Avoid instantiating Plugin class if a plugin is already unloaded

[slovdahl]

Given the following preconditions:

• PluginClassLoader is configured to use ClassLoadingStrategy PDA, DPA or DAP.
• There is a PluginStateListener that calls event.getPlugin().getPlugin().
• A plugin (plugin-a) with a missing dependency is loaded (plugin-a).
• The Plugin class in plugin-a extends, implements or references a class in either plugin-b or in the main application.

In that scenario, when plugin-a is unloaded by pf4j, and the PluginStateListener is run, the event.getPlugin().getPlugin() call will fail with a NullPointerException. This is caused by the class loading done when instantiating the Plugin class in plugin-a. Because not all classes needed are found in plugin-a itself, pf4j will continue by looking up classes in plugin-a's dependencies. Because plugin-b is missing and has no associated ClassLoader.

[slovdahl]

@decebals thoughts about this? Does it make any sense at all? 🤔

We have some internal tests that use custom-built plugins, and now that I was rebuilding them to use a base org.pf4j.Plugin class coming from the application itself, those tests started failing.

The NPE is coming from the PluginStateListener that tries to tear down the Spring application context for unloaded plugins. We can work around it there too, but I figured it could make sense to avoid this in pf4j itself too.

In our PluginStateListener we did something like this before:

if (event.getPluginState() == PluginState.UNLOADED) {
  if (pluginWrapper.getPlugin() instanceof SpringPlugin springPlugin) {
    ApplicationContext context = springPlugin.getPluginApplicationContext();
    if (childApplicationContext instanceof ConfigurableApplicationContext) {
      ((ConfigurableApplicationContext)context).close();
    }
  }
}

The workaround is to only do it if the previous state was PluginState.STOPPED:

if (event.getPluginState() == PluginState.UNLOADED && event.getOldState() == PluginState.STOPPED) {
  if (pluginWrapper.getPlugin() instanceof SpringPlugin springPlugin) {
    ApplicationContext context = springPlugin.getPluginApplicationContext();
    if (childApplicationContext instanceof ConfigurableApplicationContext) {
      ((ConfigurableApplicationContext)context).close();
    }
  }
}

I would like to write some tests for this in pf4j too, but it requires a bit of new infrastructure for being able to create plugins that actually contain fully working classes.