In this tutorial, we will build a stateful serverless API using Java and Redis on AWS Lambda. The API will simply count the page views and return it as HTTP response.

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Prerequisites

  1. Install the Serverless Framework installed with an AWS account set up.
npm i serverless@3.39.0 -g
  1. Install JDK and not Java JRE. Set your JAVA_HOME.
  2. Install Apache Maven.
  3. Create a free Serverless Redis database from Upstash as described here.

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Project Setup

Create the project: 
serverless create --template aws-java-maven --name counter-api -p aws-java-counter-api

cd aws-java-counter-api
Add jedis as dependency to the pom.xml:
pom.xml
...
    <dependency>
        <groupId>redis.clients</groupId>
        <artifactId>jedis</artifactId>
        <version>3.6.0</version>
    </dependency>
...
Update serverless.yml as below:
serverless.yml
service: counter-api

frameworkVersion: '3'

provider:
  name: aws
  runtime: java17

package:
  artifact: target/hello-dev.jar

functions:
 hello:
   handler: com.serverless.Handler
   events:
     - httpApi:
         path: /hello
         method: get

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Counter Function Setup

Update src/main/java/com/serverless/Handler.java as below:
src/main/java/com/serverless/Handler.java
package com.serverless;

import java.util.Map;

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;
import redis.clients.jedis.Jedis;

public class Handler implements RequestHandler<Map<String, Object>, ApiGatewayResponse> {
	@Override
	public ApiGatewayResponse handleRequest(Map<String, Object> input, Context context) {
		Jedis jedis = new Jedis("lasting-roughy-29092.upstash.io", 6379, true);
		jedis.auth("********");
		Long value = jedis.incr("counter");
		jedis.close();
		String message = "Hello World, Count:" + value;
		return ApiGatewayResponse.builder()
				.setStatusCode(200)
				.setObjectBody(message)
				.build();
	}
}
In the above code, you need to replace your Redis endpoint and password. You can copy Jedis connection code from the Upstash Redis Console -> Your Database -> Connect to your database -> Java.

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Build and Deploy

Build your project: 
mvn clean install
Deploy to AWS: 
serverless deploy
Visit the output URL to see the counter in action.