Java APIs

HI ...
i m trying to make own Proxy server but is thr problem in compilation

"java uses unchecked and unsafe operation...
Recompile with -Xlint: unchecked details.."

my java code :=>

import java.net.*;
import java.util.*;
import java.io.*;
import java.lang.*;


class Firewall
{
boolean checkSiteName(String fn)
{
try{
FileInputStream fis=new FileInputStream("rsite.txt");
BufferedReader br=new BufferedReader(new InputStreamReader(fis));
String str=br.readLine();
boolean b=true;
while(str!=null)
{
b=!str.startsWith(fn);
if(!b)
break;
str=br.readLine();
}
return b;
}catch(Exception e2){}

} // Firewall


boolean CheckPacket(byte b[])
{
try{
String s=new String(b);
StringTokenizer st=new StringTokenizer(s);
FileInputStream fis=new FileInputStream("rword.txt");
BufferedReader br=new BufferedReader(new InputStreamReader(fis));

Vector <String>v=new Vector<String>();

String str1=br.readLine();

while(str1.hasMoreTokens())
{
v.addElement(st.nextToken());
}

boolean b1=true;

while(str1!=null)
{
b1=!v.contains(str1);
if(!b1)
break;
str1=br.readLine();
}
return b1;
}catch(Exception e4){}

}//CheckPacket
}

public class ProxyServer
{
public static void main(String args[])
{
try{
ServerSocket ss=new ServerSocket(4000);
while(true)
{
Socket s=ss.accept();
ThreadA t=new ThreadA();
t.s1=s;
t.start();
}
}catch(Exception e)
{
System.out.println(e);
}
}
}//ProxyServer

class ThreadA extends Thread
{
Socket s1;
public void run()
{
try{
InputStream in;// =new InputStream();
BufferedReader br=new BufferedReader(new InputStreamReader(in));
String str=br.readLine();
StringTokenizer st=new StringTokenizer(str);
st.nextToken();
str=st.nextToken();
OutputStream os=s1.getOutputStream();
Firewall f=new Firewall();
boolean b=f.checkSiteName(str);
boolean b1=b;
if(b)
{
byte b2[]=new byte[1000];
URL u=new URL(str);
URLConnection con=u.getConnection();
InputStream in1=con.getInputStream();
int x=in1.read();
b1=f.CheckPacket(b2);
if(b1)
{
os.write(b,0,x);
os.flush();
}
}
if(b1)
{
String str1="This is Restricted Site";
os.write(str1.getBytes());
os.flush();
}
}catch(Exception e1)
{
System.out.println(e1);
}
}
}

where "rsite.txt" for restricted sites name file and
"rword.txt" for restricted words file.