Fixing Expecty Crashes With Java Enums

Hey guys! Ever run into a weird crash in your Scala code when using expecty with Java enums? It's a real head-scratcher, but don't worry, we're going to dive deep into what's going on and how to fix it. This guide is all about troubleshooting and providing solutions so you can get back to writing clean, reliable code. Let's get started, shall we?

The expect Crash: A Deep Dive

So, what's the deal? You're using expecty, a fantastic testing library, to verify your code, and suddenly, boom! You get a java.lang.NoClassDefFoundError at runtime. Specifically, it's a ClassNotFoundException: ExampleEnum$. It’s like the JVM can't find your enum class. This is super frustrating, especially when your code compiles just fine, but then blows up when you run it.

Let’s break down the scenario. You've got an ExampleEnum, and you're trying to use expect(actualCode == ExampleEnum.OK). The compiler is happy, but the runtime isn’t. This usually means that the class loader can't find the enum class, or something is messed up with how the class is being loaded at runtime. This often happens because of how Scala and Java interact, especially when dealing with nested or generated classes.

Here’s a simple example that recreates the issue:

//> using dep com.eed3si9n.expecty::expecty:0.17.0
import com.eed3si9n.expecty.Expecty.expect

object ExampleEnum extends Enumeration {
 val OK, ERROR = Value
}

@main def repro() = {
  val actualCode = ExampleEnum.ERROR
  expect(actualCode == ExampleEnum.OK)
}

When you run this, you'll likely see the crash. The core problem is how expecty attempts to inspect and evaluate the expression involving the enum. It’s a bit of a quirk in the interaction between expecty, Scala, and Java enums that can trip you up. The key takeaway is that the class isn’t being found when expecty tries to use it during the runtime evaluation of the expect call. This is usually due to how the class loaders are set up or how Scala generates bytecode when dealing with enums and comparisons.

Why This Happens

The root cause often lies in the way Scala and Java handle enums internally. Scala, by default, tries to optimize and potentially create these enum classes in a way that sometimes confuses the class loader. expecty, in turn, tries to dig into the expression, understand the values, and evaluate them, but it fails to load the enum class at runtime. The compiler has no issues because it's only checking syntax and type compatibility, but the runtime needs the actual class definition to execute the comparison. The error stems from expecty’s attempt to introspect the enum value. The class loader fails because it can’t find the specific generated class during the introspection process of expect.

The Problem in Detail

When expecty is called with an expression that involves an enum, it internally tries to analyze the expression to provide detailed error messages if the expectation fails. This analysis involves inspecting the types and values involved. In some cases, the way expecty accesses the enum's class information during this analysis causes the ClassNotFoundException. This is specifically because of the way expecty tries to resolve and load the enum class at runtime.

In essence, it's a timing issue. The class isn't available when expecty tries to access it. This can be exacerbated by different classloading strategies or how the Scala compiler and JVM interact.

The Workaround: A Simple Fix

Fortunately, there's a straightforward workaround. You can sidestep the issue by extracting the enum value into a separate variable before calling expect. This simple change helps the classloader by ensuring the enum is correctly loaded before expecty attempts to evaluate the expression. This is because the enum is guaranteed to be loaded when the variable is initialized.

Here's the corrected code:

//> using dep com.eed3si9n.expecty::expecty:0.17.0
import com.eed3si9n.expecty.Expecty.expect

object ExampleEnum extends Enumeration {
 val OK, ERROR = Value
}

@main def repro() = {
  val actualCode = ExampleEnum.ERROR
  val expectedCode = ExampleEnum.OK
  expect(actualCode == expectedCode)
}

Why the Workaround Works

By introducing the intermediate variable expectedCode, you ensure that ExampleEnum is loaded into the classloader before the expect call. This pre-loading gives expecty the class information it needs without causing the ClassNotFoundException. It’s like giving the class loader a heads-up, so it’s ready when expecty wants to use the enum.

Practical Application

This workaround is super easy to implement. Just make sure to assign the enum values to separate variables before your expect calls. It is simple, yet effective.

Understanding the Root Cause

Let’s dig a little deeper into why this happens. The issue is likely related to how expecty's macro system processes the code and how the JVM loads class files. When you use expect, expecty uses macros to inspect the expression you're testing. The macro system is powerful but can occasionally run into trouble when dealing with complex or dynamically generated class structures, such as those related to enums, especially when accessed directly within the expect expression.

When you use expect(actualCode == ExampleEnum.OK), the expecty macro needs to see the internal representation of ExampleEnum.OK. In doing so, it might trigger a class load, which can fail if the class isn't fully initialized or available at that exact moment. By extracting the enum value to a variable, you force the JVM to load the class before the macro tries to look at it. This ensures that the class definition is ready.

Additional Considerations

Dependency Management

Make sure you have the correct expecty dependency in your build.sbt or build.scala file. Double-check the version to ensure you're using the latest available. Sometimes, outdated dependencies can cause unexpected behavior and compatibility issues. The example provided uses com.eed3si9n.expecty::expecty:0.17.0. Check the latest version on Maven Central to be up-to-date.

Scala Version

Keep an eye on your Scala version, too. While this issue might not be strictly tied to a specific version, ensuring you use a supported Scala version can help avoid any unforeseen classloading issues or macro processing quirks.

Community Resources

If you find yourself still struggling, don’t hesitate to check out the expecty project's GitHub page. You might find existing discussions, issues, or even pull requests that address your specific problem. Many developers have encountered similar challenges, and there’s a good chance someone has already found a solution or workaround.

Conclusion: Keeping Your Tests Running

There you have it! A quick guide to fixing the expect runtime crash when working with Java enums in Scala. By understanding the root cause and applying the simple workaround, you can keep your tests running smoothly and your code reliable. Remember to extract those enum values to variables, keep your dependencies up-to-date, and leverage the community resources available. Happy coding!

This fix ensures that the class is available when expecty tries to use it during the runtime evaluation. The workaround provides an easy way to prevent the ClassNotFoundException from occurring, making your testing process much smoother. Always remember to check your project dependencies and make sure you're using the latest versions of your libraries, as this can often resolve issues.

More Advanced Troubleshooting

ClassLoader Issues

If the workaround doesn't work, and you're still getting the ClassNotFoundException, the problem might be more complex. You might need to examine your classloader setup. If you are using custom classloaders or complex module configurations, make sure that expecty can correctly access your enum classes. This can sometimes involve adjusting your build configuration or explicitly adding the necessary dependencies to the classpath.

Expecty Internals

For those who like to delve deeper, you could potentially look into the internals of expecty. Examining the source code can give you more insight into how it processes expressions and interacts with class loaders. However, this is typically only necessary if you're dealing with very specific or unusual setups. Understanding how expecty's macros work can help you understand why this issue happens and how to work around it. But, most of the time, the simple workaround is sufficient.

Testing Strategies

Consider different testing strategies. If you frequently encounter issues with enums, you might adjust your test structure to minimize direct comparisons within expect calls. For example, you can create separate test methods for each enum value or use helper functions to make the comparisons. These strategies can help make your tests more robust and less prone to runtime issues.

Alternative Approaches

Refactoring and Design Patterns

Consider refactoring your code. If you find yourself consistently struggling with enums, evaluate if they are the best design choice. Sometimes, using sealed traits or classes can provide more flexibility and avoid some of the limitations of enums, especially when interacting with testing libraries like expecty. While this might involve more initial work, it can result in more maintainable and robust code in the long run.

Debugging Tools

Make use of debugging tools. If the error persists, use a debugger to step through your code and see exactly where the ClassNotFoundException occurs. Debugging can help you pinpoint the exact line of code causing the problem and shed light on how expecty is interacting with your enum.

The Power of a Good Test Suite

Having a comprehensive test suite is crucial. Tests should cover all possible enum values and edge cases. Make sure your tests are designed to catch this type of error early in the development cycle. Regularly running your tests helps catch these issues early and avoid runtime surprises.

Conclusion Revisited

In summary, the expect crash with Java enums is a manageable issue. The easy workaround – extracting enum values to variables – often fixes it. Understanding the underlying problem – the class loading quirks between Scala, Java, and expecty – empowers you to troubleshoot more effectively. Keep your dependencies current, check the community resources, and remember that well-structured tests are your best friends.