一个JDK7的四舍五入的bug引发的思考

## 1.背景:

今天我的 [feilong-core](https://github.com/venusdrogon/feilong-core "feilong-core") 项目使用 `jdk8` 进行`maven install` 的时候,有一个测试类报错, 但是原先使用`jdk7` 进行`maven install`的时候却是正常通过,

issue 参见 [https://github.com/venusdrogon/feilong-core/issues/165](https://github.com/venusdrogon/feilong-core/issues/165)

## 2.测试类代码如下:

```JAVA


    @Test
    public void testFormat32(){
        assertEquals("1.2", NumberFormatUtil.format(1.15, "#####.#", RoundingMode.HALF_EVEN));
        assertEquals("1.2", NumberFormatUtil.format(1.25, "#####.#", RoundingMode.HALF_EVEN));
        assertEquals("1.3", NumberFormatUtil.format(1.251, "#####.#", RoundingMode.HALF_EVEN));

        assertEquals("-1.2", NumberFormatUtil.format(-1.15, "#####.#", RoundingMode.HALF_EVEN));
        assertEquals("-1.2", NumberFormatUtil.format(-1.25, "#####.#", RoundingMode.HALF_EVEN));
        assertEquals("-1.3", NumberFormatUtil.format(-1.251, "#####.#", RoundingMode.HALF_EVEN));
    }


    @Test
    public void testFormat321(){
        assertEquals("1.2", NumberFormatUtil.format(1.15, "#####.#", null));
        assertEquals("1.3", NumberFormatUtil.format(1.25, "#####.#", null));
        assertEquals("1.3", NumberFormatUtil.format(1.251, "#####.#", null));

        assertEquals("-1.2", NumberFormatUtil.format(-1.15, "#####.#", null));
        assertEquals("-1.3", NumberFormatUtil.format(-1.25, "#####.#", null));
        assertEquals("-1.3", NumberFormatUtil.format(-1.251, "#####.#", null));
    }


    @Test
    public void testFormat111(){
        assertEquals("1.2", NumberFormatUtil.format(1.15, "#####.#"));
        assertEquals("1.3", NumberFormatUtil.format(1.25, "#####.#"));
        assertEquals("1.3", NumberFormatUtil.format(1.251, "#####.#"));

        assertEquals("-1.2", NumberFormatUtil.format(-1.15, "#####.#"));
        assertEquals("-1.3", NumberFormatUtil.format(-1.25, "#####.#"));
        assertEquals("-1.3", NumberFormatUtil.format(-1.251, "#####.#"));
    }

```


## 3.报错信息


```XML

	Tests run: 568, Failures: 3, Errors: 0, Skipped: 2, Time elapsed: 2.194 sec <<< FAILURE! - in com.feilong.core.FeiLongSuiteTests
	testFormat111(com.feilong.core.text.NumberFormatUtilTest)  Time elapsed: 0.009 sec  <<< FAILURE!
	org.junit.ComparisonFailure: expected:<1.[2]> but was:<1.[1]>
		at com.feilong.core.text.NumberFormatUtilTest.testFormat111(NumberFormatUtilTest.java:149)
	
	testFormat321(com.feilong.core.text.NumberFormatUtilTest)  Time elapsed: 0 sec  <<< FAILURE!
	org.junit.ComparisonFailure: expected:<1.[2]> but was:<1.[1]>
		at com.feilong.core.text.NumberFormatUtilTest.testFormat321(NumberFormatUtilTest.java:135)
	
	testFormat32(com.feilong.core.text.NumberFormatUtilTest)  Time elapsed: 0.001 sec  <<< FAILURE!
	org.junit.ComparisonFailure: expected:<1.[2]> but was:<1.[1]>
		at com.feilong.core.text.NumberFormatUtilTest.testFormat32(NumberFormatUtilTest.java:121)
	
	
	Results :
	
	Failed tests: 
	  NumberFormatUtilTest.testFormat111:149 expected:<1.[2]> but was:<1.[1]>
	  NumberFormatUtilTest.testFormat32:121 expected:<1.[2]> but was:<1.[1]>
	  NumberFormatUtilTest.testFormat321:135 expected:<1.[2]> but was:<1.[1]>
	
	Tests run: 568, Failures: 3, Errors: 0, Skipped: 2
	
	[INFO] ------------------------------------------------------------------------
	[INFO] BUILD FAILURE

```


## 4.原因


经过搜索,发现 

- https://bugs.openjdk.java.net/browse/JDK-7131459
- https://bugs.openjdk.java.net/browse/JDK-8029896
- http://stackoverflow.com/questions/22797964/is-inconsistency-in-rounding-between-java-7-and-java-8-a-bug

`jdk7 `上是bug , `jdk8` 修复了

![image](https://cloud.githubusercontent.com/assets/3479472/17660670/3cadd984-630d-11e6-9d31-747d610ef110.png)


## 5.思考

- 不要使用 `float` 或者 `double` 浮点数来表述货币这样的精确数量的值,会导致舍入误差。使用浮点数来进行元和分计算会得到**灾难性的后果**。
- 浮点数最好用来表示象测量值这类数值，这类值从一开始就不怎么精确。(PS:一般情况在你的工作中理论上是用不到的)
- 尽量不要用 `float` 或者 `double` ,来做 +-*/ 运算,以及format,小数请使用  `BigDecimal`
- `BigDecimal` 的构造函数有
	- `java.math.BigDecimal.BigDecimal(double)` 
	- `java.math.BigDecimal.BigDecimal(String)`
  
	请使用  `java.math.BigDecimal.BigDecimal(String)`)(PS:如果你使用 `sonar` 进行代码扫描的话,它会给你提示和建议)
- 比较两个 `BigDecimal` 大小,请使用 `java.math.BigDecimal.compareTo(BigDecimal)` 方法,而**不要**使用 `java.math.BigDecimal.equals(Object)`方法,
  
  因为
	> equals()方法认为，两个表示同一个数但换算值不同（例如， 100.00 和 100.000 ）的 BigDecimal 值是不相等的。
	> 然而， compareTo() 方法会认为这两个数是相等的，所以在从数值上比较两个 BigDecimal 值时，应该使用 compareTo() 而不是 equals() 。

  **参见源码:**

	![](http://i.imgur.com/uT03nUf.png)

## 6.参考

- https://bugs.openjdk.java.net/browse/JDK-7131459
- https://bugs.openjdk.java.net/browse/JDK-8029896
- http://stackoverflow.com/questions/22797964/is-inconsistency-in-rounding-between-java-7-and-java-8-a-bug
- http://www.ibm.com/developerworks/cn/java/j-jtp0114/index.html#4