BonkersJavacOptimisationsTest

Came across these issues while working on a Minecraft plugin that softly depends on other plugins. It hadn't been an issue since those plugins were always present, but as soon as one was missing, the plugin exploded with NoClassDefFoundError exceptions.

The problem arises from the assumption that you can directly reference potentially-missing classes if the process of execution never 'reaches' that code, eg:

class ChestLockerListener implements Listener {
    
    @EventHandler(ignoreCancelled = true)
    public void onChestOpen(FictionalChestOpenEvent event) {
        if (Bukkit.getPluginManager().isPluginEnabled("SomePlugin")) {
            if (SomePlugin.isChestLocked(event.getChest())) {
                event.setCancelled(true);
                return;
            }
        }
    }
    
}

And yes, this does work. However, there are cases where javac optimises your code ways that break this assumption. This repo demonstrates three instances of this (see SoftDependencyTests):

KEEP IN MIND THAT THE init METHOD IN ALL THE TESTS IS NEVER INVOKED!

Tests

Lambda Hoisting

If you've played around with any Java decompilers, you may have noticed that lambdas are often compiled as sibling classes where, for example, lambdas in ExampleClass will produce ExampleClass$1 as a standalone class within the same package. This is not always true, something that lambdaHoistTest() demonstrates. Instead, the lambda gets hoisted as a sibling method, eg:

public final class Demonstration {
    public void init() {
        ActionRegistry.registerActionProvider(
            ExampleAction.IDENTIFIER,
            this::lambda$init$0
        );
    }

    public Action lambda$init$0(UUID uuid) {
        return new ExampleAction(uuid.toString(), 0, 0, 0);
    }
}

And because Action is a class from a missing soft-dependency, the Demonstration class fails upon initialisation.

Const `new`

However, manually doing what you wish the compiler had done and creating a sibling class isn't helpful either.

public final class Demonstration {
    public void init() { // Remember that this is never invoked
        ActionRegistry.registerActionProvider(
            ExampleAction.IDENTIFIER,
            new ExampleActionProvider() // Why does this cause a NoClassDefFoundError?! ಠ_ಠ
        );
    }
}

You'd think that, based on the ChestLockerListener example above, that this would be fine. But something is happening here that's causing the Demonstration class to fail upon initialisation that I can't put my finger on... but whatever it is, the issue is 'fixed' by replacing it with a static-method reference, even if that static method is on an missing class.

public final class Demonstration {
    public void init() { // Remember that this is never invoked
        ActionRegistry.registerActionProvider(
            ExampleAction.IDENTIFIER,
            ExampleAction::provider // This fixes it... for some reason
        );
    }
}

Const fields

Now, let's say you have an event listener like so:

public final class Demonstration {
    private final Object events = new Object() { // This is never registered nor is any of its methods invoked
        @Subscribe
        public void handleSomeEvent(
            final Object event
        ) {
            for (final ActionUser user : ActionUser.getUsers()) {
                user.acceptAction(new ExampleAction(UUID.randomUUID().toString(), 0, 0, 0));
            }
        }
    };

    public void init() { // Remember that this is never invoked
        Something.eventBus.register(this.events);
    }
}

That anonymous class is compiled as a sibling class, as expected. And yet, despite none of the missing classes being 'reached'. Fixing this means making the field no longer final (or effectively final) and re-assigning it within the init method, like so:

public final class Demonstration {
    private final Object events = null;

    public void init() { // Remember that this is never invoked
        Something.eventBus.register(this.events = new Object() {
            @Subscribe
            public void handleSomeEvent(
                final Object event
            ) {
                for (final ActionUser user : ActionUser.getUsers()) {
                    user.acceptAction(new ExampleAction(UUID.randomUUID().toString(), 0, 0, 0));
                }
            }
        });
    }
}

Which is bizarre because this changes seemingly nothing about how the anonymous class gets compiled as a sibling class.

How to run

Clone this repo and execute sh run.sh

Results

openjdk 17.0.9 2023-10-17 LTS
OpenJDK Runtime Environment Corretto-17.0.9.8.1 (build 17.0.9+8-LTS)
OpenJDK 64-Bit Server VM Corretto-17.0.9.8.1 (build 17.0.9+8-LTS, mixed mode, sharing)

> Task :testing:test

SoftDependencyTests > constParameterTest() STARTED

SoftDependencyTests > constParameterTest() PASSED

SoftDependencyTests > lambdaHoistTest() STARTED

SoftDependencyTests > lambdaHoistTest() PASSED

SoftDependencyTests > constFieldTest() STARTED

SoftDependencyTests > constFieldTest() PASSED

Deprecated Gradle features were used in this build, making it incompatible with Gradle 9.0.

You can use '--warning-mode all' to show the individual deprecation warnings and determine if they come from your own scripts or plugins.

For more on this, please refer to https://docs.gradle.org/8.5/userguide/command_line_interface.html#sec:command_line_warnings in the Gradle documentation.

BUILD SUCCESSFUL in 1s
7 actionable tasks: 7 executed

Protonull/BonkersJavacOptimisationsTest