Wait for the end of subscribe method before returning value in Flux java

I am developing a custom Spring cloud gateway predicate factory that will take decision based on some value in the request body.

I have written the following code.

public class OperatorIdPredicateFactory extends AbstractRoutePredicateFactory<OperatorIdPredicateFactory.Config> {

    public OperatorIdPredicateFactory(Class<Config> configClass) {
        super(configClass);

    }

    @Override
    public Predicate<ServerWebExchange> apply(Config config) {

        return (ServerWebExchange webExchange) -> {
            
            return webExchange.getRequest().getBody().as((Flux<DataBuffer> body) ->{
                
                var returnValue = new Object() {boolean value = false;};    // wrapper anonymous class for return value;
                        
                body.subscribe((DataBuffer buffer) ->{
                    
                    CharBuffer charBuffer = StandardCharsets.UTF_8.decode(buffer.asByteBuffer());
                    
                    DataBufferUtils.release(buffer);
                    
                    String requestJson = charBuffer.toString();
                    ObjectMapper mapper = new ObjectMapper();
                    RouteRequestPayload requestPayload = null;
                    
                    try {
                        requestPayload = mapper.readValue(requestJson, RouteRequestPayload.class);
                    } catch (JsonProcessingException e) {
                        e.printStackTrace();
                    }
                    
                    if(config.getPtoId().equalsIgnoreCase(requestPayload.getRouteRequest().getOperatorId()))
                        returnValue.value = true;


                });

                return returnValue.value;
            });
        };
    }

// config class
}

But the problem is that the method is returning before the subscribe method finishes.

How can I make sure that the subscribe method finished before returning the value.