When you create REST services with JAX-RS, you typically either return nothing (so HTTP 201/2/4 etc) or some data, potentially in JSON format (so HTTP 200), or some Exception / Error (so HTTP 4xx or 5xx).

We usually translate a Runtime Exception into some HTTP 5xx and a Checked Exception into some 4xx.

Because we want to keep our boundary clean, we do not include the full Java stacktrace in the body of the response when we translate an Exception to a HTTP response. We usually just add a “REASON” Header with the HTTP 5xx (or sometimes 4xx) response. However, this means that most of our ExceptionMappers looks pretty much the same (something like this):


    @Provider
    public class SomeExceptionMapper implements ExceptionMapper<SomeException> {

        @Override
        public Response toResponse(SomeException exception) {
            return Response.status(500).header("reason", exception.getMessage()).build();
        }

    }

Using MicroProfile Config API

We can use MicroProfile Config API to create a configurable Exception Mapper, that allows the consumer to configure the Exception to HTTP Response Code mapping.

Our @Provider will handle all Runtime Exceptions:


    @Provider
    public class RuntimeExceptionMapper implements ExceptionMapper<RuntimeException> {
        // ...
    }

We @Inject both the Config and the Providers:


    @Inject
    private Config config;
    
    @Context 
    private Providers providers;

When we implement the toResponse method, we see if there is a mapping for this Exception class in our config:


    @Override
    @Produces({MediaType.APPLICATION_JSON,MediaType.APPLICATION_XML})
    public Response toResponse(RuntimeException exception) {
        return handleThrowable(exception);
    }
    
    private Response handleThrowable(Throwable exception) {
        if(exception instanceof WebApplicationException) {
            return ((WebApplicationException) exception).getResponse();
        }
        if(exception!=null){
            String configkey = exception.getClass().getName() + STATUS_CODE_KEY;
            Optional<Integer> possibleDynamicMapperValue = config.getOptionalValue(configkey,Integer.class);
            if(possibleDynamicMapperValue.isPresent()){
                int status = possibleDynamicMapperValue.get();
                // You switched it off
                if(status<0)return handleNotMapped(exception);
                String reason = getReason(exception);
                log.log(Level.FINEST, reason, exception);
                return Response.status(status).header(REASON, reason).build();
            } else if(exception.getCause()!=null && providers!=null){
                final Throwable cause = exception.getCause();
                return handleThrowable(cause);
            } else {
                return handleNotMapped(exception);
            }
        }
        return handleNullException();
    }

(full example here)

We also go up the exception chain until we get a mapping, or then default to a normal 500 error.

So we can add configuration for mappings like this:

    ## 503 Service Unavailable: The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.
    org.eclipse.microprofile.faulttolerance.exceptions.CircuitBreakerOpenException/mp-jaxrs-ext/statuscode=503
    
    ## 401 Unauthorized (RFC 7235): Similar to 403 Forbidden, but specifically for use when authentication is required and has failed or has not yet been provided.
    javax.ws.rs.NotAuthorizedException/mp-jaxrs-ext/statuscode=401

In the above example we will map a CircuitBreakerOpenException (from MicroProfile Fault tolerance API) to a 503 and NotAuthorizedException to a 401.

Example screenshot

Use it as a library.

You can also bundle all of this in a jar file to be used by any of your projects. I made the above available in maven central and github, so you can also use that directly.

Just add this to your pom.xml

    <dependency>
        <groupId>com.github.phillip-kruger.microprofile-extensions</groupId>
        <artifactId>jaxrs-ext</artifactId>
        <version>1.0.9</version>
    </dependency>

It comes with a few pre-defined mappings, but you can override it in your configuration.