ConditionalOnMissingProperty.java

package com.acme.condition;

import org.springframework.boot.autoconfigure.condition.ConditionMessage;
import org.springframework.boot.autoconfigure.condition.ConditionOutcome;
import org.springframework.boot.autoconfigure.condition.SpringBootCondition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.context.annotation.Conditional;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import org.springframework.core.type.AnnotatedTypeMetadata;
import org.springframework.util.MultiValueMap;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD})
@Documented
@Conditional(ConditionalOnMissingProperty.OnMissingPropertyCondition.class)
public @interface ConditionalOnMissingProperty {

    String[] value();

    @Order(Ordered.HIGHEST_PRECEDENCE + 40)
    class OnMissingPropertyCondition extends SpringBootCondition {

        @Override
        public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {

            MultiValueMap<String, Object> annotationAttributes
                    = metadata.getAllAnnotationAttributes(ConditionalOnMissingProperty.class.getName());
            for (Object values : annotationAttributes.get("value")) {
                for (String propertyName : (String[]) values) {
                    if (context.getEnvironment().containsProperty(propertyName)) {
                        // return NO match if there is a property of the given name:
                        return ConditionOutcome.noMatch(ConditionMessage.of("Found property " + propertyName));
                    }
                }
            }

            // return match if no matching property was found:
            return ConditionOutcome.match(ConditionMessage.of("None of the given properties found"));
        }
    }
}

Could you explain please, why did you specify 40 in @Order(Ordered.HIGHEST_PRECEDENCE + 40) code?