Define the HitCounter API

Create a new file for our hit counter construct #

Create two new files under src/main/java/com/myorg called HitCounterProps.java with the following content:

package com.myorg;

import software.amazon.awscdk.services.lambda.IFunction;

public interface HitCounterProps {
    // Public constructor for the props builder
    public static Builder builder() {
        return new Builder();
    }

    // The function for which we want to count url hits
    IFunction getDownstream();

    // The builder for the props interface
    public static class Builder {
        private IFunction downstream;

        public Builder downstream(final IFunction function) {
            this.downstream = function;
            return this;
        }

        public HitCounterProps build() {
            if(this.downstream == null) {
                throw new NullPointerException("The downstream property is required!");
            }

            return new HitCounterProps() {
                @Override
                public IFunction getDownstream() {
                    return downstream;
                }
            };
        }
    }
}

AND HitCounter.java with this content:

package com.myorg;

import software.constructs.Construct;

public class HitCounter extends Construct {
    public HitCounter(final Construct scope, final String id, final HitCounterProps props) {
        super(scope, id);
        // TODO
    }
}

Save the files.

What’s going on here? #

We declared a new construct class called HitCounter.
As usual, constructor arguments are scope, id and props, and we propagate them to the Construct base class.
The props argument is of type HitCounterProps which includes a single property getDownStream of type IFunction and a Builder to create the props. This is where we are going to “plug in” the Lambda function we created in the previous chapter so it can be hit-counted.

Next, we are going to write the handler code of our hit counter.