Amazon RDSへのJDBCコネクションが取得できない

Question

前提・実現したいこと

Spring Frameworkで制作中のWebアプリから、Amazon RDS(MySQL)へのDBコネクションを取得したいです。
コネクションプールはdbcp2を使用しており、ORMはMyBatisです。

データソースに関する設定は下記のDataSourceConfig.javaに記載しています。

発生している問題・エラーメッセージ

The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.)
	at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
	at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:149)
	at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:140)
	at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:76)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:426)
	... 53 common frames omitted
Caused by: org.springframework.jdbc.CannotGetJdbcConnectionException: Failed to obtain JDBC Connection; nested exception is java.sql.SQLException: Cannot create PoolableConnectionFactory (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.)
	at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:82)
	at org.mybatis.spring.transaction.SpringManagedTransaction.openConnection(SpringManagedTransaction.java:80)
	at org.mybatis.spring.transaction.SpringManagedTransaction.getConnection(SpringManagedTransaction.java:67)
	at org.apache.ibatis.executor.BaseExecutor.getConnection(BaseExecutor.java:336)
	at org.apache.ibatis.executor.SimpleExecutor.prepareStatement(SimpleExecutor.java:85)
	at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:62)
	at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:324)
	at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156)
	at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109)
	at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:83)
	at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:147)
	... 60 common frames omitted
Caused by: java.sql.SQLException: Cannot create PoolableConnectionFactory (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.)
	at org.apache.commons.dbcp2.BasicDataSource.createPoolableConnectionFactory(BasicDataSource.java:669)
	at org.apache.commons.dbcp2.BasicDataSource.createDataSource(BasicDataSource.java:544)
	at org.apache.commons.dbcp2.BasicDataSource.getConnection(BasicDataSource.java:753)
	at org.springframework.jdbc.datasource.DataSourceUtils.fetchConnection(DataSourceUtils.java:158)
	at org.springframework.jdbc.datasource.DataSourceUtils.doGetConnection(DataSourceUtils.java:116)
	at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:79)
	... 70 common frames omitted
Caused by: 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.
	at com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:174)
	at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64)
	at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:836)
	at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:456)
	at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:246)
	at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:199)
	at org.apache.commons.dbcp2.DriverConnectionFactory.createConnection(DriverConnectionFactory.java:55)
	at org.apache.commons.dbcp2.PoolableConnectionFactory.makeObject(PoolableConnectionFactory.java:355)
	at org.apache.commons.dbcp2.BasicDataSource.validateConnectionFactory(BasicDataSource.java:115)
	at org.apache.commons.dbcp2.BasicDataSource.createPoolableConnectionFactory(BasicDataSource.java:665)
	... 75 common frames omitted
Caused by: com.mysql.cj.exceptions.CJCommunicationsException: 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.
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
	at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:61)
	at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:105)
	at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:151)
	at com.mysql.cj.exceptions.ExceptionFactory.createCommunicationsException(ExceptionFactory.java:167)
	at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:91)
	at com.mysql.cj.NativeSession.connect(NativeSession.java:144)
	at com.mysql.cj.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:956)
	at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:826)
	... 82 common frames omitted
Caused by: java.net.ConnectException: Connection timed out: connect
	at java.base/java.net.PlainSocketImpl.connect0(Native Method)
	at java.base/java.net.PlainSocketImpl.socketConnect(PlainSocketImpl.java:101)
	at java.base/java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:399)
	at java.base/java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:242)
	at java.base/java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:224)
	at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
	at java.base/java.net.Socket.connect(Socket.java:609)
	at com.mysql.cj.protocol.StandardSocketFactory.connect(StandardSocketFactory.java:155)
	at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:65)
	... 85 common frames omitted

該当のソースコード

（※ソースコード内の"hostname"には、エンドポイントのURLが格納されています）

package org.step.helper.config;

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

import org.apache.commons.dbcp2.BasicDataSource;
import org.apache.ibatis.session.SqlSessionFactory;
import org.mybatis.spring.SqlSessionFactoryBean;
import org.mybatis.spring.annotation.MapperScan;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.io.ClassPathResource;
import org.springframework.jdbc.datasource.DataSourceTransactionManager;
import org.springframework.transaction.annotation.EnableTransactionManagement;

@Configuration
@EnableTransactionManagement
@MapperScan("org.step.helper.mapper")
public class DataSourceConfig {
  @Bean
  public BasicDataSource dataSource() {
    Properties prop = new Properties();

    try {
      InputStream resource = new ClassPathResource("datasource.properties").getInputStream();
      prop.load(resource);
    } catch (IOException e) {
      e.printStackTrace();
    }

    String hostname = prop.getProperty("hostname");
    String portNo = prop.getProperty("portNo");
    String dbName = prop.getProperty("dbName");
    String userName = prop.getProperty("userName");
    String password = prop.getProperty("password");

    BasicDataSource basicDataSource = new BasicDataSource();
    basicDataSource.setDriverClassName("com.mysql.cj.jdbc.Driver");
    basicDataSource.setUrl(
        "jdbc:mysql://"
            + hostname
            + ":"
            + portNo
            + "/"
            + dbName
            + "?characterEncoding=UTF-8&serverTimezone=JST");
    basicDataSource.setUsername(userName);
    basicDataSource.setPassword(password);
    return basicDataSource;
  }

  @Bean
  public DataSourceTransactionManager transactionManager() {
    return new DataSourceTransactionManager(dataSource());
  }

  @Bean
  public SqlSessionFactory sqlSessionFactory() throws Exception {
    SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();
    factoryBean.setDataSource(dataSource());
    org.apache.ibatis.session.Configuration config = new org.apache.ibatis.session.Configuration();
    config.setMapUnderscoreToCamelCase(true);
    config.setDefaultFetchSize(100);
    factoryBean.setConfiguration(config);
    return factoryBean.getObject();
  }
}

試したこと

EC2で構築したCentOSサーバより、下記コマンドでデータベースへの接続が出来ることは確認しました。
mysql -h (エンドポイント) -P 3306 -u admin -p

補足情報（FW/ツールのバージョンなど）

Apache Commons DBCP: 2.7.0

Answer 1

RDSがプライベートサブネットに配置されていたため、外部からアクセスが出来ない状態だったことが発覚いたしました。
パブリックサブネットに配置し、Publicly Accessibleを有効にすることでアクセスできました。