How to use environment-specific configuration in Spring, activated with a Java system property: -Denv=dev

Spring environment-aware configuration

As a Java developer, I want to be able to inject configuration properties into my application as simply
and easily as possible. I want these properties to be different depending on which environment (prod
vs. stage vs. dev) the system is running in. I don't want to generate separate, non-portable build
artifacts for each environment, because they will get confused. I don't want to redefine property values
that are the same across different environments (dev should inherit and override prod, not duplicate
prod). I want the default environment to be prod, so any operations person can be completely oblivious to
this system and still get the correct behavior.

In this example, a servlet application connects to a Mongo database. We want the database hostname to be
different depending on the environment; on prod, Mongo is hosted locally, but in dev, Mongo is on a VM
and the servlet is run from a workstation IDE.

1. META-INF⁄prod⁄db.properties

db.host=127.0.0.1
db.port=27017
db.name=partitionmagick

2. META-INF⁄dev⁄db.properties

db.host=skadi.bluejeansnet.com

3. startJetty.sh

#!/bin/bash

# production mode
java -jar start.jar

4. startJetty-dev.sh

#!/bin/bash

# development mode
java -Denv=dev -jar start.jar

5. DatabaseConfig.java

package vc.bjn.partitionmagick.config;

import com.mongodb.Mongo;
import com.mongodb.MongoOptions;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.data.mongodb.MongoDbFactory;
import org.springframework.data.mongodb.core.MongoFactoryBean;
import org.springframework.data.mongodb.core.MongoTemplate;
import org.springframework.data.mongodb.core.SimpleMongoDbFactory;
import org.springframework.data.mongodb.repository.config.EnableMongoRepositories;

@Configuration
@EnableMongoRepositories(basePackages = "vc.bjn.partitionmagick")
public class DatabaseConfig {

	@Value("${db.host}") private String dbHost;
	@Value("${db.port}") private int dbPort;
	@Value("${db.name}") private String dbName;

	@Bean public MongoFactoryBean mongoConnection() {
		final MongoFactoryBean mongoFactoryBean = new MongoFactoryBean();

		mongoFactoryBean.setHost(dbHost);
		mongoFactoryBean.setPort(dbPort);
		final MongoOptions mongoOptions = new MongoOptions();
		mongoOptions.autoConnectRetry = true;
		mongoOptions.j = true;
		mongoFactoryBean.setMongoOptions(mongoOptions);

		return mongoFactoryBean;
	}

	@Bean public MongoDbFactory mongoDbFactory(final Mongo mongoConnection) {
		return new SimpleMongoDbFactory(mongoConnection, dbName);
	}

	@Bean public MongoTemplate mongoTemplate(final MongoDbFactory mongoDbFactory) {
		return new MongoTemplate(mongoDbFactory);
	}
}

6. applicationContext.xml

<?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-3.2.xsd
        	http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.2.xsd">

	<!-- This is impossible in Spring JavaConfig because the list of filenames has a wildcard -->
	<context:property-placeholder location="classpath*:META-INF/common/*.properties,classpath*:META-INF/prod/*.properties,classpath*:META-INF/${env}/*.properties" ignore-unresolvable="true" />

	<!-- This file is not the primary entry point to Spring context configuration. 
  	   It is imported by AppConfig.java, which is loaded by web.xml -->
</beans>

I know databases connections can be made infinitely portable by removing them entirely from code and configuring them using a JNDI definition in the servlet container's XML. This is a more lightweight solution that is well-suited to simple parameters, such as "the SOAP client should not verify SSL certificates in development mode, because no development server is going to have a valid cert." Plus, who uses Mongo with JNDI, seriously.

Eclipse WTP settings:

1. Double-click Server instance
2. Click Open launch configuration
3. Go to Arguments tab
4. Append -Denv=dev to VM Arguments
5. OK
6. Save

What if a configuration item is not of String type, e.g. an enum?

For some context (prod) we need to set mongo db username/password, and for others dev/test, we do not need them. How to handle this case?