用Java实现的多线程下载类，后面会加入gui和断点续传之类的功能改进为一个较为完善的Java程序。 代码如下：
import java.io.IOException; import java.io.InputStream; import java.io.RandomAccessFile; import java.net.*; public class t { public static void main(String[] args) throws IOException { URL url = new URL("http://baiduliulanqi.00791.com/style/images/img-1-1.png"); URLConnection urlConnection = url.openConnection(); int contentLength = urlConnection.getContentLength(); System.out.println(contentLength); RandomAccessFile file = new RandomAccessFile("/Users/caohao/IdeaProjects/threadlearn/src/main/resources/targetfile.png","rw"); file.setLength(contentLength); file.close(); int concurrentThreadSize=4;//设置为四个线程同时下载 int threadFileSize = contentLength/concurrentThreadSize+1;//每个线程块所负责的大小 for (int i = 0; i < concurrentThreadSize; i++) { FIleDownLoader fIleDownLoader = new FIleDownLoader(i * threadFileSize, threadFileSize, "http://baiduliulanqi.00791.com/style/images/img-1-1.png"); new Thread(fIleDownLoader).start(); } } } class FIleDownLoader implements Runnable{ private int order;//所负责文件块的第一个下标 private RandomAccessFile concurrentFileBlock;//所负责的文件 private int fileSize;//所负责的总大小 String url;//url private int nowFileSize=0;//当前所完成的大小 public FIleDownLoader(){ } public FIleDownLoader(int order, int fileSize,String url) { this.order = order; this.concurrentFileBlock = concurrentFileBlock; this.fileSize = fileSize; this.url=url; } @Override public void run() { try { URL url = new URL(this.url); URLConnection connection = url.openConnection(); connection.setConnectTimeout(1000); InputStream inputStream = connection.getInputStream(); inputStream.skip(order); concurrentFileBlock = new RandomAccessFile("/Users/caohao/IdeaProjects/threadlearn/src/main/resources/targetfile.png","rw"); concurrentFileBlock.seek(order); byte[] buffer = new byte[1024]; int hashRead = 0; while (nowFileSize<fileSize&&(hashRead=inputStream.read(buffer))!=-1){ concurrentFileBlock.write(buffer,0,hashRead); nowFileSize+=hashRead; } concurrentFileBlock.close(); inputStream.close(); } catch (MalformedURLException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } }大致的思路就是，通过URLconnection来建立连接并按照线程个数对目标文件进行分配。
