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Comment by gred | Hacker News Reader

My experience with very simple Jersey web apps is ~1.5 seconds to start up. Much less than his reported ~8 seconds with Spring Boot, but still not in the 100 ms range he reports with Go. I assume one second or so is about as low as you can go with a mainstream Java framework without AOT, though I'd be happy to be corrected.

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I worked on an app in kotlin a while back, and am currently working in a dotnet app. We can run our entire unit test suite faster than the JVM started up on that project.

Also, 8 seconds is quick in my experience for Java - I’ve seen more like 15-30

The JVM starts in milliseconds

Probably you're loading many thousands of classes...

It certainly tells me it does. i can hit 'run unit test', sit back in my chair and zone out for 5 seconds, then come back and read that the test took 22ms.

Yeah that’s been my experience too.

haha well your test did take 22ms, but starting the JVM and loading all the classes and all your dependency injection stuff probably took 5s. You can test it yourself with a tiny hello world, it's pretty fast to start.

But that's our point - you're saying java is fast (and it is ripping fast once it gets going) and the startup is fast, unless it's not.

Well no, startup is fast, period. You're probably just giving the class loader a ton of work on startup? I would start with checking that I think.

It could also be when he's hitting "run test" it's actually "compile and run"...

> Well no, startup is fast, period. You're probably just giving the class loader a ton of work on startup? I would start with checking that I think.

Ok, maybe I misspoke about the "JVM" startup. But the time between `mvn test` and it actually logging the first line of user code was in the region of 15 seconds. Every java project I've worked on has had startup time issues once they're bigger than a toy project.

Saying "the JVm startup is fine, it's just the class loader that's slow" reinforces the point of "it's slow to startup"

> It could also be when he's hitting "run test" it's actually "compile and run"...

This would be true in C# too, and in go. Both of those tools have much quicker compilation times IME than java, which is just a nice plus.

That's mostly because nobody cares enough to optimize it, but if you split code into modules so your number of classes, including dependencies, is smaller, then your tests will start quickly. Usually it's dependencies and DI that's the cause of this I think.

Aren't Go's compile times about the same as Java's? Of course it depends on what tooling you use to package the jars.

BTW, I wonder if any test frameworks are using the hot reloading JVMs yet? Would also solve this problem.

Well I could switch to a programming language that isn't so slowed down by ahead-of-time compilation. Maybe a JIT language?

Thanks for bringing up some painful memories. Now I'm working in C and tests take about 100ms end-to-end (multicore, no frameworks). Sometimes I re-run them for no reason just to enjoy how fast everything is.

Can confirm with Vert.x. I used to have a JRebel subscription for code hot swapping in another project but in my current Vert.x thing I simply don’t need it. It’s just fast.

Also, you don't have to continuously restart the server for every change. Can hot swap code. And for tests there are solutions to keep the jvm running between the code->test->fix cycle. But I've never felt the need for those, running a single test is often plenty fast as one doesn't need to instantiate the whole app, only the dependencies needed for the test. So plenty fast.

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