Troubleshooting Empty Output in WebTestClient

Detailed guidance for diagnosing and resolving empty Flux responses

Key Highlights

Proper Error and Exception Handling: Ensure that all errors are logged and handled.
Accurate Response Body Extraction: Confirm that the response class matches the server output.
Configuration and Content-Type Matching: Verify client configuration and header consistency.

Understanding the Issue

In a Spring WebFlux application, when using WebTestClient to send requests, receiving an empty output (Flux) may be caused by several factors. The method sendSetLicenseConsumptionRequest as provided sends a PUT request and processes the response as a Flux of the desired type. However, an empty output might occur because of issues including unexpected error handling behaviors, content type mismatches, or even the absence of data in the response.

Examining the Flow of the Request

1. Request Initialization and Headers

The client is configured to use specific headers:

Accept is set to APPLICATION_NDJSON_VALUE, which means the client expects newline-delimited JSON.
Content-Type is set to APPLICATION_JSON_VALUE.
An Authorization header is also provided.

These headers ensure that the request payload is correctly interpreted by the server; however, any mismatch or incorrect header might result in data not being returned.

2. Response Status Verification

Immediately after sending the request, the code asserts that the returned status matches expectedStatus (in this case, BAD_REQUEST). If the status does not match, an error might be thrown, leading to an empty stream.

3. Response Body Extraction

Even when a response is received, its body might not be correctly translated into the declared responseClass. This is particularly important if the API does not return any body when a BAD_REQUEST is issued. Validate that the server’s response structure aligns with your ApiClientError class.

4. Logging for Diagnosis

Logging using doOnSubscribe, doOnNext, and doOnError helps diagnose what is happening during the call lifecycle. Ensure that:

The initial request payload is logged.
The streamed data is logged upon reception.
Any error that occurs is logged with meaningful context.

These detailed logs provide insights into whether the call fails or completes without data.

Potential Causes for an Empty Flux and Their Solutions

Analysis and Troubleshooting Steps

Possible Cause	Explanation	Recommended Solution
Response Status Mismatch	If the server returns a status code different from `expectedStatus`, the test might fail before processing the response body.	Verify the server behavior by testing the same request with tools such as Postman. Adjust your test expectations accordingly.
Empty Flux from Server	The API might not be sending any body with a `BAD_REQUEST` response. If the Flux is empty, the `switchIfEmpty` operator could be used to handle this condition.	Consider adding `.switchIfEmpty(Mono.error(new RuntimeException("Flux is empty")))` to handle the empty response case.
Content-Type Mismatch	The actual content type provided by the server might differ from `APPLICATION_NDJSON_VALUE`, causing deserialization issues.	Confirm the content type on the server side and match it with the header in your request. Alternatively, adjust the expectation to `APPLICATION_JSON_VALUE` if applicable.
Error Handling Interruptions	Unhandled exceptions (such as `WebClientResponseException`) may interrupt the Flux processing.	Integrate enhanced error-handling using `onErrorResume` or `onErrorReturn` to ensure that the exception's details are captured and optionally logged or substituted.
Improper Message Converters	The deserialization framework might not correctly convert the response body into the expected class structure.	Double-check that all message converters for JSON are properly configured and that your `ApiClientError` structure matches the actual JSON output.

Enhanced Example with Additional Debugging and Error Handling

Code Revision

Below is an updated version of the method, incorporating more comprehensive logging and error handling:

// Enhanced method with added logging and error handling
private <T> Flux<T> sendSetLicenseConsumptionRequest(String requestPayload, HttpStatus expectedStatus, Class<T> responseClass) {
    return webTestClient.put()
            .uri(endPoint)
            .header(HttpHeaders.ACCEPT, MediaType.APPLICATION_NDJSON_VALUE)
            .header(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
            .header(HttpHeaders.AUTHORIZATION, authorizationToken)
            .bodyValue(requestPayload)
            .exchange()
            .doOnNext(response -> {
                log.info("Received status: {}", response.getStatusCode());
                // Log full response details if needed
            })
            .expectStatus().isEqualTo(expectedStatus)
            .returnResult(responseClass)
            .getResponseBody()
            .doOnSubscribe(sub -> log.info("Request payload: \n{}", requestPayload))
            .doOnNext(response -> log.info("Received Streamed Data: \n{}", JsonUtil.toJsonString(response)))
            .doOnError(error -> {
                log.error("Error occurred.", error);
                if (error instanceof WebClientResponseException) {
                    WebClientResponseException wcre = (WebClientResponseException) error;
                    log.error("Response status: {}", wcre.getStatusCode());
                    log.error("Response body: {}", wcre.getResponseBodyAsString());
                }
            })
            .onErrorResume(error -> {
                // Optionally propagate a specific error message or leave empty
                return Flux.empty();
            });
}

In this revised method, observe the following improvements:

The initial subscription logs the request payload, keeping track of what is sent to the server.
On receiving a response, the status code is logged, which is useful for debugging status assertion failures.
Detailed logging within the doOnError block captures any exceptions (with additional checks for WebClientResponseException).
The use of onErrorResume ensures that even if an error occurs, the Flux completes gracefully albeit empty.

Additional Debugging Tips

Practical Recommendations

Server-Side Logging

Make sure to check your server logs to confirm that it is receiving the request as expected. If the server is returning an error status without a body, the client might interpret it as an empty Flux.

Testing with Alternative Tools

Testing the endpoint using tools like Postman or cURL to see the raw response can help identify if the problem is inherent to the API or in how the WebTestClient processes the response.

Using Reactive Debug Operators

Operators such as doOnComplete or doOnTerminate can be added to log when the stream completes, providing further clarity on whether the Flux finishes without emitting items:


// Example logging for stream termination
apiClientErrorFlux
    .doOnComplete(() -> log.info("Flux completed"))
    .doOnTerminate(() -> log.info("Flux terminated"))
    .subscribe();