Tuesday, September 18, 2012

Spring 3.1 - Loading Properties For XML Configuration From Database

Spring makes it easy to inject values obtained from properties files via its PropertyPlaceholderConfigurer and (pre-Spring 3.1) PropertySourcesPlaceholderConfigurer (Spring 3.1). These classes implement the BeanFactoryPostProcessor interface, which enables them to manipulate the values within the Spring XML configuration file before the beans are initialized. So if you specify ${jdbc.driverClassName} to be set to the property 'driverClassName', this variable will be replaced/swapped with the value with the key 'jdbc.driverClassName' in a properties file.

Apart from properties files, the database table can also be a place to get key-value pairs. Great, so just extend the PropertySourcesPlaceholderConfigurer, and have it read a table containing the key-value pairs, populate them and we're done!

However, there's a slight problem. If the DataSource bean also relies on values obtained from a properties file (e.g. JDBC URL, username, password), and being good Springers, inject this bean to the bean class extending PropertySourcesPlaceholderConfigurer, the bean container will fail to startup properly, because the 'jdbc.driverClassName' variable cannot be resolved. Strange, but true.

The reason for this is that any bean injected into a BeanFactoryPostProcessor class will trigger bean initialization BEFORE the BeanFactoryPostProcessor classes are run. You know, dependency injection...all depending beans have to be ready before being injected into the consumer. So this creates a cyclic-dependency kind of thing. All dependencies in the XML configuration are resolved first before the BeanFactoryPostProcessor classes are run.

So, how to go about this? Well, there's a trick you can employ. A BeanFactoryPostProcessor class has access to the ConfigurableListableBeanFactory object via the 'postProcessBeanFactory' method. From this object, you can do a 'getBean' and get a reference of any bean with an id. And guess what, you can get the vaunted DataSource bean without triggering premature bean initialization.

Let's say there's a table 'sys_param' with the following data:

 --------------  --------------
 service.charge  1.5          
 rebate.amount   15.99        

The DbPropertySourcesPlaceholderConfigurer is shown here:

package org.gizmo.labs.utils.spring;

import javax.sql.DataSource;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.context.support.PropertySourcesPlaceholderConfigurer;

public class DbPropertySourcesPlaceholderConfigurer extends PropertySourcesPlaceholderConfigurer
 public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException
  DataSource dataSource = beanFactory.getBean(DataSource.class);
  DbProperties dbProps = new DbProperties(dataSource);

The DbProperties class will make use of the DataSource reference and queries the database to get the key-value pairs:

package org.gizmo.labs.utils.spring;

import java.util.List;
import java.util.Map;
import java.util.Properties;

import javax.sql.DataSource;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.jdbc.core.JdbcTemplate;

public class DbProperties extends Properties
 private final Logger logger = LoggerFactory.getLogger(DbProperties.class);
 private static final long serialVersionUID = 1L;

 public DbProperties(DataSource dataSource)
  JdbcTemplate jdbcTemplate = new JdbcTemplate(dataSource); 
  List> l = jdbcTemplate.queryForList("select param_cd, param_value from sys_param");
  for(Map m: l)
   logger.debug("Loading from DB: [{}:{}]", m.get("PARAM_CD"), m.get("PARAM_VALUE"));
   setProperty((m.get("PARAM_CD")).toString(), (m.get("PARAM_VALUE")).toString());

To demonstrate that the values from the table are properly injected, here's the class which acts as the consumer:

package org.gizmo.labs.utils.spring;

import java.math.BigDecimal;

import org.apache.commons.lang.builder.ReflectionToStringBuilder;
import org.apache.commons.lang.builder.ToStringStyle;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.InitializingBean;

public class DbPropConsumer implements InitializingBean
 private final Logger logger = LoggerFactory.getLogger(DbPropConsumer.class);

 private BigDecimal serviceCharge;
 private double rebateAmount;
 private String smtpIp;
 public void afterPropertiesSet() throws Exception
  logger.debug("I have consumed: {}", this);

 public String toString()
  return ReflectionToStringBuilder.toString(this, ToStringStyle.MULTI_LINE_STYLE);
 public BigDecimal getServiceCharge() {
  return serviceCharge;

 public void setServiceCharge(BigDecimal serviceCharge) {
  this.serviceCharge = serviceCharge;

 public double getRebateAmount() {
  return rebateAmount;

 public void setRebateAmount(double rebateAmount) {
  this.rebateAmount = rebateAmount;

 public String getSmtpIp() {
  return smtpIp;

 public void setSmtpIp(String smtpIp) {
  this.smtpIp = smtpIp;


Last but not least, the Spring configuration (DataSource bean not shown, simplified for clarity):




The first 2 bean definitions are the BeanFactoryPostProcessor classes, and to ensure the first one is run first, the 'order' property is set (lower means higher precedence).

For the DbPropertySourcesPlaceholderConfigurer, a different placeholder prefix and suffix is used for clarity (notice the placeholders for DbPropConsumer).

So, upon Spring container startup, you should be able to view a similar output as below:
2012-09-18 00:03:14, DEBUG, org.gizmo.labs.utils.spring.DbProperties, Loading from DB: [service.charge:1.5]
2012-09-18 00:03:14, DEBUG, org.gizmo.labs.utils.spring.DbProperties, Loading from DB: [rebate.amount:15.99]
2012-09-18 00:03:14, DEBUG, org.gizmo.labs.utils.spring.DbProperties, Loading from DB: [smtp.ip:]

2012-09-18 00:03:14, DEBUG, org.gizmo.labs.utils.spring.DbPropConsumer, I have consumed: org.gizmo.labs.utils.spring.DbPropConsumer@189b939[

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