNT; i++){
if(leftLength >0){
endPos ++;
leftLength--;
}
service.execute(new DownLoadThread(url, file, pos, endPos));
pos = endPos;
endPos = pos + packageLength;
}
System.out.println("waiting........................................");
long begin = System.currentTimeMillis();
latch.await();
file.close();
System.out.println("end........................................");
System.out.println(System.currentTimeMillis() - begin + "ms");
service.shutdown();
}
class DownLoadThread implements Runnable{
private URL url;
private RandomAccessFile file;
private long from;
private long end;
DownLoadThread(URL url, RandomAccessFile file, long from, long end){
this.url = url;
this.file = file;
this.from = from;
this.end = end;
}
public void run() {
long pos = from;
byte[] buf = new byte[512];
try {
HttpURLConnection cn = (HttpURLConnection) url.openConnection();
cn.setRequestProperty("Range", "bytes=" + from + "-" + end);
if(cn.getResponseCode() != 200 && cn.getResponseCode()!=206){
run();
return;
}
BufferedInputStream bis = new BufferedInputStream(cn.getInputStream());
int len ;
while((len = bis.read(buf)) != -1){
// synchronized(file){
file.seek(pos);
file.write(buf, 0, len);
// }
pos += len;
completeLength +=len;
System.out.println("threadName: " + Thread.currentThread().getName()
+ "persent: " + completeLength * 100 /fileLength + "%");
}
cn.disconnect();
latch.countDown();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}