ErrorHandler.java

@Singleton
public class ErrorHandler extends DefaultHttpErrorHandler {
    
    private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
    
    @Inject
    public ErrorHandler(Configuration configuration, Environment environment,
                        OptionalSourceMapper sourceMapper, Provider<Router> routes) {
        super(configuration, environment, sourceMapper, routes);
    }
    
    @Override
    protected CompletionStage<Result> onNotFound(Http.RequestHeader request, String message) {
        String errorResponse="The requested resource does not exist.";
        
        JsonNode jsonNode = OBJECT_MAPPER.valueToTree(errorResponse);
        
        return CompletableFuture.completedFuture(Results.notFound(jsonNode));
    }
}