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Comment by haddr | Hacker News Reader

I'm deeply missing hot swapping. I believed it could be included in java 9 but i think we have to wait...

ZeroTurnaround have a payware product (that's highly acclaimed in this space) called JRebel. It swaps out bytecode directly as opposed to using Java's (somewhat broken) classloader mechanism.

Are you aware of DCEVM? https://github.com/DCEVM/DCEVM#readme

What is wrong with restarting your application?

Edit: Real Hotswapping will requires profound changes to the memory model. It would introduce too much complexity. I rather restart my application than to suffer from a fragile and leaking platform.

It can take up to 5 minutes to restart my application.

Can't you just break things up to see them in action? If not, you should question your architecture.

So run multiple instances of the app, and put some kind of load balancer / proxy in front of the instances.

Unfortunately, this will require externalization of state data, but if you can't be down for 5 minutes, that's what you need to do.

You probably only need one large server running the instances, rather than a cluster. Java apps don't seem to do well -- longer GC pauses -- when they have over 2 GB of memory, anyway, so run multiple instances of "small" apps on a server with say 12 cores. A single server with a large pool of RAM shared between a large number of cores can do pretty amazing things with a collection of "small" programs on it (DB, web service apps, message queue server, web servers)

Of course, I'm assuming said server isn't Windows...

I don't think he meant hotswapping in productive environments but during development. Load balancing doesn't help here.

The webapp I'm working on currently has a startup time of 50s on my macbook. If I restart the webapp 100 times a day, thats a total waiting time of 30+ minutes. Our team is 6 developers, so we loose 3h per day.

Real hotswapping may be extremely complex but IMO it would be absolutely worth the effort.

Ah, gotcha. It's funny, I used to do very frequent tests like that when I worked in C (particularly the first few years in the language), but tend to do much fewer test runs in Java. Fewer fatalities, but longer build/start times, I guess.

of course you can restart your app, but it's not sufficient answer to the problem. Apparently long time ago hot swapping was scheduled around Java 9, but they didn't progress with it since then. See JEP-159 for more details: http://openjdk.java.net/jeps/159

What is hot swapping?

Replacing code while it's running. Currently this is possible but quite limited in the official JDK, you can only replace the contents of method bodies.

Edit: Demo of it in action here, notch is rapidly developing a game in Eclipse by changing the code while it's running: https://www.youtube.com/watch?v=rhN35bGvM8c

There's always JRebel!

Not really.

http://zeroturnaround.com/blog/goodbye-liverebel/

Oh, damn! I just started using jrebel.. (I know I asked what hotswapping was but I was curious if the OP meant hotswapping in the same way as jrebel or something else)

edit: Wait, that is liverebel, not jrebel

Ah, sorry! I though liverebel was the new name.

notch is rapidly developing a game in Eclipse

He's modifying the Swing object tree from the main thread! Don't do that, Swing isn't thread safe ;-)

https://www.youtube.com/watch?v=rhN35bGvM8c#t=1454

http://docs.oracle.com/javase/7/docs/api/javax/swing/package...

Isn't this basically what Spring is for? I get that it's not included in the language itself, but that seems more like a feature that should be provided by a framework rather than the language itself...

No, that's not part of Spring. It would be implemented by a container. OSGi was a sort of failed attempt at standardizing it. Right now you have to pay for something like jrebel (worth it, IMO) to get hot swapping.

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