Speeding up the incremental extraction of web jars

[adpi2]

The incremental compilation of my project (scalacenter/scaladex) is slowed down by the webJars task.

If I run server/Test/compile twice without changing any source, it takes about 3 sec each and the time is spent on extracting the web jars and coyping the resources. That's kind of slow for a no-op incremental compilation.

$ sbt -Dsbt.task.timings=true
sbt:scaladex> server / Test / compile
[info] Total time: 2481 ms
[info]   server / Web-assets / webJars                                     : 824 ms
[info]   server / Web-assets-test / webJars                                : 678 ms
[info]   server / Compile / copyResources                                  : 444 ms
[info]   server / Web-assets / assets                                      : 297 ms
...
[success] Total time: 3 s, completed May 19, 2023, 2:31:57 PM
sbt:scaladex> server / Test / compile
[info] Total time: 2431 ms
[info]   server / Web-assets / webJars                                     : 787 ms
[info]   server / Web-assets-test / webJars                                : 677 ms
[info]   server / Compile / copyResources                                  : 469 ms
[info]   server / Web-assets / assets                                      : 283 ms
...
[success] Total time: 3 s, completed May 19, 2023, 2:34:03 PM

After some investigation I found out that most of the time is spent in:

sbt-web/src/main/scala/com/typesafe/sbt/web/SbtWeb.scala

Line 450 in ba7674e

e.extractAllWebJarsTo(to)

Would it be possible to use some sort of caching to speed up this step?

[mkurz]

Did you test with 1.5.0-M1? I just released that 3 days ago: https://github.com/sbt/sbt-web/releases/tag/1.5.0-M1

[adpi2]

It is significantly faster:

[info]   server / Compile / copyResources                                  : 481 ms
[info]   server / Web-assets / webJars                                     : 419 ms
[info]   server / Web-assets / assets                                      : 361 ms
[info]   server / Web-assets-test / webJars                                : 359 ms

But still, would it be possible to not touch the managed resources, so that sbt doesn't need to copy them?

[mkurz]

@adpi2 Maybe. If you provide a pull request, I am happy to review it. I don't currently have time to take a closer look here myself.