Configure a module with an initializer

Use an initializer when you write a library of components and want it to set itself up in any application that puts the jar on its classpath: mount its resources, register listeners, add settings. Code for one application belongs in its init() method instead.

Write the initializer

Implement IInitializer. Wicket calls init() when the application starts and destroy() when it stops:

ChartsInitializer.java

package com.example.charts;

public class ChartsInitializer implements IInitializer {

    @Override
    public void init(Application application) {
        application.getSharedResources().add("chartData", new ChartDataResource());
        application.getComponentInstantiationListeners().add(new ChartLicenseCheck());
    }

    @Override
    public void destroy(Application application) {
        // release what init() acquired, such as threads or connections
    }

    @Override
    public String toString() {
        return "Charts library";
    }
}

Wicket logs each initializer it runs at info level with its toString(), so a readable name helps.

Register it with the ServiceLoader

Wicket finds initializers with java.util.ServiceLoader. Create the file src/main/resources/META-INF/services/org.apache.wicket.IInitializer and put the fully qualified class name in it:

com.example.charts.ChartsInitializer

The file may list several initializers, one per line.

If your library is a Java module with a module-info.java, declare the service there as well:

module com.example.charts {
    requires org.apache.wicket.core;
    provides org.apache.wicket.IInitializer with com.example.charts.ChartsInitializer;
}

When the initializer runs

Application.initApplication() runs these steps in order:

1. Wicket’s internal setup.
2. The init() method of every initializer found.
3. The application’s own init().

Because the application’s init() runs last, an application can change any setting an initializer made. When the application is destroyed, Wicket calls destroy() on every initializer, after the application’s onDestroy().

Wicket uses the same mechanism for its own modules: wicket-jmx, for example, registers its MBeans from an initializer as soon as the jar is on the classpath (see Inspect and change settings with JMX). Each Wicket application in a web application runs every initializer once.

Wicket does not read the META-INF/wicket/*.properties files with an initializer= entry that older documentation describes; only the ServiceLoader registration works.