BigDecimal默认用四舍五入方式

import java.math.BigDecimal;

target.setWeight(source.getWeight().setScale(3, BigDecimal.ROUND_HALF_UP));

不同舍入模式下的舍入操作汇总

	根据给定的舍入模式将输入数字舍入为一位数的结果
输入数字	UP	DOWN	CEILING	FLOOR	HALF_UP	HALF_DOWN	HALF_EVEN	UNNECESSARY
5.5	6	5	6	5	6	5	6	抛出 ArithmeticException
2.5	3	2	3	2	3	2	2	抛出 ArithmeticException
1.6	2	1	2	1	2	2	2	抛出 ArithmeticException
1.1	2	1	2	1	1	1	1	抛出 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	抛出 ArithmeticException
-1.6	-2	-1	-1	-2	-2	-2	-2	抛出 ArithmeticException
-2.5	-3	-2	-2	-3	-3	-2	-2	抛出 ArithmeticException
-5.5	-6	-5	-5	-6	-6	-5	-6	抛出 ArithmeticException