New Relic Java Agent & Kotlin Coroutines: UnsupportedOperationException Fix

Hey there, fellow developers! Have you ever hit a roadblock where your cutting-edge Kotlin Coroutines application suddenly throws a mysterious UnsupportedOperationException the moment you introduce a New Relic Java agent? If you're running into issues with your coroutine-based application, specifically an UnsupportedOperationException within a delay() call, you're not alone. This particular problem surfaces when using the New Relic Java agent v8.25.0 alongside kotlinx-coroutines-core:1.9.0, and it can be quite baffling to troubleshoot. The core of the issue lies in a defensive check within the Kotlin coroutines runtime, which detects what it perceives as a "third-party implementation" of CancellableContinuation. This detection is triggered because the New Relic agent, in its effort to instrument your application for monitoring, modifies or wraps crucial coroutine internal classes. Let's dive deep into understanding this conflict, identifying its root causes, and, most importantly, finding a practical solution to get your applications running smoothly while still enjoying the benefits of comprehensive application performance monitoring (APM).

This article aims to unravel the intricacies of this specific UnsupportedOperationException, which manifests as java.lang.UnsupportedOperationException: third-party implementation of CancellableContinuation is not supported. This error message is a direct signal from the kotlinx.coroutines library that something isn't right with the way its internal continuation mechanisms are being handled. When an APM tool like New Relic steps in to observe and instrument, it often injects code or modifies bytecode to capture metrics, trace transactions, and monitor performance. While this is generally beneficial, in this specific scenario, the instrumentation applied by New Relic Java agent v8.25.0 seems to alter the CancellableContinuation class in a way that violates Kotlin coroutines' strict internal expectations. This leads to the runtime throwing an exception, effectively halting your coroutine execution. We'll explore why this happens, how to confirm if you're facing this exact problem, and what steps you can take to mitigate it, ensuring your Kotlin Coroutines continue to operate flawlessly under the watchful eye of New Relic.

Understanding the Core Problem: Kotlin Coroutines and New Relic Agent Conflicts

When you're building modern, high-performance applications with Kotlin Coroutines, you're embracing a powerful paradigm for asynchronous programming. Coroutines make complex concurrency feel intuitive, allowing you to write sequential-looking code that performs non-blocking operations efficiently. However, integrating such a sophisticated system with an Application Performance Monitoring (APM) tool like New Relic Java agent can sometimes lead to unexpected interactions. Our main keyword here, the UnsupportedOperationException, becomes a glaring red flag that something is amiss in this integration. Specifically, the error message third-party implementation of CancellableContinuation is not supported points to a deeply technical conflict within the heart of Kotlin's coroutine dispatcher.

At its core, a CancellableContinuation is a fundamental building block within the kotlinx.coroutines library. Think of it as the mechanism that allows a coroutine to pause its execution (suspend) and then resume it later, often after an asynchronous operation completes or a delay elapses. When you call a suspension function like delay(), the current coroutine suspends, and the CancellableContinuation associated with it is passed around. This continuation holds the context needed to resume the coroutine later. Kotlin's coroutines runtime is highly optimized and relies on internal, specific implementations of these continuations, usually CancellableContinuationImpl. It has robust, defensive checks in place to ensure that these internal objects are indeed what they're expected to be, preventing subtle bugs and maintaining the integrity of the coroutine lifecycle. If an external entity, like an APM agent performing bytecode instrumentation, modifies or wraps this CancellableContinuation in a way that deviates from its expected internal structure, Kotlin's runtime will detect this as a "third-party implementation" and throw an UnsupportedOperationException rather than risking undefined behavior. This is precisely what happens with the New Relic Java agent v8.25.0 when it interacts with kotlinx-coroutines-core:1.9.0.

The impact of this UnsupportedOperationException is significant. Your application, designed for seamless asynchronous operations, will crash during routine delay() calls or other suspension points. This means critical functionalities relying on timed operations, network requests, or database interactions that leverage coroutines will fail. The error at kotlinx.coroutines.CancellableContinuationKt.invokeOnCancellation further pinpoints the exact location in the coroutines library where this check occurs. This method is crucial for handling what happens when a coroutine is canceled, and any non-standard implementation of CancellableContinuation could lead to resource leaks or incorrect cancellation behavior. Thus, Kotlin's runtime takes a strong stance, prioritizing stability and correctness over allowing potentially problematic external modifications. This situation highlights a common challenge in modern software development: ensuring compatibility between highly optimized runtime frameworks and powerful observability tools that operate by modifying the application's internal workings. The detailed stack trace, showing the exception originating from kotlinx.coroutines.DelayKt.delay, provides a clear indicator that the problem is directly linked to how coroutine suspension points are being handled under the New Relic agent's influence. This specific version interaction is key, as earlier agent versions did not exhibit this behavior, suggesting a change in New Relic's instrumentation strategy. Understanding this core conflict is the first step towards a stable, monitored Kotlin Coroutines application.

Diving Deeper: Why This Conflict Arises

To truly grasp why your application throws an UnsupportedOperationException when using Kotlin Coroutines with the New Relic Java agent, we need to talk about bytecode instrumentation. This is the magic (and sometimes problematic) behind how APM tools like New Relic work. When you start your Java Virtual Machine (JVM) with the -javaagent flag, New Relic loads itself and begins to modify your application's compiled code—its bytecode—at runtime. It's like a highly skilled surgeon performing delicate operations on your application's internals while it's running, injecting code to capture metrics, trace method calls, and monitor resource usage without requiring you to change your source code. This process is incredibly powerful for observability but can also be incredibly sensitive, especially when dealing with highly optimized and state-dependent frameworks like Kotlin Coroutines.

The conflict arises because Kotlin's coroutine runtime, particularly its CancellableContinuation mechanism, is designed with very specific expectations about its internal components. The CancellableContinuation object is critical for managing the lifecycle of a suspended coroutine, including how it gets resumed or canceled. The kotlinx.coroutines library uses an internal class, typically CancellableContinuationImpl, for this purpose. When the New Relic Java agent v8.25.0 instruments your code, it might wrap, proxy, or subclass CancellableContinuation to insert its monitoring logic. For instance, it might try to intercept calls to invokeOnCancellation or other lifecycle methods to record timing or context. However, the kotlinx.coroutines.CancellableContinuationKt.invokeOnCancellation method, as seen in the source code (e.g., on GitHub), contains a defensive check. This check explicitly verifies that the CancellableContinuation instance it's operating on is not a