Code snippets from: http://adtennant.co.uk/blog/2014/07/14/component-controller-communication-using-interfaces-part-2/

gistfile1.cls

public class InnerClassCounterPageController {
    private Boolean showAlert;
    
    public InnerClassCounterPageController() {
        showAlert = false;
    }
    
    public Boolean getShowAlert() {
        return showAlert;
    }
    
    public CounterComponentListener getComponentListener() {
        return new InnerClassComponentListener(this);
    }
    
    private class InnerClassComponentListener implements CounterComponentListener {
        private InnerClassCounterPageController controller;
        
        public InnerClassComponentListener(InnerClassCounterPageController controller) {
            this.controller = controller;
        }
        
        public void onChange(Integer newValue) {
            controller.showAlert = (newValue > 2);
        }
    }
}

gistfile2.html

<apex:page controller="InnerClassCounterPageController">
    <apex:pageMessage severity="warning" strength="3" summary="It's over 2!" rendered="{!ShowAlert}"/>
    <c:CounterComponent componentListener="{!ComponentListener}"/>
</apex:page>

gistfile3.cls

public class MultipleCounterPageController {
    private class ComponentListener1 implements CounterComponentListener {
        private Boolean overTwo;
        
        public void onChange(Integer newValue) {
            overTwo = (newValue > 2);
        }
    }
    
    private class ComponentListener2 implements CounterComponentListener {
        private Boolean equalsOne;
        
        public void onChange(Integer newValue) {
            equalsOne = (newValue == 1);
        }
    }
    
    private ComponentListener1 listener1;
    private ComponentListener2 listener2;
    
    public MultipleCounterPageController() {
        listener1 = new ComponentListener1();
        listener2 = new ComponentListener2();
    }
    
    public Boolean getShowAlert1() {
        return listener1.overTwo;
    }
    
    public Boolean getShowAlert2() {
        return listener2.equalsOne;
    }
    
    public CounterComponentListener getComponentListener1() {
        return listener1;
    }
    
    public CounterComponentListener getComponentListener2() {
        return listener2;
    }
}

gistfile4.html

<apex:page controller="MultipleCounterPageController">
    <apex:pageMessage severity="warning" strength="3" summary="It's over 2!" rendered="{!ShowAlert1}"/>
    <c:CounterComponent componentListener="{!ComponentListener1}"/>
    
    <apex:pageMessage severity="error" strength="3" summary="ONE!!!!" rendered="{!ShowAlert2}"/>
    <c:CounterComponent componentListener="{!ComponentListener2}"/>
</apex:page>