多线程下载

//计算每条线程负责下载的数据量
int block = length % threadsize == 0 ? length / threadsize : length / threadsize +1;

public class MulThreadDownloader {

public static void main(String[] args) throws Exception {
String path = "http://192.168.1.100:8080/web/QQWubiSetup.exe";
int threadsize = 3;
new MulThreadDownloader().download(path, threadsize);

}

注意【 RandomAccessFile(file, "rwd");//生成本地文件，不给定目录默认生成在该项目底下 】的用法，write（）写进内存，并不是立即写进卡里，断掉的话数据丢失。元数据表示作者等信息

private void download(String path, int threadsize) throws Exception {
URL downpath = new URL(path);
HttpURLConnection conn = (HttpURLConnection) downpath.openConnection();
conn.setConnectTimeout(5000);
conn.setRequestMethod("GET");
if(conn.getResponseCode() == 200){
int length = conn.getContentLength();//获取网络文件的长度
File file = new File(getFileName(path));
RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");//生成本地文件
accessFile.setLength(length);
accessFile.close();
//计算每条线程负责下载的数据量
int block = length % threadsize == 0 ? length / threadsize : length / threadsize +1;
for(int threadid = 0 ; threadid < threadsize ; threadid++){
new DownloadThread(threadid, downpath, block, file).start();
}
}

}

从图片右侧获得网络文件的长度

//负责下载操作
private final class DownloadThread extends Thread{
private int threadid;
private URL downpath;
private int block;
private File file;

public DownloadThread(int threadid, URL downpath, int block, File file) {
this.threadid = threadid;
this.downpath = downpath;
this.block = block;
this.file = file;
}
public void run() {
int startposition = threadid * block;//从网络文件的什么位置开始下载数据
int endposition = (threadid+1) * block - 1;//下载到网络文件的什么位置结束
//指示该线程要从网络文件的startposition位置开始下载，下载到endposition位置结束
//Range:bytes=startposition-endposition
try{
RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");
accessFile.seek(startposition);//移动指针到文件的某个位置
HttpURLConnection conn = (HttpURLConnection) downpath.openConnection();
conn.setConnectTimeout(5000);
conn.setRequestMethod("GET");
conn.setRequestProperty("Range", "bytes="+ startposition+ "-"+ endposition);
InputStream inStream = conn.getInputStream();
byte[] buffer = new byte[1024];
int len = 0;
while( (len = inStream.read(buffer)) != -1 ){
accessFile.write(buffer, 0, len);
}
accessFile.close();
inStream.close();
System.out.println("第"+ (threadid+1)+ "线程下载完成");
}catch (Exception e) {
e.printStackTrace();
}
}
}
/**
* 获取文件名称
* @param path 下载路径
* @return
*/
private static String getFileName(String path) {
return path.substring(path.lastIndexOf("/")+ 1);
}}