java异常捕获问题

通过DEBUG追踪了下。
Scanner最后是调用 public String next(Pattern pattern) 方法获取一个String对象，通过Pattern表示要获取对象的类型。
代码如下

   public String next(Pattern pattern) {
        ensureOpen();
        if (pattern == null)
            throw new NullPointerException();

        // Did we already find this pattern?
        if (hasNextPattern == pattern)
            return getCachedResult();
        clearCaches();

        // Search for the pattern
        while (true) {
            String token = getCompleteTokenInBuffer(pattern);//尝试获取符合类型的结果
            if (token != null) { 
                matchValid = true;
                skipped = false;
                return token;
            }
            if (needInput) //是否需要input数据
                readInput();
            else
                throwFor();
        }
    }

问题就出在
getCompleteTokenInBuffer(pattern)
和
`if (needInput)
readInput();`
首先初次尝试失败后，会判断是否需要input数据，此时你还没有输入数据，因此通过readInput();得到数据的数据，并把needInput设置为了false
然后再次调用
getCompleteTokenInBuffer(pattern)；
然而调试里面的代码发现，当pattern指定的类型与你实际输入的类型不匹配时，并不会讲needInput设置为false，
查看该方法的说明：

 /*
     * Returns a "complete token" that matches the specified pattern
     *
     * A token is complete if surrounded by delims; a partial token
     * is prefixed by delims but not postfixed by them
     *
     * The position is advanced to the end of that complete token
     *
     * Pattern == null means accept any token at all
     *
     * Triple return:
     * 1. valid string means it was found
     * 2. null with needInput=false means we won't ever find it
     * 3. null with needInput=true means try again after readInput (意味着这是readInput后的又一次尝试)
     */
    private String getCompleteTokenInBuffer(Pattern pattern) {
            /*
                ...
            */
        }

因此，当你循环调用next()方法时，只会不停的尝试将你上次输入的数据中获取你想要的int类型数据。