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  4. A Custom Property in Spring

A Custom Property in Spring

Rob Gordon user avatar by
Rob Gordon
·
Apr. 22, 12 · Interview
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<context:property-placeholder> is a really easy way to provide property replacements in Spring configurations with values from a standard Java Properties file. But what if you don’t want a property hard coded into a file – a clear text password for instance? Spring provides all the bits and pieces to write your own property replacement. Let me introduce my CustomPropertyConfigurer.

I’ll demonstrate using a variation on the theme of the Spring JDBC Template. MyQuery is a simple extension of org.springframework.jdbc.core.JDBCTemplate that gets the current timestamp from a MySQL database. Here’s the, hopefully familiar, configuration:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context"
  xsi:schemaLocation="
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd">
 
  <context:property-placeholder location="classpath:jdbc.properties" />
 
  <bean id="myQuery" class="rob.MyQuery">
    <property name="dataSource" ref="dataSource" />
  </bean>
 
  <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"
    destroy-method="close">
    <property name="driverClassName" value="${jdbc.driverClassName}" />
    <property name="url" value="${jdbc.url}" />
    <property name="username" value="${jdbc.username}" />
    <property name="password" value="${jdbc.password}" />
  </bean>
 
</beans>

Except the jdbc.properties file does not contain the password:

jdbc.driverClassName=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://rob-7
jdbc.username=rob

I will set the jdbc.password property myself from what is entered on the command line.

public static void main(String... args) {
 
    char[] password = System.console().readPassword("Password: ");
 
    Properties properties = new Properties();
    properties.setProperty("jdbc.password", new String(password));
 
    ConfigurableApplicationContext context =
        new ClassPathXmlApplicationContext(
            new String[] {
                "rob/MyQuery.xml"},
            false);
 
    context.addBeanFactoryPostProcessor(
            new CustomPropertyConfigurer(properties));
    context.refresh();
 
    MyQuery myQuery = context.getBean(MyQuery.class);
 
    myQuery.run();
 
    context.close();
}

Where the CustomPropertyConfigurer is:

import java.util.Properties;
 
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanDefinitionStoreException;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.BeanDefinitionVisitor;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.util.PropertyPlaceholderHelper;
import org.springframework.util.StringValueResolver;
 
public class CustomPropertyConfigurer implements BeanFactoryPostProcessor {
 
    private final Properties properties;
 
    public CustomPropertyConfigurer(Properties properties) {
        this.properties = properties;
    }
 
    public void postProcessBeanFactory(
            ConfigurableListableBeanFactory beanFactoryToProcess)
    throws BeansException {
 
        BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(
                new BeanDirectoryResolver());
 
        String[] beanNames =
                beanFactoryToProcess.getBeanDefinitionNames();
        for (int i = 0; i < beanNames.length; i++) {
 
            BeanDefinition bd = beanFactoryToProcess.getBeanDefinition(
                    beanNames[i]);
            try {
                visitor.visitBeanDefinition(bd);
            }
            catch (BeanDefinitionStoreException ex) {
                throw new BeanDefinitionStoreException(
                        bd.getResourceDescription(), beanNames[i],
                        ex.getMessage());
            }
        }
    }
 
    class BeanDirectoryResolver implements StringValueResolver {
 
        private final PropertyPlaceholderHelper helper;
 
        public BeanDirectoryResolver() {
            helper = new PropertyPlaceholderHelper("${", "}");
        }
 
        public String resolveStringValue(String strVal) {
            return helper.replacePlaceholders(strVal, properties);
        }
    }
}

The CustomPropertyConfigurer gets applied first. It leaves any properties it can’t resolve (all but the password) for the standard <context:property-configurer> to resolve. Unit tests running against a different jdbc.propeties file can continue to provide the password as before.

Here it is running:

Cusom Propery Configurer Running

There are many other examples of configuration values that might only be discovered at runtime – file names, schedule dates, form values etc. So long as the value can be a String, a CustomPropertyConfigurer provides a simple way of passing these values to Spring.

 

Property (programming) Spring Framework

Published at DZone with permission of Rob Gordon, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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