I'm currently running a Spring Boot Pod in Kubernetes. There's a side car in the pod for the cloud SQL proxy.

Below is my spring Boot application.properties configuration:

server.port=8081
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
spring.jpa.hibernate.ddl-auto=create-drop
spring.datasource.continue-on-error=true
spring.datasource.url=jdbc:mysql://localhost:3306/<database_name>
spring.datasource.username=<user_name>
spring.datasource.password=<password>

Below is my pom.xml extract with plugins and dependencies:

<properties>
    <java.version>1.8</java.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-actuator</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-devtools</artifactId>
        <scope>runtime</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>com.jayway.jsonpath</groupId>
        <artifactId>json-path</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-hateoas</artifactId>
    </dependency>

    <dependency>
        <groupId>com.h2database</groupId>
        <artifactId>h2</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>ca.performance.common</groupId>
        <artifactId>common-http</artifactId>
        <version>1.1.1</version>
    </dependency>
    <dependency>
        <groupId>com.google.cloud.sql</groupId>
        <artifactId>mysql-socket-factory</artifactId>
        <version>1.0.10</version>
    </dependency>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-jdbc</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-gcp-starter-sql-mysql</artifactId>
        <version>1.1.0.RELEASE</version>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

And this is my deployment.yaml file:

apiVersion: v1
kind: Service
metadata:
  name: app-dummy-name
spec:
  selector:
    app: app-dummy-name
  ports:
  - port: 81
    name: http-app-dummy-name
    targetPort: http-api
  type: LoadBalancer
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: app-dummy-name
spec:
  replicas: 1
  selector:
    matchLabels:
      app: app-dummy-name
  template:
    metadata:
      labels:
        app: app-dummy-name
    spec:
      containers:
      - name: app-dummy-name
        image: <image url>
        ports:
        - containerPort: 8081
          name: http-api
        env:
        - name: DB_HOST
          value: 127.0.0.1:3306
        - name: DB_USER
          valueFrom:
            secretKeyRef:
              name: cloudsql-db-credentials
              key: username
        - name: DB_PASSWORD
          valueFrom:
            secretKeyRef:
              name: cloudsql-db-credentials
              key: password
      - name: cloudsql-proxy
        image: gcr.io/cloudsql-docker/gce-proxy:1.11
        command: ["/cloud_sql_proxy",
                    "-instances=<INSTANCE_CONNECTION_NAME>=:3306",
                    "-credential_file=/secrets/cloudsql/credentials.json"]
        securityContext:
          runAsUser: 2  # non-root user
          allowPrivilegeEscalation: false
        volumeMounts:
          - name: cloudsql-instance-credentials
            mountPath: /secrets/cloudsql
            readOnly: true
      volumes:
        - name: cloudsql-instance-credentials
          secret:
            secretName: cloudsql-instance-credentials

I followed the instructions from this link, so I created the secrets and the service account. However, I'm constantly getting connection refusal errors when I deploy the previous yaml file in Kubernetes after creating the secrets:

org.springframework.jdbc.support.MetaDataAccessException: Could not get Connection for extracting meta-data; 
nested exception is org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; 
nested exception is com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure. 
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.

I even tested the Spring boot application locally using the proxy and the same application.properties configuration and it was working fine.