@Nullable
    @Override
    public CircuitBreakerCallback tryRequest(ClientRequestContext ctx, Request req) {
        final CircuitBreaker circuitBreaker;
        try {
            circuitBreaker = requireNonNull(mapping.get(ctx, req), "circuitBreaker");
        } catch (Throwable t) {
            logger.warn("Failed to get a circuit breaker from mapping ({}) for context ({})", mapping, ctx, t);
            return null;
        }
        try {
            circuitBreaker.acquirePermission();
        } catch (CallNotPermittedException e) {
            throw UnprocessedRequestException.of(e);
        }
        final long startTimestamp = circuitBreaker.getCurrentTimestamp();
        return new Resilience4JCircuitBreakerCallback(circuitBreaker, startTimestamp);
    }
Creates a default {@link CircuitBreakerClientHandler} which uses the provided
{@link CircuitBreakerMapping} to handle requests.
resilience4j2/src/main/java/com/linecorp/armeria/resilience4j/circuitbreaker/client/Resilience4JCircuitBreakerClientHandler.java