浏览器能访问 Java却返回Http500

想将目标网址的源代码抓过来，浏览器能够正常访问网页，可是Java连接却报错：
Exception in thread "main" java.io.IOException: Server returned HTTP response co
de: 500 for URL:
at sun.net.www.protocol.http.HttpURLConnection.getInputStream(Unknown So
urce)
at Test.main(Test.java:21)
上网搜了一下，然后设置了User-Agent，可是结果不变。
User-Agent的值是ieHttpHeaders获取的，IE能够正常访问页面。

import java.io.*;

import java.util.*;

import java.net.*;

import java.sql.*;



public class Test

{

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

	{

		URL u = new URL("网址");

		HttpURLConnection con = (HttpURLConnection)u.openConnection();

    con.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; InfoPath.3; .NET4.0C)");

		InputStream is = con.getInputStream();

		BufferedInputStream bis = new BufferedInputStream(is);

		BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream("test.html"));

		byte[] arr = new byte[512];

		int temp;

		while((temp = bis.read(arr, 0, 512)) != -1)

		{

			bos.write(arr, 0, temp);

		}

		bos.flush();

		bos.close();

		bis.close();

	}

}