Java实现多线程下载

思路：

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。

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。

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;
}