What problems should be avoided when using BigDecimal type? (glory Collection Edition)

Catalog

Preface

1、 background

2、 accident

2.1. Problem description

2.2. Accident level

P0 level

2.3. Accident process

2.4. The cause of the problem

summary

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Preface

Hello everyone , I am procedural -- Handong , I'm here again ！！

Java stay java.math Provided in the package API class BigDecimal, Used for more than 16 The number of significant bits of a bit is calculated precisely . Double precision floating point variables double Can handle 16 Bit significant number . in application , We need to calculate and process larger or smaller numbers .float and double It can only be used for scientific calculation or engineering calculation , Use... In Business Computing java.math.BigDecimal.BigDecimal What is created is an object , We can't use the traditional +、-、*、/ etc. Arithmetic operator Directly perform mathematical operations on its objects , You have to call its corresponding method . The parameter in the method must also be BigDecimal The object of . A constructor is a special method of a class , Specifically for creating objects , Especially objects with parameters .

When BigDecimal Improper use will cause serious consequences , I remember that I was still doing payment projects , The result is something wrong ,

But it's not a big problem , Procedure , Yes bag Normal , I'm not panicking at all .....

1、 background

When we use the amount to calculate or display the amount, we often use BigDecimal, It is also a very recommended type when it comes to the amount .

BigDecimal It also provides many constructor methods , Improper use of these constructor methods may cause unnecessary trouble or even loss of money , Thus causing accident asset loss .

2、 accident

Next, let's look at an accident at the cashier .

2.1. Problem description

The cashier reported an error in calculating the amount of goods , The order cannot be paid .

2.2. Accident level

P0 level

  The heart is almost broken ........

2.3. Accident process

as follows ：

• 13:44, Get the alarm , Order payment failed , Payment availability reduced to 60%

• 13:50, Quickly roll back online code , Back to normal

• 14:20,review Code , Problems found in pre release verification

• 14:58, Modify the problem code to go online , Online recovery

2.4. The cause of the problem

BigDecimal Lost precision in amount calculation .

Cause analysis

First, let's use a piece of code to reproduce the root cause of the problem , As shown below ：

public static void main(String[] args) {
    BigDecimal bigDecimal=new BigDecimal(88);
    System.out.println(bigDecimal);
    bigDecimal=new BigDecimal("8.8");
    System.out.println(bigDecimal);
    bigDecimal=new BigDecimal(8.8);
    System.out.println(bigDecimal);
}

The results are as follows ：

Found by test , When using double perhaps float These floating-point data types , Will lose precision ,String、int Will not be , Why is that ？

Let's open the constructor method to see the source code ：

public static long doubleToLongBits(double value) {
    long result = doubleToRawLongBits(value);
    // Check for NaN based on values of bit fields, maximum
    // exponent and nonzero significand.
    if ( ((result & DoubleConsts.EXP_BIT_MASK) ==
          DoubleConsts.EXP_BIT_MASK) &&
         (result & DoubleConsts.SIGNIF_BIT_MASK) != 0L)
        result = 0x7ff8000000000000L;
    return result;
}

The problem is doubleToRawLongBits In this way , stay jdk in double class （float And int Corresponding ） Provided in double And long transformation ,doubleToRawLongBits Will be double Convert to long, This method is the original method （ The bottom is not java Realization , yes c++ Realized ）.

double The reason why something goes wrong , It's because the decimal point to binary lost precision .

 BigDecimal Expand decimal number in processing N Times make it count on integers , And keep the corresponding precision information .

1. float and double type , It is mainly designed for scientific calculation and engineering calculation , The reason for performing binary floating-point operations , It is designed to provide accurate and fast approximate sum calculation over a wide range of numerical values .
2. It doesn't provide completely accurate results , So it should not be used for precise results .
3. When the floating-point number reaches a certain large number , Will automatically use scientific counting , Such a representation is only approximate to the real number but not equal to the real number .
4. When decimals are converted to binary, there will be infinite cycles or the length of the floating-point mantissa will be exceeded .

summary

therefore , In the process of precision calculation , We try to use String Type to convert .

This thing lost my performance for the whole month .......

about amount of money This sensitive field is better to be cautious ... Lesson from blood ....

Because computers use binary to process data , But a lot of decimals , Such as 0.1 The binary of is a wireless circular decimal , And this kind of number can't be accurately expressed in the computer .

BigDecimal It adopts a way of approximate value to express in the computer , So there are single precision floating-point numbers and double precision floating-point numbers .

BigDecimal As a single precision floating-point number float And double precision floating point numbers double, It's just an approximation when it comes to decimals , It's not true .

When using BigDecimal(Double) When you create one , Got BigDecimal It's a loss of precision .

And using a number that loses precision , The results are also imprecise .

Want to avoid this problem , Can pass BigDecimal(String) The way to create BigDecimal, In this case ,0.1 It will be expressed exactly .

It takes the form of a scale-free value 1, And a scale 1 The combination of .

You can see it here , Just click on it. Well .