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jimfs
Java2014–2025
google / jimfs

jimfs

An in-memory file system for Java

Systems & Infrastructure
Explore on GitHubRead Hacker News discussions
5Distinct people
7Mentions
0Explicit recommenders
2.6KGitHub stars
REPOSITORY AT A GLANCE

What to know before exploring

What it is

An in-memory file system for Java

GitHub

Where it is used

No documented product use has been verified yet.

What people are saying

No recommendation phrase was found in the indexed comments.

No classified warning is attached to this page. Read the discussions before treating silence as approval.

Also in these discussions

No other repository is repeatedly linked in the same discussions.

This week on Hacker News

Nov 7–Nov 14 UTC · Direct repository links and separately labeled related project stories. Indexed through 11/14/2026, 1:49:46 PM UTC.

No direct repository links were found in the indexed HN discussions over the past seven days. Older discussions are available below.

Mentions over time

HN mentions by year7 dated mentions

Observed mentions: 2014-03-29 06:42:33 UTC to 2025-02-25 18:23:10 UTC.

01232014-01-01 00:00:00 UTC to 2015-01-01 00:00:00 UTC (exclusive): 1 mentions; 1 submissions, 0 comments20142015-01-01 00:00:00 UTC to 2016-01-01 00:00:00 UTC (exclusive): 2 mentions; 0 submissions, 2 comments2016-01-01 00:00:00 UTC to 2017-01-01 00:00:00 UTC (exclusive): 0 mentions; 0 submissions, 0 comments2017-01-01 00:00:00 UTC to 2018-01-01 00:00:00 UTC (exclusive): 0 mentions; 0 submissions, 0 comments2018-01-01 00:00:00 UTC to 2019-01-01 00:00:00 UTC (exclusive): 0 mentions; 0 submissions, 0 comments2019-01-01 00:00:00 UTC to 2020-01-01 00:00:00 UTC (exclusive): 0 mentions; 0 submissions, 0 comments20192020-01-01 00:00:00 UTC to 2021-01-01 00:00:00 UTC (exclusive): 0 mentions; 0 submissions, 0 comments2021-01-01 00:00:00 UTC to 2022-01-01 00:00:00 UTC (exclusive): 0 mentions; 0 submissions, 0 comments2022-01-01 00:00:00 UTC to 2023-01-01 00:00:00 UTC (exclusive): 0 mentions; 0 submissions, 0 comments2023-01-01 00:00:00 UTC to 2024-01-01 00:00:00 UTC (exclusive): 0 mentions; 0 submissions, 0 comments2024-01-01 00:00:00 UTC to 2025-01-01 00:00:00 UTC (exclusive): 3 mentions; 1 submissions, 2 comments2025-01-01 00:00:00 UTC to 2026-01-01 00:00:00 UTC (exclusive): 1 mentions; 0 submissions, 1 comments2025

Counts come from the repository’s dated source evidence. Index cutoff: 2026-11-02 08:16:59 UTC.

View mention counts and date ranges
UTC ranges include their start and exclude their end. Select a period to filter the source evidence.
PeriodStart (UTC)End (exclusive, UTC)MentionsSubmissionsComments
20142014-01-01 00:00:00 UTC2015-01-01 00:00:00 UTC110
20152015-01-01 00:00:00 UTC2016-01-01 00:00:00 UTC202
20162016-01-01 00:00:00 UTC2017-01-01 00:00:00 UTC000
20172017-01-01 00:00:00 UTC2018-01-01 00:00:00 UTC000
20182018-01-01 00:00:00 UTC2019-01-01 00:00:00 UTC000
20192019-01-01 00:00:00 UTC2020-01-01 00:00:00 UTC000
20202020-01-01 00:00:00 UTC2021-01-01 00:00:00 UTC000
20212021-01-01 00:00:00 UTC2022-01-01 00:00:00 UTC000
20222022-01-01 00:00:00 UTC2023-01-01 00:00:00 UTC000
20232023-01-01 00:00:00 UTC2024-01-01 00:00:00 UTC000
20242024-01-01 00:00:00 UTC2025-01-01 00:00:00 UTC312
20252025-01-01 00:00:00 UTC2026-01-01 00:00:00 UTC101

What Hacker News says

7 sources
bhelx·Feb 25, 2025
WASI does open up some holes you should be considerate of. But it's still much safer than other implementations. We don't allow you direct access to the FS we use jimfs: https://github.com/google/jimfs I typically recommend people don't allow wasm plugins to talk to the filesystem though, unless they really need to read some things from disk like a python interpreter. You don't usually need to.
In Chicory: A JVM native WebAssembly runtime
bhelx·Oct 15, 2024
Chicory has some partial wasip1 support. https://github.com/dylibso/chicory/tree/main/wasi. We use jimfs to keep things simple and secure (and not worry about exposing the real filesystem): https://github.com/google/jimfs When I did this experiment a few months ago, what we could accomplish was pretty limited. I could load and query databases, but not write to them. However the Chicory wasip1 implementation is advancing. BTW, we've borrowed a few ideas from wazero so thanks for your work there :)
In Sqlite3 WebAssembly
pron·Jun 22, 2015
In-memory implementations are very useful, especially for larger tests, but, again, I don't see anything in Haskell that makes it easier. For example, on the JVM tests often use Google's Jimfs[1] in-memory file system (the JVM has pluggable file systems implemented as Java interfaces), and for relational databases they use H2[2]. None of this requires any code changes, perhaps thanks to the JVM's extreme pluggability. The JVM uses SPIs for almost everything, from encryption to filesystems to sockets -- you can fake the entire OS (almost) in Java, and what you can't fake easily, you can still fake with an agent (unrelated, but starting in Java 9, even the JIT is pluggable, so you can control machine-code generation at any granularity you want). Just since my conversation here with chadaustin, I've written a small library that uses an agent
Read the rest
to make the entire JVM use a virtual clock[3] -- again, with no code changes necessary in the tested code. [1]: https://github.com/google/jimfs [2]: http://www.h2database.com/html/main.html [3]: https://github.com/puniverse/timewarp
In Testable IO in Haskell
pron·Jun 21, 2015
> Haskell's ability to define restricted subsets of generalize IO is extremely powerful and sidesteps this entire problem. Except it doesn't, precisely where that problem is most annoying. For example, suppose you want to test a scheduler, or any mechanism that places a timeout on an operation. You could abstract away the entire timing mechanism, but that is exactly what you're trying to test: maybe the developer used some system function wrong, and is now waiting 5 seconds instead of 5 milliseconds? This approach (which is the same as the mocking approach -- there's nothing special about Haskell here) doesn't do too well when you want to test timing, and those tests are usually the most brittle (hosted continuous integration services often introduce really long pauses). Of course, you could weave virtual time services throughout your cod
Read the rest
e, but then you have the third-party library problem again. Mocking (or IO isolating as you call it in Haskell) ends up producing really good unit-tests, but often the more important tests span more than one small unit, and making those non-flaky is very hard. In theory, you could replace all references to time in a JVM language -- even in libraries -- using an agent or a bootstrap classloader in order to make larger tests run predictably, but I'm not aware of people going to such great lengths (maybe Google does it; I know, for example, that when it comes to accessing the filesystem, they replace the JVM's filesystem provider with an in-memory file-system[1] when they don't mock, but I don't know how the deal with clocks). EDIT: Apparently somebody has done just that as a research project[2]. It's actually given me an idea for a nice afternoon-project. [1]: https://github.com/google/jimfs [2]: http://www3.imperial.ac.uk/pls/portallive/docs/1/55125696.PD...
In Testable IO in Haskell

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