BigDecimal有八种RoundingMode，其中最基本的有两种，它们是：
ROUND_UP：Always increments the digit prior to a nonzero discarded fraction.
总是将要丢掉的数字前一位的数字加1.
例如：
Rounding mode UP Examples Input Number | Input rounded to one digit
with UP rounding
—|—
5.5 | 6
2.5 | 3
1.6 | 2
1.1 | 2
1.0 | 1
-1.0 | -1
-1.1 | -2
-1.6 | -2
-2.5 | -3
-5.5 | -6
比方说5.5的scale是1，那么需要丢掉的数字的0.5, 将0.5丢掉，然后给5加1，就变成6了；
需要注意的是-5.5，这里的负号是不参加运算的，将它当成5.5，先进行Round up，然后将负号加上；
ROUND_DOWN：Never increments the digit prior to a discarded fraction (i.e.,
truncates).将要丢掉的数字直接丢掉。
Rounding mode DOWN Examples Input Number | Input rounded to one digit
with DOWN rounding
—|—
5.5 | 5
2.5 | 2
1.6 | 1
1.1 | 1
1.0 | 1
-1.0 | -1
-1.1 | -1
-1.6 | -1
-2.5 | -2
-5.5 | -5
比方说5.5的scale是1，那么需要丢掉的数字的0.5, 直接将0.5丢掉就可以了；
需要注意的是-5.5，这里的负号是不参加运算的，将它当成5.5，先进行Round Down，然后将负号加上；
然后其它五中都是根据Round Down/Up来推出来的；
例如：
ROUND_CEILING：If the result is positive, behaves as for RoundingMode.UP ;
if negative, behaves as for RoundingMode.DOWN
.如果参数是正数，使用UP规则，如果参数是负数，使用DOWN规则；
Rounding mode CEILING Examples Input Number | Input rounded to one digit
with CEILING rounding
—|—
5.5 | 6
2.5 | 3
1.6 | 2
1.1 | 2
1.0 | 1
-1.0 | -1
-1.1 | -1
-1.6 | -1
-2.5 | -2
-5.5 | -5
ROUND_FLOOR：If the result is positive, behave as for RoundingMode.DOWN ;
if negative, behave as for RoundingMode.UP
.如果参数是正数，使用DOWN规则，如果参数是负数，使用UP规则；
Rounding mode FLOOR Examples Input Number | Input rounded to one digit
with FLOOR rounding
—|—
5.5 | 5
2.5 | 2
1.6 | 1
1.1 | 1
1.0 | 1
-1.0 | -1
-1.1 | -2
-1.6 | -2
-2.5 | -3
-5.5 | -6
ROUND_HALF_UP: Behaves as for RoundingMode.UP if the discarded fraction is ≥
0.5; otherwise, behaves as for RoundingMode.DOWN.
如果要丢掉的数字大于等于0.5，使用UP规则；否则使用DOWN规则；
Rounding mode HALF_UP Examples Input Number | Input rounded to one digit
with HALF_UP rounding
—|—
5.5 | 6
2.5 | 3
1.6 | 2
1.1 | 1
1.0 | 1
-1.0 | -1
-1.1 | -1
-1.6 | -2
-2.5 | -3
-5.5 | -6
ROUND_HALF_DOWN: Behaves as for RoundingMode.UP if the discarded fraction is
0.5; otherwise, behaves as for RoundingMode.DOWN.
如果要丢掉的数字大于0.5，使用UP规则；否则使用DOWN规则；
Rounding mode HALF_DOWN Examples Input Number | Input rounded to one
digit
with HALF_DOWN rounding
—|—
5.5 | 5
2.5 | 2
1.6 | 2
1.1 | 1
1.0 | 1
-1.0 | -1
-1.1 | -1
-1.6 | -2
-2.5 | -2
-5.5 | -5
ROUND_HALF_EVEN：Rounding mode to round towards the “nearest neighbor” unless
both neighbors are equidistant, in which case, round towards the even
neighbor. Behaves as for RoundingMode.HALF_UP if the digit to the left of
the discarded fraction is odd; behaves as for RoundingMode.HALF_DOWN if
it’s even.
如果要丢掉的数字不是5的话，需要看这个数字前面的数字的奇偶性，如果是奇数，使用HALF_UP规则；如果是偶数使用HALF_DOWN规则；如果要丢掉的数字是5的话，round到它的偶数邻居上；
Rounding mode HALF_EVEN Examples Input Number | Input rounded to one
digit
with HALF_EVEN rounding
—|—
5.5 | 6
2.5 | 2
1.6 | 2
1.1 | 1
1.0 | 1
-1.0 | -1
-1.1 | -1
-1.6 | -2
-2.5 | -2
-5.5 | -6
ROUND_UNNECESSARY：不需要ROUND
最后来一张总表，
Summary of Rounding Operations Under Different Rounding Modes | Result of
rounding input to one digit with the given rounding mode
—|—
Input Number | UP | DOWN | CEILING | FLOOR | HALF_UP
| HALF_DOWN | HALF_EVEN | UNNECESSARY
5.5 | 6 | 5 | 6 | 5 | 6 | 5 | 6 | throw ArithmeticException
2.5 | 3 | 2 | 3 | 2 | 3 | 2 | 2 | throw ArithmeticException
1.6 | 2 | 1 | 2 | 1 | 2 | 2 | 2 | throw ArithmeticException
1.1 | 2 | 1 | 2 | 1 | 1 | 1 | 1 | throw ArithmeticException
1.0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1
-1.0 | -1 | -1 | -1 | -1 | -1 | -1 | -1 | -1
-1.1 | -2 | -1 | -1 | -2 | -1 | -1 | -1 | throw ArithmeticException
-1.6 | -2 | -1 | -1 | -2 | -2 | -2 | -2 | throw ArithmeticException
-2.5 | -3 | -2 | -2 | -3 | -3 | -2 | -2 | throw ArithmeticException
-5.5 | -6 | -5 | -5 | -6 | -6 | -5 | -6 | throw ArithmeticException
refs：
jdk document
http://en.wikipedia.org/wiki/Floor_and_ceiling_functions
http://www.opentaps.org/docs/index.php/How_to_Use_Java_BigDecimal:_A_Tutorial