JPA

import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import java.util.HashMap;
import java.util.Map;

/**
 * @author zolw
 */
public class CustomEntityManagerFactory {

    private static EntityManagerFactory emf;
    private static final Object mutex = new Object();

    public static EntityManagerFactory createFactory(CustomConfig config) {
        Map<String, String> properties = new HashMap<>();
        properties.put("javax.persistence.jdbc.url", config.get(String.class, "h2", "url"));
        properties.put("javax.persistence.jdbc.password", config.get(String.class, "h2", "password"));
        properties.put("javax.persistence.jdbc.user", config.get(String.class, "h2", "user"));
        return Persistence.createEntityManagerFactory(config.getPersistanceUnitName(), properties);
    }

    public static EntityManagerFactory getInstance() {
        if (emf == null) {
            synchronized (mutex) {
                if (emf == null) {
                    CustomConfig config = CustomConfig.getInstance();
                    emf = createFactory(config);
                }
            }
        }
        return emf;
    }
}