package generic.util;

import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.Properties;

public class DBConnection {

    private static final String FILE_NAME = "db";
    private static final String URL = "url";
    private static final String USER = "user";
    private static final String PASSWORD = "password";

    private static Connection conn;

    private DBConnection(){

    }

    public static synchronized Connection getConnection() throws SQLException, ClassNotFoundException, IOException {

        if(DBConnection.conn == null || DBConnection.conn.isClosed()){
            Properties props = PropertiesReader.readPropertyFile(FILE_NAME);

            String url = props.getProperty(URL);
            String user = props.getProperty(USER);
            String password = props.getProperty(PASSWORD);

            Connection conn = DriverManager.getConnection(url, user, password);

            DBConnection.conn = conn;
        }

        return DBConnection.conn;
    }

}

<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>groupid</groupId>
    <artifactId>artifactid</artifactId>
    <version>1.0</version>
    <properties>
        <skipTests>true</skipTests>
    </properties>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.7.0</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>2.12.4</version>
                <configuration>
                    <skipTests>${skipTests}</skipTests>
                </configuration>
            </plugin>
        </plugins>
    </build>
    <dependencies>

        <!-- https://mvnrepository.com/artifact/org.postgresql/postgresql -->
        <dependency>
            <groupId>org.postgresql</groupId>
            <artifactId>postgresql</artifactId>
            <version>42.2.2</version>
        </dependency>

    </dependencies>
</project>