Java实现多线程下载

public class Download {
// 服务器的下载地址
//	public static String FILEPATH = "http://192.168.86.1:8080/1-9.zip";
public static String FILEPATH = "http://dldir1.qq.com/invc/tt/QQBrowserSetup.exe";
// 本地保存的路径
public static String DESTINATION = "D:\\";
// 下载的线程数
public static int THREADCOUNT = 5;

public static void main(String[] args) throws Exception {

URL url = new URL(FILEPATH);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
// 连接超时
conn.setConnectTimeout(5 * 1000);
// 下载通过GET请求
conn.setRequestMethod("GET");
// 设置代理
conn.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.63 Safari/537.36");
conn.setRequestProperty("Connection", "keep-alive");
if (conn.getResponseCode() == 200) {
// 文件的大小
int contentLength = conn.getContentLength();
// 将文件分成5份
int blocksize = contentLength / THREADCOUNT;
// 此类的实例支持对随机访问文件的读取和写入
// 并建立一个空的文件
RandomAccessFile randomAccessFile = new RandomAccessFile(new File(Download.DESTINATION + Download.getFileName(Download.FILEPATH)), "rwd");
randomAccessFile.setLength(contentLength);
randomAccessFile.close();
conn.disconnect();
for (int i = 0; i < THREADCOUNT; i++) {
// 少下载一个字节
int startpos = i * blocksize;
int endpos = (i + 1) * blocksize - 1;
if (i == THREADCOUNT - 1) {
endpos = contentLength;
}
new Thread(new DownloadTask(i, startpos, endpos)).start();
}
} else {

System.out.println("服务器出错!");
}

}

// 截取文件名
public static String getFileName(String path) {

return path.substring(path.lastIndexOf("/") + 1, path.length());
}
}

// 多线程的任务
class DownloadTask implements Runnable {

public DownloadTask(int id, int startpos, int endpos) {
this.id = id;
this.startpos = startpos;
this.endpos = endpos;
}

int id;
int startpos;
int endpos;

@Override
public void run() {
try {
URL url = new URL(Download.FILEPATH);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setReadTimeout(5 * 1000);
conn.setRequestMethod("GET");
// Request only part of an entity. Bytes are numbered from 0.
// eg:Range: bytes=500-999
conn.setRequestProperty("Range", "bytes=" + startpos + "-" + endpos + "");
conn.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.63 Safari/537.36");
conn.setRequestProperty("Connection", "keep-alive");

// HTTP Status-Code 206: Partial Content.
if (conn.getResponseCode() == 206) {
InputStream is = conn.getInputStream();
RandomAccessFile randomAccessFile = new RandomAccessFile(new File(Download.DESTINATION + Download.getFileName(Download.FILEPATH)), "rwd");
// 设置到此文件开头测量到的文件指针偏移量，在该位置发生下一个读取或写入操作。
randomAccessFile.seek(startpos);
int len = 0;
byte[] buffer = new byte[1024];
while ((len = is.read(buffer)) != -1) {
randomAccessFile.write(buffer, 0, len);
}
is.close();
randomAccessFile.close();
conn.disconnect();
System.out.println("线程" + id + ":下载完毕");
}
} catch (IOException e) {
e.printStackTrace();
}
}

}