Kotlin Coroutines based Spring EL Evaluation failed: A Comprehensive Guide to Troubleshooting

If you’re reading this article, chances are you’ve encountered the frustrating error “Kotlin Coroutines based Spring EL Evaluation failed”. Don’t worry, you’re not alone! In this article, we’ll delve into the world of Kotlin coroutines and Spring EL evaluation, and provide you with a step-by-step guide to identifying and resolving this pesky issue.

Understanding the Basics: Kotlin Coroutines and Spring EL

Before we dive into the troubleshooting process, let’s take a quick look at the basics. Kotlin coroutines are a way to write asynchronous code in Kotlin, allowing you to perform tasks concurrently without blocking threads. Spring EL (Expression Language) is a powerful expression language used in Spring-based applications to evaluate expressions at runtime.

Kotlin Coroutines: A Brief Introduction

Kotlin coroutines are built on top of the Kotlin language and provide a way to write asynchronous code that’s both concise and efficient. With coroutines, you can write code that’s asynchronous by design, without the need for callbacks or threads. This allows you to write more efficient and scalable code that’s easier to maintain.

import kotlinx.coroutines.*

fun main() {
    GlobalScope.launch {
        delay(1000L)
        println("World!")
    }
    println("Hello, ")
    Thread.sleep(2000L)
}

In this example, we’re using the `launch` function to start a coroutine that will print “World!” after a 1-second delay. Meanwhile, the main thread will continue to execute and print “Hello, “.

Spring EL: A Brief Introduction

Spring EL is a powerful expression language used in Spring-based applications to evaluate expressions at runtime. It allows you to inject values into your application configuration, making it more flexible and dynamic.

<bean id="myBean" class="com.example.MyBean">
    <property name="myProperty" value="${my.property}" />
</bean>

In this example, we’re using Spring EL to inject a value from a properties file into a bean property.

The Error: Kotlin Coroutines based Spring EL Evaluation failed

Now that we’ve covered the basics, let’s talk about the error. The “Kotlin Coroutines based Spring EL Evaluation failed” error typically occurs when you’re trying to use Kotlin coroutines in a Spring-based application that relies on Spring EL for expression evaluation.

The error message might look something like this:

org.springframework.expression.spel.SpelEvaluationException: 
    EL1001E: Type cannot be found - 'kotlinx.coroutines.Dispatchers'
    at org.springframework.expression.spel.ast.TypeReference.resolveType(TypeReference.java:134)
    at org.springframework.expression.spel.standard.SpelExpressionParser.doParseExpression(SpelExpressionParser.java:94)
    ...

This error occurs because Spring EL is trying to evaluate an expression that references a Kotlin coroutine, but it can’t find the required type. This is because Kotlin coroutines are not automatically available to Spring EL.

Troubleshooting: Identifying the Cause

Before we dive into the solution, let’s identify the causes of this error. There are a few common scenarios that can lead to this error:

• Missing dependencies: Make sure you have the correct dependencies in your project’s build configuration (e.g., Maven or Gradle).
• Incompatible versions: Verify that your Spring and Kotlin versions are compatible.
• Incorrect configuration: Check your Spring configuration to ensure that Kotlin coroutines are properly enabled.
• Type resolution issues: Spring EL might have trouble resolving the types required for Kotlin coroutines.

Troubleshooting: Resolving the Issue

Now that we’ve identified the possible causes, let’s walk through the steps to resolve the issue:

Step 1: Verify Dependencies

Make sure you have the correct dependencies in your project’s build configuration. For Maven, add the following dependencies:

<dependencies>
    <dependency>
        <groupId>org.springframework></groupId>
        <artifactId>spring-context</artifactId>
    </dependency>
    <dependency>
        <groupId>org.jetbrains.kotlinx></groupId>
        <artifactId>kotlinx-coroutines-core</artifactId>
    </dependency>
</dependencies>

For Gradle, add the following dependencies:

dependencies {
    implementation 'org.springframework:spring-context:5.3.10'
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.5.0'
}

Step 2: Verify Version Compatibility

Verify that your Spring and Kotlin versions are compatible. You can check the official Spring and Kotlin documentation for version compatibility matrices.

Step 3: Enable Kotlin Coroutines in Spring

To enable Kotlin coroutines in Spring, add the following configuration to your Spring configuration file:

<bean id="coroutineContext" class="kotlinx.coroutines.ExperimentalCoroutinesApi">
    <constructor-arg><value>kotlinx.coroutines.Dispatchers.IO</value></constructor-arg>
</bean>

This configuration enables Kotlin coroutines in Spring and specifies the dispatcher to use (in this case, `Dispatchers.IO`).

Step 4: Resolve Type Resolution Issues

To resolve type resolution issues, add the following configuration to your Spring configuration file:

<bean id="expressionFactory" class="org.springframework.expression.spel.standard.SpelExpressionFactory">
    <property name="typeLocator" ref="typeLocator" />
</bean>

<bean id="typeLocator" class="org.springframework.expression.spel.support.StandardTypeLocator">
    <property name="typeConverter" ref="typeConverter" />
</bean>

<bean id="typeConverter" class="org.springframework.expression.spel.support.StandardTypeConverter">
    <property name="conversionService" ref="conversionService" />
</bean>

<bean id="conversionService" class="org.springframework.core.convert.support.GenericConversionService">
    <property name="converters">
        <list>
            <bean class="org.springframework.core.convert.support.StringToBooleanConverter" />
            <bean class="org.springframework.core.convert.support.StringToIntegerConverter" />
            <bean class="org.springframework.core.convert.support.StringToLongConverter" />
            <!-- Add more converters as needed -->
        </list>
    </property>
</bean>

This configuration sets up the expression factory, type locator, type converter, and conversion service to resolve type issues.

Conclusion

In conclusion, the “Kotlin Coroutines based Spring EL Evaluation failed” error can be frustrating, but with the right steps, it’s easy to resolve. By verifying dependencies, ensuring version compatibility, enabling Kotlin coroutines in Spring, and resolving type resolution issues, you can get your application up and running in no time.

Remember to stay calm, follow the troubleshooting steps, and you’ll be back to coding in no time!

Troubleshooting Step
Verify Dependencies	Ensure correct dependencies in project build configuration
Verify Version Compatibility	Check Spring and Kotlin version compatibility
Enable Kotlin Coroutines in Spring	Add configuration to enable Kotlin coroutines in Spring
Resolve Type Resolution Issues	Add configuration to resolve type resolution issues

We hope this article has been helpful in resolving the “Kotlin Coroutines based Spring EL Evaluation failed” error. Happy coding!

Frequently Asked Question

Are you stuck with the annoying “Kotlin Coroutines based Spring EL Evaluation failed” error? Don’t worry, we’ve got you covered! Here are the top 5 questions and answers to help you troubleshoot and fix this issue.