1.基本思路

2.源码

package cn.itcast.download;import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.RandomAccess;public class MulThreadDownload {
public static void main(String[] args) throws Exception {String path = "http://localhost:8080/web/Dreamweaver.exe";new MulThreadDownload().download(path,3);
}
/*** ************************* *0*1*2*3*4*5*6*7*8*9*10* 文件字节* ************************* threadid:0 1 2* int block = 4 每一个下载多少* int start = threadid*block 开始位置* int end = (threadid+1)*block-1 结束位置*/
public void download(String path,int threadsize) throws Exception{URL url = new URL(path);HttpURLConnection conn = (HttpURLConnection) url.openConnection();conn.setConnectTimeout(5000);conn.setRequestMethod("GET");if(conn.getResponseCode()==200){//网络资源长度int length = conn.getContentLength();//new File中的参数如果直接写名字,那么表示绝对路径,会存放在当前项目下;File file = new File(getFilename(path));//RandomAccessFile是用来访问那些保存数据记录的文件的,其大小和位置必须是可知的,它可以指定seek,搜索指定的位置,在指定位置写入数据;//rwd 参数是权限,表示将读取到的资源立马写入文件中;//多线程下载先要生成一个大小一样的文件,在进行写入;RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");accessFile.setLength(length);accessFile.close();int block = length%threadsize==0?length/threadsize:length/threadsize+1;for (int threadid = 0; threadid < threadsize; threadid++) {new DownloadThread(threadid,block,url,file).start();}}else{System.out.println("下载失败");}}
private class DownloadThread extends Thread{
private int threadid;
private int block;
private URL url;
private File file;public DownloadThread(int threadid, int block, URL url, File file) {this.block=block;this.threadid=threadid;this.url=url;this.file=file;}@Overridepublic void run() {int start = threadid*block;int end = (threadid+1)*block-1;try {RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");accessFile.seek(start);HttpURLConnection conn = (HttpURLConnection) url.openConnection();conn.setConnectTimeout(5000);conn.setRequestMethod("GET");//设置头信息,表明要读取网络资源的从哪个地方开始-哪个地方结束conn.setRequestProperty("Range", "bytes="+start+"-"+end);if(conn.getResponseCode()==200){InputStream inputStream = conn.getInputStream();byte[] buffer = new byte[1024];int len = 0;while ((len=inputStream.read(buffer))!=-1) {accessFile.write(buffer,0,len);}accessFile.close();}accessFile.close();System.out.println("第"+threadid+"线程已经下载完成");} catch (Exception e) {e.printStackTrace();}}}
private String getFilename(String path) {return path.substring(path.lastIndexOf("/")+1);
}
}