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How to retrieve the custom error message from a webservice call in a error handler

This will show how to access the webservice message when a error occurs during a webservice call.

When you make a webservice call in your code you usually process the response functionally in a map and have a onErrorResume handler to handle the call failure. The following is an example of a http call in a Spring Reactive stack with error handler:

1. final String endpoint = localHost+"/api/health/throwerror";

2. WebClient.ResponseSpec responseSpec = webClient.get().uri(endpoint).accept(MediaType.APPLICATION_JSON).retrieve();
3. return responseSpec.bodyToMono(Map.class).
        flatMap(s ->
                    ServerResponse.ok().contentType(MediaType.APPLICATION_JSON)
                            .bodyValue(getMap(Pair.of("message", s.toString()))))
    .onErrorResume(throwable -> {
       LOG.error("endpoint: '{}' rest call failed: {}", endpoint, throwable.getMessage());
        String errorMessage = throwable.getMessage();

4.        if (throwable instanceof WebClientResponseException) {
                WebClientResponseException webClientResponseException = (WebClientResponseException) throwable;
                LOG.error("error body contains: {}", webClientResponseException.getResponseBodyAsString());
                errorMessage = webClientResponseException.getResponseBodyAsString();
            }
            return Mono.error(new SecurityException(errorMessage));
        });   

In the event of a error when a webservice call is made you would have to cast the Throwable object into a WebClientResponseException class to get the actual webservice message. In the above example, you can see the throwable object is casted to a WebClientResponseException to get the actual mesage. For an example of this usage you can follow this example from the jwt-validator project.

The example I have shown is from a test case. In this example, the proxied service returns a bad request or http 400 error invoking onErrorResume block in the caller code. I print both the thorwable.getMessage() vs the webClientResponseException.getResponseBodyAsString() output to show the output.

Test output

When I run the test case, the proxied error http message is captured here:

EndpointHandler        : throwing error
2023-04-03 10:41:14.484 ERROR 60979 --- EndpointHandler        : endpoint: 'http://localhost:10001/api/health/throwerror' rest call failed: 400 Bad Request from GET http://localhost:10001/api/health/throwerror
2023-04-03 10:41:14.484 ERROR 60979 --- EndpointHandler        : error body contains: throwing error