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)

I've never heard of hot swap for command line apps, only for web apps. — duffymo, Jun 19 '11 at 12:05
Coming from eclipse, which does this I would expect intellij to be able to do this as well. Am I missing something? — Maxim Veksler, Jun 19 '11 at 12:09
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 — Ashish Shetkar, Jun 01 '20 at 04:07

score 125 · Accepted Answer · answered Jun 19 '11 at 12:21

125

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.

answered Jun 19 '11 at 12:21

joschi

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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). – Maxim Veksler Jun 19 '11 at 16:01
4

See http://www.jetbrains.com/idea/webhelp/reloading-classes.html and http://www.jetbrains.com/idea/webhelp/debugger-hotswap.html – joschi Jun 19 '11 at 16:13
2

Thanks a lot, looked for this a lot of time! – Simcha Sep 07 '17 at 14:11
For Mac version only was necessary `Build -> Recompile 'MainTest.java'`, a little different. – Deoxyseia Nov 09 '21 at 17:13
What helped me really was Save Actions plugin https://plugins.jetbrains.com/plugin/7642-save-actions. – Tamilselvan Feb 03 '22 at 13:08

Berk · Answer 2 · 2020-07-28T12:51:47.380

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.

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? — Dmitriy Korobskiy, Apr 07 '17 at 14:17
@DKroot, yes - in IntelliJ you must be in Debug mode for hot swap to work. — Woodchuck, Feb 20 '19 at 21:26

score 8 · Answer 3 · answered Aug 23 '12 at 22:55

8

There is a Debugger > HotSwap option in intellij 11.

answered Aug 23 '12 at 22:55

Pratik Khadloya

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There is an "Eclipse Mode" plugin, which works exactly like eclipse's background incremental compilation. – Pratik Khadloya Aug 24 '12 at 00:38
1

Good comment, sadly the plugin does not seem to be maintained any more http://plugins.intellij.net/plugin/?id=3822 – Maxim Veksler Aug 24 '12 at 08:58
3

In intellij 13 use: File->Settings->(type HotSwap in search field) – Ghandhikus Oct 30 '14 at 06:33

score 6 · Answer 4 · answered Aug 19 '16 at 05:10

6

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

answered Aug 19 '16 at 05:10

Yasir Shabbir Choudhary

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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. – Henno Vermeulen Oct 20 '16 at 13:04

score 4 · Answer 5 · answered Jan 19 '19 at 19:49

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

Created a macro that will run on save: https://stackoverflow.com/a/5581992/14731
Added Action: Hotswap at the end of the macro (Run -> Reload Changed Classes)

score 3 · Answer 6 · answered Sep 10 '20 at 09:56

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.

score 3 · Answer 7 · answered Nov 03 '20 at 11:26

3

I think you are asking about this,

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

answered Nov 03 '20 at 11:26

Rajitha Bhanuka

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score 0 · Answer 8 · answered Feb 01 '18 at 16:51

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:

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
```
Then run this command to apply the change:

sudo sysctl -p --system

Now my React components rebuild and hot swap just fine.

Source

I have similar issue. however this (increasing file watcher) didn't help me. — Rejeev Divakaran, Aug 10 '18 at 17:22

How to enable Intellij hot code swap

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