1、基本思路是将文件分段切割、分段传输、分段保存。
2、分段切割用到HttpUrlConnection对象的setRequestProperty("Range", "bytes=" + start + "-" + end)方法。
3、分段传输用到HttpUrlConnection对象的getInputStream()方法。
4、分段保存用到RandomAccessFile的seek(int start)方法。
5、创建指定长度的线程池，循环创建线程，执行下载操作。
首先，我们要先写一个方法，方法的参数包含URL地址，保存的文件地址，切割后的文件开始位置和结束位置，这样我们就能把分段文件下载到本地。并且这个方法要是run方法，这样我们启动线程时就直接执行该方法。
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public class DownloadWithRange implements Runnable { private String urlLocation; private String filePath; private long start; private long end; DownloadWithRange(String urlLocation, String filePath, long start, long end) { this.urlLocation = urlLocation; this.filePath = filePath; this.start = start; this.end = end; } @Override public void run() { try { HttpURLConnection conn = getHttp(); conn.setRequestProperty("Range", "bytes=" + start + "-" + end); File file = new File(filePath); RandomAccessFile out = null; if (file != null) { out = new RandomAccessFile(file, "rwd"); } out.seek(start); InputStream in = conn.getInputStream(); byte[] b = new byte[1024]; int len = 0; while ((len = in.read(b)) != -1) { out.write(b, 0, len); } in.close(); out.close(); } catch (Exception e) { e.getMessage(); } } public HttpURLConnection getHttp() throws IOException { URL url = null; if (urlLocation != null) { url = new URL(urlLocation); } HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setReadTimeout(5000); conn.setRequestMethod("GET"); return conn; } } |
然后我们创建线程池，线程池的长度可以自定义，然后循环创建线程来执行请求，每条线程的请求开始位置和结束位置都不同，本地存储的文件开始位置和请求开始位置相同，这样就可以实现多线程下载了。
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public class DownloadFileWithThreadPool{ public void getFileWithThreadPool(String urlLocation,String filePath, int poolLength) throws IOException { Executor threadPool = Executors.newFixedThreadPool(poolLength); long len = getContentLength(urlLocation); for(int i=0;i<poolLength;i++) { long start=i*len/poolLength; long end = (i+1)*len/poolLength-1; if(i==poolLength-1) { end =len; } DownloadWithRange download=new DownloadWithRange(urlLocation, filePath, start, end); threadPool.execute(download); } } public static long getContentLength(String urlLocation) throws IOException { URL url = null; if (urlLocation != null) { url = new URL(urlLocation); } HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setReadTimeout(5000); conn.setRequestMethod("GET"); long len = conn.getContentLength(); return len; } |
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持服务器之家。
原文链接：http://www.cnblogs.com/stsinghua/p/6418796.html
