How to use SpEL to read payload and header content

Technologies in my project

Spring Boot 2
Spring Integration (XML style)
Java 8
Tomcat 9.x/Liberty 19.0.0.1

As a part of my Spring Integration project (REST API with an inbound-http-gateway, that takes an XML input and produces an XML output), I am writing the following components:

A service-activator that performs basic validation of the incoming XML against its XSD.
If validation is successful, a POJO (a builder-pattern Java object) is built with a Boolean instance variable/property called "isError" set to true.
If validation is un-successful, the POJO (a builder-pattern Java object) is built with a Boolean instance variable/property called "isError" set to false.
After building the POJO, it is wrapped into a Message and sent to an output-channel.
There is a router component that gets the message from the outout-channel and decides the target route/channel based on the value of the "isError" property in the payload/POJO.

A draft of the POJO class (in reality, the POJO will have more properties but for our example, kept it short):

public class Composite {
    private Boolean isError;

    private Composite(CompositeBuilder compositeBuilder) {
            this.isError = miCompositeBuilder.isError;
        }

    public boolean isError() {
            return isError;
        }

    //Builder
    public static class CompositeBuilder {
        private Boolean isError;

        public CompositeBuilder(Boolean isError) {
            this.isError = isError;
        }      

        public Composite build() {
            return new Composite(this);
        }
    }
}

The validator service-activator component in XML:

<!-- SERVICE ACTIVATOR FOR REQUEST VALIDATION -->
    <int:service-activator ref="myService"
                           method="validateMYRequest"
                           input-channel="myGatewayRequests"
                           output-channel="compositesPostRequestValidation" />

The router component in XML:

<!-- ROUTER POST-REQUEST VALIDATION -->
    <int:router input-channel="compositesPostRequestValidation" expression="payload.isError">
        <int:mapping value="true" channel="upstreamResponses"/>
        <int:mapping value="false" channel="downstreamValidatedRequests"/>
    </int:router>

Finally coming to my questions, in this "router",

Question 1) Something seems to be wrong with the SpEL expression (payload.isError) since I got an exception during the invocation of the router that went like:

org.springframework.expression.spel.SpelEvaluationException: EL1008E: Property or field 'isError' cannot be found on object of type 'com.amb.restSample.core.model.Composite' - maybe not public or not valid? at org.springframework.expression.spel.ast.PropertyOrFieldReference.readProperty(PropertyOrFieldReference.java:217) ~[spring-expression-5.1.5.RELEASE.jar:5.1.5.RELEASE] ... ...

Can you please tell me how to access the Boolean "isError" property inside the Composite POJO? This is assuming that the Message being sent to the router is Message

Question 2) In case I wish to accommodate the "isError" property as a Message header (String) before sending it to the router (instead of using it as a Boolean property inside the Composite POJO), is there an expression to access it?

Looks like the paylaod is MIComposite not Composite.

In any case, SpEL uses the JavaBean conventions; so isError() is a getter for a boolean property error. So payload.error should work (as long as MIComposite exposes it). Or you can use payload.isError().

If it's in a header, the expression would be headers['myHeader'].