feature flags made easy

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"
  xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

  <bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
    <property name="systemPropertiesModeName" value="SYSTEM_PROPERTIES_MODE_OVERRIDE"/>
    <property name="properties">
      <map>
        <entry key="imaginaryFeature.implementation.bean" value="oldImaginaryFeature"/>
      </map>
    </property>  
  </bean>

  <bean id="application" class="com.eranharel.SpringPolymorphicApplication">
    <constructor-arg ref="imaginaryFeature"/>
  </bean>
  
  <alias name="${imaginaryFeature.implementation.bean}" alias="imaginaryFeature"/>
  
  <bean id="newImaginaryFeature" class="com.eranharel.NewImaginaryFeature" lazy-init="true"/>
  
  <bean id="oldImaginaryFeature" class="com.eranharel.OldImaginaryFeature" lazy-init="true"/>
</beans>

IfElseApplication.java

  public void runApplication() {

    // ...
    if (useNewImplementation) {
      executeNewImaginaryFeatureImplementation();
    } else {
      executeOldImaginaryFeatureImplementation();
    }
    // ...
  }

ImaginaryFeature.java

public interface ImaginaryFeature {
  public void executeFeature();
}

class OldImaginaryFeature implements ImaginaryFeature {
  @Override
  public void executeFeature() {
    System.out.println("old feature implementation");
  }
}

class NewImaginaryFeature implements ImaginaryFeature {
  @Override
  public void executeFeature() {
    System.out.println("new feature implementation");
  }
}

PolymorphicApplication.java

public class PolymorphicApplication {
  
  private final ImaginaryFeature imaginaryFeature;
  
  public PolymorphicApplication() {
    this.imaginaryFeature = createImaginaryFeature();
  }

  private ImaginaryFeature createImaginaryFeature() {
    final String featureClass = System.getProperty("PolymorphicApplication.imaginaryFeature.class");
    try {
      return (ImaginaryFeature) Class.forName(featureClass).newInstance();
    } catch (final Exception e) {
      throw new IllegalStateException("Failed to create ImaginaryFeature of class " + featureClass, e);
    }
  }

  public void runApplication() {

    // ...
    imaginaryFeature.executeFeature();
    // ...
  }
}

SpringPolymorphicApplication.java

public class SpringPolymorphicApplication {

  private final ImaginaryFeature imaginaryFeature;

  public SpringPolymorphicApplication(final ImaginaryFeature imaginaryFeature) {
    this.imaginaryFeature = imaginaryFeature;
  }

  public void runApplication() {

    // ...
    imaginaryFeature.executeFeature();
    // ...
  }
}