Spring ApplicationContextInitializer class which adds a new PropertySource to Spring's DefaultEnvironment.

PropertySourceInitializer.java

import java.util.HashMap;
import java.util.Map;

import org.apache.commons.lang3.exception.ExceptionUtils;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.mapping.Environment;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.apache.ibatis.transaction.jdbc.JdbcTransactionFactory;
import org.springframework.context.ApplicationContextInitializer;
import org.springframework.core.env.PropertySource;
import org.springframework.jdbc.datasource.lookup.JndiDataSourceLookup;
import org.springframework.web.context.ConfigurableWebApplicationContext;

/**
 * Adds a new PropertySource(settings table in db) to Spring's
 * DefaultEnvironment before application context is refreshed. This class should
 * be registered as a servlet context param(contextInitializerClasses) in
 * web.xml
 * 
 * @see <a
 *      href="http://blog.springsource.org/2011/02/15/spring-3-1-m1-unified-property-management/">
        SPRINGSOURCE BLOG: UNIFIED PROPERTY MANAGEMENT</a>
 * 
 * @author Inanc Sevinc
 * 
 */
public class PropertySourceInitializer implements ApplicationContextInitializer<ConfigurableWebApplicationContext> {

	public void initialize(ConfigurableWebApplicationContext applicationContext) {
		applicationContext.getEnvironment().getPropertySources()
				.addLast(new PortalSettingsPropertySource("portalsettings"));
	}
	
	private interface SettingsMapper {
		@Select("select PROPERTY_VALUE from PORTAL_SETTINGS where PROPERTY_KEY = #{key}")
		String selectProperty(String key);
	}

	private static class PortalSettingsPropertySource extends
			PropertySource<String> {
		
		private static final String JNDI_DATASOURCE_NAME = "jdbc/verim";
		private static final Map<String,String> settingsMap = new HashMap<String,String>();

		public PortalSettingsPropertySource(String name) {
			super(name);
		}

		@Override
		public String getProperty(String key) {
			String value = settingsMap.get(key);
			if(value!=null)
				return value;
			synchronized (settingsMap) {
				value = getPropValueFromDB(key);
				settingsMap.put(key, value);
			};
			return value;
		}

		private String getPropValueFromDB(String key) {
			SqlSession sqlSession = null;
			try {
				// programmatically create mybatis sessionfactory
				Configuration configuration = new Configuration(
												new Environment("verim",
														new JdbcTransactionFactory(),
														new JndiDataSourceLookup().getDataSource(JNDI_DATASOURCE_NAME)
														));
				// add mapper interface before executing query
				configuration.addMapper(SettingsMapper.class);
				SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
						.build(configuration);
				sqlSession = sqlSessionFactory.openSession();
				SettingsMapper mapper = sqlSession.getMapper(SettingsMapper.class);
				String result = mapper.selectProperty(key);
				
				if (result != null) {
					System.out.println("INFO: Configuration value is retrieved from DB. key="+key+" value="+result);
					return result;
				} else {
					System.out.println("ERROR: Configuration value could not be found in DB. key="+key);
				}
			} catch (Exception e) {
				System.err.print("Exception in "+this.getClass().getCanonicalName()+".getPropValueFromDB()\n"+ExceptionUtils.getStackTrace(e));
			} finally {
				if (sqlSession != null)
					sqlSession.close();
			}
			return null;
		}
	}
}