如何在不加载时区的情况下实现LocalDateTime与UTC的互转？

Nice question! I totally get wanting to skip the manual timezone loading steps while handling local-to-UTC (and vice versa) conversions with threetenbp on Android. Let's break down how to do this cleanly, while keeping things efficient.

First, a quick note: LocalDateTime is inherently timezone-agnostic, so converting between local time and UTC does require associating timezone info under the hood—but we can leverage built-in threetenbp constants to avoid manual "loading" (like parsing timezone strings) entirely.

Local Time to UTC #

Your original code works, but we can simplify it using ZoneOffset.UTC—a pre-defined static constant in threetenbp that doesn't require any extra timezone parsing or loading. Here's the streamlined version:

// Get current local time, then convert to UTC LocalDateTime
LocalDateTime localNow = LocalDateTime.now(ZoneId.systemDefault());
LocalDateTime utcNow = localNow.atOffset(ZoneOffset.UTC).toLocalDateTime();

Or even more concise:

LocalDateTime utcNow = LocalDateTime.now(ZoneId.systemDefault())
    .atOffset(ZoneOffset.UTC)
    .toLocalDateTime();

UTC to Local Time #

For the reverse conversion, we can use the same ZoneOffset.UTC constant to avoid manual UTC timezone creation:

// Start with a UTC LocalDateTime, convert to local time
LocalDateTime utcTime = LocalDateTime.now(ZoneOffset.UTC);
LocalDateTime localTime = utcTime.atOffset(ZoneOffset.UTC)
    .atZoneSameInstant(ZoneId.systemDefault())
    .toLocalDateTime();

Why This Counts as "No Explicit Timezone Loading" #

• ZoneOffset.UTC is a built-in constant in threetenbp—no need to call ZoneId.of("UTC") which involves string parsing and timezone data loading. It's lighter and faster.
• ZoneId.systemDefault() pulls the device's default timezone directly, which is a lightweight operation (threetenbp is already optimized for Android's system timezone handling).

Bonus: Using Instant for Even Cleaner Code #

If your use case allows it, consider using Instant instead of LocalDateTime—Instant is inherently a UTC timestamp, so conversions become even simpler:

// Local time to UTC Instant
Instant utcInstant = LocalDateTime.now(ZoneId.systemDefault())
    .atZone(ZoneId.systemDefault())
    .toInstant();

// UTC Instant to local time
LocalDateTime localTime = LocalDateTime.ofInstant(utcInstant, ZoneId.systemDefault());

This avoids dealing with LocalDateTime's timezone-agnostic nature entirely, which can reduce confusion in time-sensitive code.

内容的提问来源于stack exchange，提问作者android developer