spring tool configure

If you use Spring in your Java projects, you are probably familiar with the PropertyPlaceholderConfigurer. Using this BeanFactoryPostProcessor, you can pull property values from a properties file. An example XML context snippet from the Spring JavaDocs:

<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
    <property name="driverClassName"><value>${driver}</value></property>
    <property name="url"><value>jdbc:${dbname}</value></property>
</bean>

And the corresponding properties file:

driver=com.mysql.jdbc.Driver
dbname=mysql:mydb

Such a technique separates configuration from the context file. You could simply use a different properties file to configure development, QA, staging, and production environments.

What, however, if you would like to use the same technique to pull property values from a database table? It's actually relatively easy to achieve. First, you override the default PropertyPlaceholderConfigurer:

import ...;
 
/**
 * Used to load configurtion properties from a database table. 
 * Changes the default placeholderPrefix to '#{'.
 * 
 * <property name="url" value="#{website.url}"/>
 * 
 * @author David Norton
 *
 */
public class DatabasePropertyPlaceholderConfigurer extends PropertyPlaceholderConfigurer {
    public DatabasePropertyPlaceholderConfigurer() {
        super();
        setPlaceholderPrefix("#{");
    }
}

Then, you add some database code and override the loadProperties() function:

private JdbcTemplate jdbcTemplate;
 
@Override
protected void loadProperties(final Properties props) throws IOException {
    jdbcTemplate.query("SELECT "+nameColumn+", "+valueColumn+" FROM "+propsTable,new RowCallbackHandler() {
 
        public void processRow(ResultSet rs) throws SQLException {
            String name = rs.getString(nameColumn);
            String value = rs.getString(valueColumn);
            props.setProperty(name, value);
        }
 
    });
}
 
public void setDataSource(DataSource dataSource) {
    this.jdbcTemplate = new JdbcTemplate(dataSource);
}
 
private String nameColumn = "name";
private String valueColumn = "value";
private String propsTable = "config";

In order to use this, place the following in your Spring XML (assuming you already have a configured datasource bean named 'dataSource'):

<bean id="propertyConfigurer" class="DatabasePropertyPlaceholderConfigurer">
    <property name="dataSource" ref="dataSource"/>
</bean>
 
<bean id="someBean" class="org.my.ConfigurableService">
    <property name="website.url" value="#{website.url}"/>
</bean>

Note the departure from the traditional "${propertyname}" usage to "#{propertyname}".

We can add some setters to make the table and column names configurable:

/**
 * Specify a custom column for the property name. Default is 'name'.
 * @param nameColumn
 */
public void setNameColumn(String nameColumn) {
    this.nameColumn = nameColumn;
}
/**
 * Specify a custom column for the property value. Default is 'value'.
 * @param valueColumn
 */
public void setValueColumn(String valueColumn) {
    this.valueColumn = valueColumn;
}
 
/**
 * Specify a custom table name. Default is 'config'.
 * @param propsTable
 */
public void setPropsTable(String propsTable) {
    this.propsTable = propsTable;
}

Used such as:

<bean id="propertyConfigurer" class="DatabasePropertyPlaceholderConfigurer">
    <property name="dataSource" ref="dataSource"/>
    <property name="nameColumn" ref="property_name"/>
</bean>

Some future refactorings could include: move the actually data access code out of this class and into a more generic properties data access object. Of course, comments and criticism are welcome.

About David

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Tagged as: java, spring Leave a comment

Comments (1) Trackbacks (0) ( subscribe to comments on this post )

1. miluch
   January 19th, 2010 - 07:13

   Hi

   Great Post,
   I tried to use your solution in my project but i bumped into a one big inconvenience.
   Next to DatabasePropertyPlaceholderConfigurer i wanted to use “standard”
   PropertyPlaceholderConfigurer to resolve properties on dataSource bean.
   These properties must be loaded from property file.

   bean id=”dataSource”
   class=”org.springframework.jdbc.datasource.DriverManagerDataSource”
   property name=”driverClassName” value=”${db.driver}”"
   property name=”url” value=”${url}”
   property name=”username” value=”${db.username}”
   property name=”password” value=”${db.password}”
   bean

   The problem is that Spring first creates ALL PropertyPlaceholderConfigurer beans and after that it invokes their postProcessBeanFactory() methods.
   In this way dataSource bean is created before any placeholder resolving takes place.

   To cope with it i decided to inject dataSource bean name instead of dataSource bean itself.
   Additionally, let DatabasePropertyPlaceholderConfigurer implements BeanFactoryAware
   (PropertyPlaceholderConfigurer implements BeanFactoryAware but it stores BeanFactory in private filed and can not access it from subclasses)
   and retrieve dataSource (by “dataSourceBeanName” name from BeanFactory) within loadProperties method.
   This way i am able to retrieve datasource bean “lazily” after standard PropertyPlaceholderConfigurer resolving took place.