How to enable Intellij hot code swap

Question

Intellij does not seem to be doing basic hot code swap on my installation.

For this code:

public class MainTest {
    public void method1() {
        System.out.println("Breakpoint here");
    }

    public void method2() {
        System.out.println("Line that will get 24 modified");
    }

    public static void main(String[] args) {
        System.out.println("First print here");
        MainTest mainTest = new MainTest();
        mainTest.method1();
        mainTest.method2();
        System.out.println("Line that I do not modify");
    }
}

I put a breakpoint on mainTest.method1(); then modify the string at method2(), hit ctrl+s and continue to step by step. Unfortunately the runtime is not updated, old string is being printed. Full stop - compile - run does print the new string. So my conclusion is hot-swap not working.

Is there any settings I need to set to enable hot code swap?

• Ubuntu, JDK 1.6
• Intellij 10.5 Ultimate (evaluation)

Daysidayspring

I've never heard of hot swap for command line apps, only for web apps. – Santamaria

Coming from eclipse, which does this I would expect intellij to be able to do this as well. Am I missing something? – Daysidayspring

i was also facing lot of issues with this, and then i got to know that if you are using VM - then hot swap won't work – Desolation

IDEA 2021.2 : Settings ->Advanced Settings -> Allow auto-make to start even if developed application is currently running. Adding dependency: spring-boot-devtools – Sidelight

In IntelliJ you must be in Debug mode for hot swap to work. – Debutante

Answer 1

After saving your class while waiting on your breakpoint just recompile your class with Build -> Compile 'MainTest.java' or press Ctrl+Shift+F9 with the standard key bindings.

IntelliJ IDEA will then show a small dialog in which it asks you if you want to reload that class.

Citronellal

Great, thanks. Can I make intellij auto compile? (a much better feature would be to allow to: save without auto compile then to have compile on demand turned on by default). – Daysidayspring

See jetbrains.com/idea/webhelp/reloading-classes.html and jetbrains.com/idea/webhelp/debugger-hotswap.html – Citronellal

Thanks a lot, looked for this a lot of time! – Hairtail

For Mac version only was necessary Build -> Recompile 'MainTest.java', a little different. – Groh

What helped me really was Save Actions plugin plugins.jetbrains.com/plugin/7642-save-actions. – Anaxagoras

You saved my days. I prefer the shortcut instead of restarts at every change, even if minor. – Doss

For Mac. command + shift + (Fn) + F9 – Theriault

Answer 2

After below modifications and enabling hot swap, a change in a Java file took 1-2 seconds of restart time. (Initial start time is around 7 seconds for me).

I hope below method helps...

---

First, you need to check “Make project automatically” in preferences menu.

To open preferences menu;

you can go to top menu and click;

IntelliJ IDEA -> Preferences

or you can type below shortcut via keyboard;

cmd + option + s

Then, you can check Make project automatically as in below picture;

Secondly, you need to modify compiler.automake.allow.when.app.running registry setting as true.

To open registry, you need to click below keyboard shortcut:

cmd + shift + a

Type registry as in below picture, select Registry, and hit enter button of keyboard;

After Registry window opened, type compiler.automake to see compiler.automake.allow.when.app.running option, and check it as in below picture;

Then, you need to restart IntelliJ to make registry changes work.

Slab

HotSwap works for me when I debug the application and then build the project or just compile a file. However, it does not work on save even after the registry setting above was done. Is it because I'm in Debug and not Run? – Belay

@DKroot, yes - in IntelliJ you must be in Debug mode for hot swap to work. – Inexpressive

Answer 3

Go to Setting --> Select Debug --> HotSwap

Mortarboard

Even when using this I still have to manually instruct intelliJ to recompile. This does make it a bit easier because you don't have to answer the question whether you want to reload the classes. – Standstill

Answer 4

There is a Debugger > HotSwap option in intellij 11.

Banket

There is an "Eclipse Mode" plugin, which works exactly like eclipse's background incremental compilation. – Banket

Good comment, sadly the plugin does not seem to be maintained any more plugins.intellij.net/plugin/?id=3822 – Daysidayspring

In intellij 13 use: File->Settings->(type HotSwap in search field) – Foulness

Answer 5

I think you are asking about this,

1. Right-click on your java class
2. Select Compile And Reload File

Kiri

Answer 6

I wanted hotswap to automatically run every time I saved a Java file. None of the other answers did this so I just:

1. Created a macro that will run on save: https://mcmap.net/q/80704/-intellij-reformat-on-file-save
2. Added Action: Hotswap at the end of the macro (Run -> Reload Changed Classes)

Carver

Answer 7

For anyone experiencing the same problem as me, you might have incurred in the following recent bug affecting the 2020.2 update: https://youtrack.jetbrains.com/issue/IDEA-247530?_ga=2.116305920.729836936.1599654650-984156210.1599654650

TL;DR As a workaround, you need to re-activate the Groovy plugin.

Saied

Answer 8

I encountered this problem on Ubuntu. I also had an apparently unrelated issue where IntelliJ gave an error about the file watch limit (my apologies that I no longer have the exact error message). To solve this later issue I did the following steps:

1. Add the following line to either /etc/sysctl.conf file or a new *.conf file (e.g. idea.conf) under /etc/sysctl.d/ directory:

   fs.inotify.max_user_watches = 524288
   

2. Then run this command to apply the change:

   sudo sysctl -p --system

Now my React components rebuild and hot swap just fine.

Source

Freeswimming

I have similar issue. however this (increasing file watcher) didn't help me. – Necaise