java调用外部控制台出现的小问题？至今没想明白

public static void main(String[] args0) {
try{
Process process = Runtime.getRuntime().exec ("ipconfig");
InputStreamReader ir=new InputStreamReader(process.getInputStream());
BufferedReader input = new BufferedReader (ir);
String line;
StringBuffer str = new StringBuffer();
int exitVal = process.waitFor();

while ((line = input.readLine()) != null) {
String temp =line;
str.append(temp+"/r/n");
System.out.println(line);
//System.out.println(input.readLine());

}




System.out.println(exitVal);
System.out.println(str.toString());
}catch(Exception e){
System.err.println ("IOException " + e.getMessage());
}

}



为什么直接输出input.readline()，控制台输出的都是空呢？是传说中的缓冲区问题？赋值一下就可以了呢？