Force the local datetime to a given value, in order to perform a JUnit test

Question

In my Java application I have to write an Unit Test and I have to check that this test works in both time of the year Daylight savings time / Normal time. To do that, I should set an hardcoded datetime that override the server datetime; execute my test; and restore the real datetime.

How can I do that? I found this solution https://www.baeldung.com/java-override-system-time but is not what I really need (it only define a Instant entity to use in specific instruction, but I need to fix the desired date time and then to perform a test by using some methods not directly connected to an Instant).

Java.time would be preferred.

Edit

The test assigned to me should only read a data cell from an Excel file (formatted in Excel as Datetime) and make a comparison with a fixed date.

This is a detailed description:

1. There is an Excel file with a cell formatted as "Datetime"

2. I read this cell by

    Workbook excelWorkbook = new Workbook(excelFileStream, excelOptions);
    Cells cells = worksheet.getCells();
   

3. A cell of the sheet if formatted by dateTime

4. A method in my implementation read the cell and creates a Datetime var

That's all. My task is, create an Unit Test in order to check if the given value is always correct independent from the Daylight saving / Normal time of the year. I should "force" the local date time of the server, implement the test by creating something like

assertThat(ExcelCellDateTime).isEqualTo("<given date time>");

And restore the datetime of the Java application to the correct Date Time of the server. So the question is:

How can I simulate a certain datetime inside a unit test of my Java application, independent from the real date time of the server, such that the application 'believes' that the current datetime is the one I forced and the test can be executed 'as' now would be equal to the datetime I forced?

Comment on possible duplicate

I don't think this is a duplicate of the suggested question. I don't want to force a date and to compare the new date with something else, but I want to force the date and execute a test without the new date as parameter.

Answer 1

Your business logic should be given a Clock instance to provide the current time (and time zone, when necessary) instead of using static methods like LocalDateTime.now() which hide a dependency on the environment. It's like a pluggable factory for various notions of "now."

When your logic uses a Clock, it is simple for tests to use clocks with a fixed time to test for correct behavior during daylight saving transitions, leap days, and other date-time corner cases.

Well-designed code is easy to use for any purpose, including testing. This sort of difficulty in testing is a symptom of poor design that indicates the code will be hard to reuse in other ways.