Java：BufferedReader的readLine()中的IOEXceptions是什么？

>为什么“in.readLine()”部分会连续点燃IOExceptions？
>抛出此类异常的目的是什么,目标可能不仅仅是更多的副作用？

代码和IOExceptions

$javac ReadLineTest.java 
ReadLineTest.java:9: unreported exception java.io.IOException; must be caught or declared to be thrown
  while((s=in.readLine())!=null){
                      ^
1 error
$cat ReadLineTest.java 
import java.io.*;
import java.util.*;

public class ReadLineTest {
 public static void main(String[] args) {
  String s;
  BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
  // WHY IOException here?
  while((s=in.readLine())!=null){
   System.out.println(s);
  }
 }
}