Fix: StackOverflowError With Self-Referencing ConfigMapping

Nov 12, 2025 by Alex Johnson 60 views

StackOverflowError: Self-Referencing Types in @ConfigMapping Cause Infinite Recursion During Build Time

Encountering a StackOverflowError when using @ConfigMapping with self-referencing types in SmallRye Config can be a frustrating experience. This article delves into the root cause of this issue, provides a minimal reproducible example, explains the observed behavior, and offers a practical workaround. Let's explore how to handle this scenario effectively.

Understanding the Problem

The core issue arises when you use @ConfigMapping with types that reference themselves, typically through a generic container like Optional<List<T>>. SmallRye Config's build-time annotation processor gets stuck in an infinite loop when it encounters these self-referencing types, ultimately leading to a StackOverflowError. This happens because the processor continuously tries to resolve the type, leading to unbounded recursion.

When working with configuration management in modern applications, tools like SmallRye Config provide a convenient way to map configuration properties directly to Java interfaces. However, certain complex type structures, especially those involving self-references, can expose limitations in these systems. The StackOverflowError is a manifestation of the configuration processor's inability to handle infinitely nested structures. Therefore, understanding the boundaries of what these tools can manage is crucial for designing robust and maintainable configuration setups. The challenge often lies in balancing the simplicity of declarative configuration mapping with the need to represent complex data structures. When faced with such errors, it is also important to refer to the documentation of SmallRye Config, to identify and implement alternative configuration strategies, such as custom parsing or splitting configurations into simpler, manageable parts. In summary, while self-referencing types might seem like a convenient way to model complex data, they often push the boundaries of automated configuration systems, necessitating manual intervention or workarounds.

Expected Behavior vs. Actual Behavior

Expected Behavior

Ideally, SmallRye Config should handle self-referencing types in one of the following ways:

Detect Circular References: The configuration processor should be intelligent enough to detect circular type references. When it finds one, it should gracefully stop the recursion to prevent the StackOverflowError.
Document Limitations: If self-referencing types are inherently not supported, this limitation should be clearly documented. This would save developers time and effort by preventing them from attempting unsupported configurations.

Actual Behavior

Instead of the expected behavior, what actually happens is a runtime exception during the build process. Here's the typical error message you might encounter:

java.lang.RuntimeException: io.quarkus.builder.BuildException: Build failure:
Build failed due to errors
[error]: Build step io.quarkus.deployment.steps.ConfigGenerationBuildStep#generateMappings 
threw an exception: java.lang.StackOverflowError

This error indicates that the generateMappings build step, responsible for processing @ConfigMapping annotations, has failed due to a StackOverflowError. The root cause is the infinite recursion triggered by the self-referencing type.

The observed behavior highlights a critical gap in the config mapping process when dealing with recursive type definitions. Tools like SmallRye Config are designed to automate the binding of configuration properties to Java interfaces, simplifying the development workflow. However, the presence of self-referencing types introduces a level of complexity that these tools are not always equipped to handle. The StackOverflowError is a direct consequence of the processor's attempt to infinitely resolve the type, indicating a need for either improved error handling or more sophisticated recursion detection. Documenting this limitation and providing alternative strategies, such as manual parsing or splitting the configuration into manageable chunks, is crucial for preventing developers from encountering this issue. Therefore, understanding the boundaries and limitations of automated configuration mapping tools is essential for building robust and maintainable applications, and proactive documentation can play a significant role in guiding developers towards appropriate solutions.

Minimal Reproducible Example (MRE)

To illustrate the problem, consider the following minimal reproducible example.

File: `example-config.yaml`

schemas:
  default:
    fields:
      - id: root_field
        name: root_field
        type: array
        children:
          - id: child_field
            name: child_field
            type: object
            children:
              - id: grandchild_field
                name: grandchild_field
                type: string

File: `ExampleConfig.java`

import io.smallrye.config.ConfigMapping;
import io.smallrye.config.WithName;
import java.util.List;
import java.util.Map;
import java.util.Optional;

@ConfigMapping(prefix = "schemas")
public interface ExampleConfig {

    @WithParentName
    Map<String, SchemaDefinition> schemas();

    interface SchemaDefinition {
        Optional<List<FieldDefinition>> fields();
    }

    /**
     * Self-referencing interface - causes StackOverflowError
     * even though it only supports 3 levels deep in practice
     */
    interface FieldDefinition {
        @WithName("id")
        String id();

        @WithName("name")
        String name();

        @WithName("type")
        String type();

        @WithName("children")
        Optional<List<FieldDefinition>> children();  //  Causes infinite recursion
    }
}

In this example, the FieldDefinition interface contains an Optional<List<FieldDefinition>> field named children. This self-reference is what triggers the StackOverflowError during the build process.

Creating a minimal reproducible example (MRE) is crucial for effectively diagnosing and addressing software issues. In this context, the MRE highlights the problematic interaction between SmallRye Config's annotation processing and self-referencing types. By providing a concise and self-contained example, developers can quickly isolate the issue and confirm that it is indeed related to the recursive type definition. This saves time and effort in troubleshooting, as it eliminates the need to sift through larger, more complex codebases. Furthermore, the MRE serves as a valuable tool for communicating the problem to the SmallRye Config development team, allowing them to reproduce the error and develop a targeted solution. The clarity and simplicity of the example also make it easier for other developers to understand the issue and potentially contribute to the resolution. In essence, the MRE acts as a precise and unambiguous representation of the problem, facilitating efficient collaboration and problem-solving.

Build Output

When you build the project with the above code, you'll see a StackOverflowError similar to this:

java.lang.StackOverflowError
  at java.base/java.util.HashSet.add(HashSet.java:216)
  at io.smallrye.config.ConfigMappingInterface.resolveTypeVariables(ConfigMappingInterface.java:XXX)
  at io.smallrye.config.ConfigMappingInterface.resolveTypeVariables(ConfigMappingInterface.java:XXX)
  [... repeats ...]

The repeated calls to resolveTypeVariables indicate the infinite recursion.

Workaround

Currently, the primary workaround is to avoid self-referencing types altogether. Here are a couple of strategies you can use:

Separate Interfaces for Nesting Levels: Create distinct interfaces for each level of nesting. For example, you could have FieldDefinition, NestedFieldDefinition, and GrandchildFieldDefinition.
Manual Parsing: Instead of relying on SmallRye Config proxies, you can manually parse the YAML/JSON configuration using a library like Jackson.

Let's look at the first workaround in more detail.

Separate Interfaces Example

import io.smallrye.config.ConfigMapping;
import io.smallrye.config.WithName;
import java.util.List;
import java.util.Map;
import java.util.Optional;

@ConfigMapping(prefix = "schemas")
public interface ExampleConfig {

    @WithParentName
    Map<String, SchemaDefinition> schemas();

    interface SchemaDefinition {
        Optional<List<FieldDefinition>> fields();
    }

    interface FieldDefinition {
        @WithName("id")
        String id();

        @WithName("name")
        String name();

        @WithName("type")
        String type();

        @WithName("children")
        Optional<List<NestedFieldDefinition>> children();
    }

    interface NestedFieldDefinition {
        @WithName("id")
        String id();

        @WithName("name")
        String name();

        @WithName("type")
        String type();

        @WithName("children")
        Optional<List<GrandchildFieldDefinition>> children();
    }

    interface GrandchildFieldDefinition {
        @WithName("id")
        String id();

        @WithName("name")
        String name();

        @WithName("type")
        String type();
    }
}

By explicitly defining each level, you break the self-referential loop and allow SmallRye Config to process the configuration correctly.

Implementing effective workarounds is critical when facing limitations in software tools or libraries. In the case of SmallRye Config and self-referencing types, avoiding the problematic construct is the most immediate solution. The suggested workarounds, such as creating separate interfaces for each nesting level or manually parsing the configuration, offer alternative approaches to achieve the desired configuration mapping. Separating interfaces ensures that the configuration processor does not encounter infinite recursion, while manual parsing provides complete control over the configuration process. These workarounds not only address the immediate issue but also encourage developers to think critically about configuration design and potential limitations of automated tools. Furthermore, understanding these alternative strategies enhances the overall resilience of the application, as it is no longer solely dependent on the automated configuration mapping provided by SmallRye Config. In summary, proactive implementation of workarounds can effectively mitigate the risks associated with unsupported or problematic features, ensuring the smooth operation and maintainability of the application.

Conclusion

While SmallRye Config offers a convenient way to manage configurations, it's essential to be aware of its limitations, especially when dealing with self-referencing types. By understanding the cause of the StackOverflowError and implementing the suggested workarounds, you can avoid this issue and ensure your application's configuration is processed correctly.

For more information on SmallRye Config and its capabilities, refer to the official SmallRye Config documentation. This article can help to understand more about SmallRye Config.