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How to use

This feature allows you to write code, save the code on disk and reload the web page from browser without restart the application (from IDE).

public class PippoApplication extends Application {

    protected void onInit() {
        GET("/", routeContext -> routeContext.send("Hello"));

public class PippoLauncher {

    public static void main(String[] args) {
        Pippo pippo = new Pippo();


So, the code is almost the same with the version without hot reloading. The only one difference is that you must specify your application class name and NOT to create the application instance.

You can supply the application class name using one of the below methods:

  • create by hand a META-INF/services/ro.pippo.core.Application file with content mypackage.PippoApplication (see Java ServiceLoader)
  • add a MetaInfServices annotation on your application class (generate automatically the META-INF/services/ro.pippo.core.Application file)
  • use -Dpippo.applicationClassName system property

How works

To implement hot reloading, Pippo introduces two new concepts (classes in package ro.pippo.core.reload):

  • ReloadWatcher
  • ReloadClassLoader

The ReloadWatcher class runs a background task that checks directories for new, modified or removed files. The default directory is target/classes (standard from Maven) but you can specify other target classes directory via -Dpippo.reload.targetClasses system property. When ReloadWatcher detects a modification, the method onEvent(ReloadWatcher.Event event, Path dir, Path path) method from Pippo class is called.

By default, the onEvent method does:

  • stop server;
  • create a new instance of application using ReloadClassLoader
  • set the new application instance in PippoFilter
  • start server

You can overwrite this method (onEvent) if you wish more control (not to restart the server, only to …).

The ReloadClassLoader class is a Java ClassLoader that loads classes from files. You can specify the classes that will be loaded by this loader using -Dpippo.reload.rootPackageName system property (for example I want to reload only classes from web layer, classes with packages that start with “mycompany.web”).

By default, hot reloading mechanism is enabled if you are in dev (development) mode and you use the application class name instead of application instance. You can use -Dpippo.reload.enabled system property to enable or disable the hot reloading mechanism.

Note If you use Idea IntelliJ as IDE, it’s a good idea to remap Ctrl-Shift-F9 (compile file) to Ctrl-S (save file) to compile file on save (see post).