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> Java seems to have its own TZDB so in general your OS and application layer might disagree about zones. More precisely, the Java Virtual Machine (not the pro

by jt2190 6y ago

> Java seems to have its own TZDB so in general your OS and application layer might disagree about zones. More precisely, the Java Virtual Machine (not the programming language) will, like any operating system running inside a VM, need to be patched with the new time information. Updating the host machine’s time info is not enough.

heavenlyblue 6y ago
Why does JVM need to know about time zones?
- lou1306 6y ago
  I guess it allows you to implement a JVM that runs on bare metal, or a host without timezone support.
  - jholloway7 6y ago
    Yep, it makes it easier to port the JRE to platforms like Windows that have an entirely different TZ system. Python, for example, has the same problem. Historically, Python took the stance of providing a "naive" datetime module that provides a conversion API, but delegated the actual TZ conversion to external implementations (dateutil, pytz). Only recently (as of 3.9) have they added a stdlib implementation that uses system-level zoneinfo by default.
- adrianmonk 6y ago
  It's not the VM engine (JIT compiler, etc.) itself, of course. It's a data file that resides in the Java Runtime Environment directory, and some of the standard library code uses this file. I assume it's done this way because it's the most foolproof way to get consistency across all platforms. If Java did delegate, it might be hard (or even impossible?) to get the same behavior out of all the different underlying APIs on each platform. And it would be challenging to create the tests to validate that it really does behave consistently. Also, although it creates extra maintenance work, it has the advantage that it decouples Java-level and OS-level updates. Suppose development machines run one OS (with its TZ data) but QA and production run a different OS. To harmonize environments as closely as possible, it's helpful that you can keep the TZ data versions on all JVMs in sync without relying on multiple OS vendors' patch processes to enable that.
Kwpolska 6y ago
Java is not an operating system. Other environments, such as .NET Core, don’t have any issues using the OS timezone database: https://devblogs.microsoft.com/dotnet/cross-platform-time-zones-with-net-core/ https://devblogs.microsoft.com/dotnet/cross-platform-time-zo...

by jt2190 6y ago

heavenlyblue 6y ago

Why does JVM need to know about time zones?

lou1306 6y ago
I guess it allows you to implement a JVM that runs on bare metal, or a host without timezone support.
- jholloway7 6y ago
  Yep, it makes it easier to port the JRE to platforms like Windows that have an entirely different TZ system. Python, for example, has the same problem. Historically, Python took the stance of providing a "naive" datetime module that provides a conversion API, but delegated the actual TZ conversion to external implementations (dateutil, pytz). Only recently (as of 3.9) have they added a stdlib implementation that uses system-level zoneinfo by default.
adrianmonk 6y ago
It's not the VM engine (JIT compiler, etc.) itself, of course. It's a data file that resides in the Java Runtime Environment directory, and some of the standard library code uses this file. I assume it's done this way because it's the most foolproof way to get consistency across all platforms. If Java did delegate, it might be hard (or even impossible?) to get the same behavior out of all the different underlying APIs on each platform. And it would be challenging to create the tests to validate that it really does behave consistently. Also, although it creates extra maintenance work, it has the advantage that it decouples Java-level and OS-level updates. Suppose development machines run one OS (with its TZ data) but QA and production run a different OS. To harmonize environments as closely as possible, it's helpful that you can keep the TZ data versions on all JVMs in sync without relying on multiple OS vendors' patch processes to enable that.

Kwpolska 6y ago

Java is not an operating system. Other environments, such as .NET Core, don’t have any issues using the OS timezone database: https://devblogs.microsoft.com/dotnet/cross-platform-time-zones-with-net-core/ https://devblogs.microsoft.com/dotnet/cross-platform-time-zo...