Troubleshoot Java class loading issues on Fargate

Java applications running on Fargate may encounter class loading issues after platform updates, particularly when the application relies on non-deterministic class loading behavior. This can manifest as dependency injection errors, Spring Boot failures, or other runtime exceptions that were not present in previous deployments.

Symptoms

You might experience the following symptoms:

• Spring Boot dependency injection errors

• ClassNotFoundException or NoClassDefFoundError exceptions

• Applications that previously worked on Fargate now fail intermittently

• The same container image works on Amazon EC2 but fails on Fargate

• Inconsistent behavior across deployments with identical container images

Causes

These issues typically occur due to:

• Non-deterministic class loading: Java applications that depend on the order in which classes are loaded from JAR files may fail when the underlying platform changes how files are accessed or cached.

• Platform updates: Fargate platform version updates may change the underlying file system behavior, affecting the order in which classes are discovered and loaded.

• JAR file ordering dependencies: Applications that implicitly rely on specific JAR loading sequences without explicit dependency management.

Resolution

To resolve Java class loading issues on Fargate, implement deterministic class loading practices:

Immediate fix

If you need an immediate workaround:

1. Enforce JAR loading order: Explicitly specify the order in which JAR files should be loaded in your application's classpath configuration.

2. Use explicit dependency management: Ensure all dependencies are explicitly declared in your build configuration (Maven, Gradle, etc.) rather than relying on transitive dependencies.

Long-term best practices

Implement these practices to prevent future class loading issues:

1. Make class loading deterministic:

   • Use explicit dependency declarations in your build files

   • Avoid relying on classpath scanning order

   • Use dependency management tools to resolve version conflicts

   • Use the Java Virtual Machine (JVM) options such as -verbose:class to get information about classes loaded by JVM.

2. Spring Boot applications:

   • Use @ComponentScan with explicit base packages

   • Avoid auto-configuration conflicts by explicitly configuring beans

   • Use @DependsOn annotations to control bean initialization order

3. Build configuration:

   • Use dependency management sections in Maven or Gradle

   • Exclude transitive dependencies that cause conflicts

   • Use tools like Maven Enforcer Plugin to detect dependency issues

4. Testing:

   • Test your application with different JVM implementations

   • Run integration tests that simulate different deployment environments

   • Use tools to analyze classpath conflicts during development

Prevention

To prevent Java class loading issues in future deployments:

• Follow deterministic class loading practices: Design your application to not depend on the order in which classes are loaded from the classpath.

• Use explicit dependency management: Always explicitly declare all required dependencies and their versions in your build configuration.

• Test across environments: Regularly test your applications across different environments and platform versions to identify potential issues early.

• Monitor platform updates: Stay informed about Fargate platform updates and test your applications with new platform versions before they affect production workloads.