Testcontainers fails to find Docker environment on Windows 11 with Spring Boot 3 and Java 21

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Summary

An integration test suite failed to initialize on Windows 11 with Spring Boot 3.4.1 and Testcontainers 1.20.4. The application threw an IllegalStateException, indicating that Testcontainers could not locate a valid Docker environment. The root cause was an incorrect DOCKER_HOST configuration intended to bypass standard connection mechanisms. While the environment variable was set to tcp://localhost:2375 (unencrypted HTTP), Docker Desktop on the machine was configured to accept connections exclusively via the default Named Pipe (//./pipe/docker_engine), causing all connection strategies to fail.

Root Cause

The failure was triggered by a conflict between the user’s manual configuration and Docker Desktop’s actual listening configuration.

  • Forced TCP Connection: The user configured DOCKER_HOST=tcp://localhost:2375. This forces Testcontainers to attempt a connection over HTTP on port 2375.
  • Active Named Pipe: Docker Desktop was running with the standard default setting, which listens on the Windows Named Pipe (//./pipe/docker_engine). It was not actively listening on port 2375 despite the user’s attempt to enable it.
  • Strategy Failure:
    • EnvironmentAndSystemPropertyClientProviderStrategy: Detected the DOCKER_HOST variable, attempted to connect to tcp://localhost:2375, and received a BadRequestException (likely because the port was closed or a non-Docker service was listening).
    • NpipeSocketClientProviderStrategy: Attempted to read the Named Pipe but encountered a BadRequestException, likely due to version incompatibilities or permission issues during the manual probe, or simply because the underlying socket logic failed during the validation handshake.

Why This Happens in Real Systems

Testcontainers relies on specific auto-discovery strategies to locate the Docker daemon. In Windows environments, this complexity increases.

  • Manual Overrides: Developers often set environment variables like DOCKER_HOST to force connectivity, assuming that if one method fails, forcing another will work. This often masks the real issue.
  • Docker Desktop Instability: Docker Desktop’s internal state can sometimes desync, causing the Named Pipe to become unresponsive or “stale,” even if the UI says it’s running.
  • The “Fix-It-Yourself” Trap: When faced with connection errors, senior engineers typically check the daemon status via a direct CLI command (docker ps) before changing environment variables. Modifying global environment variables to fix a local test issue is a systemic anti-pattern that leads to “works on my machine” issues.

Real-World Impact

  • CI/CD Blockage: This issue halts all local development and CI pipelines. If the CI runner is Windows-based, the build is completely broken.
  • Cognitive Overhead: The error message Could not find a valid Docker environment is generic. It forces the engineer to debug the infrastructure (Docker) rather than the code.
  • Wasted Time: The user spent time enabling TCP ports and setting variables that are strictly unnecessary for modern Docker Desktop on Windows.

Example or Code

Testcontainers discovers the Docker environment using a set of strategies. The logic roughly resembles the following selection process. The failure occurs when all strategies return false on isAvailable().

// Conceptual representation of Testcontainers discovery logic
public class DockerClientFactory {

    public DockerClient client() {
        // List of strategies to try in order
        List strategies = Arrays.asList(
            new NpipeSocketClientProviderStrategy(), // Default for Windows
            new EnvironmentAndSystemPropertyClientProviderStrategy(), // Checks DOCKER_HOST
            new UnixSocketClientProviderStrategy() // Default for Linux/Mac
        );

        for (Strategy strategy : strategies) {
            if (strategy.isAvailable()) {
                return strategy.dockerClient();
            }
        }

        throw new IllegalStateException("Could not find a valid Docker environment");
    }
}

How Senior Engineers Fix It

The solution relies on removing custom overrides and allowing Testcontainers to use the native Windows Named Pipe transport.

  1. Remove Overrides: Delete the DOCKER_HOST environment variable (User and System) and any docker.host settings in ~/.testcontainers.properties.
  2. Verify Native Connectivity: Open a terminal and run docker ps. If this works, the daemon is healthy.
  3. Check Docker Settings: Ensure “Expose daemon on tcp://localhost:2375 without TLS” is unchecked. This reduces security risks and eliminates port conflicts.
  4. Restart Docker: Fully quit Docker Desktop (via the tray icon) and restart it. This resets the Named Pipe socket.
  5. Verify Permissions: Ensure the user account running IntelliJ/Tests is in the docker-users group (standard in Windows Docker Desktop installs).

Key Takeaway: Trust the auto-detection. NpipeSocketClientProviderStrategy is highly optimized for Windows. Only if that fails (e.g., in a legacy environment) should DOCKER_HOST be manipulated.

Why Juniors Miss It

  • “If TCP works, it’s better”: There is a misconception that TCP is more reliable than Named Pipes. In reality, Named Pipes are faster and safer for local Windows communication.
  • Copy-Pasting Fixes: Juniors often find a StackOverflow answer suggesting tcp://localhost:2375 and apply it without verifying if their specific Docker Desktop version requires it.
  • Not verifying the Daemon: They assume the Docker Desktop UI is the source of truth. If the UI says “Running,” they don’t verify if the background process is actually accepting connections on the expected socket.
  • Ignoring the Port: They ran netstat and saw port 2375 was listening, failing to realize that something else might be listening on that port, or that Docker wasn’t actually bound to it despite the UI setting.