Скачать или смотреть Converting from Instant to XMLGregorianCalendar in Java Without Losing Precision

Discover the correct method to convert `Instant` to `XMLGregorianCalendar` and vice-versa in Java without precision loss.
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Understanding Instant and XMLGregorianCalendar Conversion in Java

When working with date and time in Java, there are various classes available that serve different purposes. Two important classes are java.time.Instant and javax.xml.datatype.XMLGregorianCalendar.

However, a common challenge developers face is converting between these two types without losing any precision. In this guide, we will delve into the underlying issue of the conversion problem and provide an effective solution that ensures you keep all the precision intact.

The Conversion Dilemma

You might have encountered a scenario where you need to convert an Instant (which represents a specific moment on the timeline) to an XMLGregorianCalendar (which is often used in XML data). This conversion process sounds straightforward, but it can get tricky.

The Problem Statement

Consider the following snippet that you may have tested:

[[See Video to Reveal this Text or Code Snippet]]

Executing this test might lead to an assertion failure, giving an error message indicating an expected vs actual discrepancy in time precision:

[[See Video to Reveal this Text or Code Snippet]]

Why Is This Test Failing?

The crux of the issue is not in the conversion from Instant to XMLGregorianCalendar, as that aspect retains full precision. Instead, the loss of precision occurs during the conversion back to Instant.

When you call .toGregorianCalendar(), you're converting the XMLGregorianCalendar to a GregorianCalendar. Unfortunately, a GregorianCalendar only supports millisecond precision, which means any sub-millisecond data (like nanoseconds) will be lost.

The Solution

To fix this issue, we can use a similar approach for the reverse conversion as we used in the initial conversion—namely, converting to a String.

Step-by-Step Fix

Here's a revised way to perform the conversion back to Instant:

Instead of calling .toGregorianCalendar(), convert the XMLGregorianCalendar directly back to a String.

Use Instant.parse() to convert this String back to an Instant.

Here’s how the updated code looks:

[[See Video to Reveal this Text or Code Snippet]]

This change ensures that the conversion retains all precision, allowing your test case to pass successfully.

Final Thoughts

As a general rule, Instant has precision up to 9 decimal places on seconds, which gives it the capability to handle nanoseconds. On the other hand, while XMLGregorianCalendar offers a broader range of time formats, its handling may lead to issues without a careful conversion strategy.

Key Takeaways

Use String conversion to maintain precision when switching back and forth between Instant and XMLGregorianCalendar.

Be mindful of the precision limitations of the classes you are using.

Implementing these practices can save time and prevent frustrating errors in date and time manipulations in Java programming.