Java Scanner NoSuchElementException

今天同学问我一个Java问题，代码结构如下：

Scanner scan = new Scanner(System.in); scan.next(); scan.close(); scan = new Scanner(System.in); scan.next(); 

当程序运行到第五行是会抛出java.util.NoSuchElementException

          at java.util.Scanner.throwFor(Scanner.java:838)

          at java.util.Scanner.next(Scanner.java:1347)

于是按照错误栈的轨迹查找原因。

简要的说，就是前后两次实例化的参数System.in是用一个对象，是InputStreamReader对象，每个该对象包含一个

StreamDecoder 实例 sd

private final StreamDecoder sd;  

而scan.close()方法为

public void close() { if (closed) return; if (source instanceof Closeable) { try { ((Closeable)source).close(); } catch (IOException ioe) { lastException = ioe; } } sourceClosed = true; source = null; closed = true; }  

当执行到 ((Closeable)source).close();就会进入InputStreamReader的close()方法：

public void close() throws IOException { sd.close(); }  

这里的sd就是上面提到的InputStreamReader对象，（有查了StreamDecoder 源代码，但没更深入下去），此时sd已关闭。

当执行如错误产生代码的第5行代码 scan.next()时，

public String next() { ensureOpen(); clearCaches(); while (true) { String token = getCompleteTokenInBuffer(null); if (token != null) { matchValid <mce:script type="text/javascript" src="http://hi.images.csdn.net/js/blog/tiny_mce/themes/advanced/langs/zh.js" mce_src="http://hi.images.csdn.net/js/blog/tiny_mce/themes/advanced/langs/zh.js"></mce:script><mce:script type="text/javascript" src="http://hi.images.csdn.net/js/blog/tiny_mce/plugins/syntaxhl/langs/zh.js" mce_src="http://hi.images.csdn.net/js/blog/tiny_mce/plugins/syntaxhl/langs/zh.js"></mce:script>= true; skipped = false; return token; } if (needInput) readInput(); else throwFor(); } }  

异常是从方法throwFor();中抛出，而异常的来源是readInput()方法

private void readInput() { if (buf.limit() == buf.capacity()) makeSpace(); // Prepare to receive data int p = buf.position(); buf.position(buf.limit()); buf.limit(buf.capacity()); int n = 0; try { n = source.read(buf); } catch (IOException ioe) { lastException = ioe; n = -1; } if (n == -1) { sourceClosed = true; needInput = false; } if (n > 0) needInput = false; // Restore current position and limit for reading buf.limit(buf.position()); buf.position(p); }  

当执行到12行source.read()时，source是Reader类

public int read(java.nio.CharBuffer target) throws IOException { int len = target.remaining(); char[] cbuf = new char[len]; int n = read(cbuf, 0, len); if (n > 0) target.put(cbuf, 0, n); return n; }  

在执行InputStreamReader的read方法

public int read(char cbuf[], int offset, int length) throws IOException { return sd.read(cbuf, offset, length); }  

而该InputStreamReader实际上就是System.in，而之前的close()方法已经将sd关闭了，此处再次实行read方法，则抛出IOException，然后层层捕获，最终抛出.NoSuchElementException