#Autowired Spring #Component with #ConditionalOnProperty - spring

Question

Can I use #Autowired Spring 4.x #Components with a #ConditionalOnProperty to choose implementations of a Feature based on a featuretoggles.properties file?
public class Controller {
#Autowired
private Feature feature;
}
#Component
#ConditionalOnProperty(name = "b", havingValue = "off")
public class A implements Feature {
}
#Component
#ConditionalOnProperty(name = "b", havingValue = "on")
public class B implements Feature {
}
#Configuration
#PropertySource("classpath:featuretoggles.properties")
public class SomeRandomConfig {
}
With a src/main/resources/featuretoggles.properties file:
b = on
(That the name of the toggle "b" and the name of the class "B" match is coincidence; it's not my aim to have these equal, the toggle could have any name.)
This fails to auto-wire feature in the Controller with an UnsatisfiedDependencyException, saying "No qualifying bean of type 'Feature' available: expected at least 1 bean that qualifies as autowire candidate".
I know I can realize this with a #Configuration class that chooses a #Bean depending on the property. But when I do that I have to add a new Configuration class each time I add a feature toggle, and those Configuration classes will be highly similar:
#Configuration
#PropertySource("classpath:featuretoggles.properties")
public class FeatureConfig {
#Bean
#ConditionalOnProperty(name = "b", havingValue = "on")
public Feature useB() {
return new B();
}
#Bean
#ConditionalOnProperty(name = "b", havingValue = "off")
public Feature useA() {
return new A();
}
}

Answer 1

I did what you're trying to do by following this guide. First step was to write a Condition...
public class OnEnvironmentPropertyCondition implements Condition
{
#Override
public boolean matches(ConditionContext ctx, AnnotatedTypeMetadata meta)
{
Environment env = ctx.getEnvironment();
Map<String, Object> attr = meta.getAnnotationAttributes(
ConditionalOnEnvProperty.class.getName());
boolean shouldPropExist = (Boolean)attr.get("exists");
String prop = (String)attr.get("value");
boolean doesPropExist = env.getProperty(prop) != null;
// doesPropExist shouldPropExist result
// true true true
// true false false
// false true false
// true false true
return doesPropExist == shouldPropExist;
}
}
...then an annotation using that condition.
/*
* Condition returns true if myprop exists:
* #ConditionalOnEnvProperty("myprop")
*
* Condition returns true if myprop does not exist
* #ConditionalOnEnvProperty(value="myprop", exists=false)
*/
#Target({ElementType.TYPE, ElementType.METHOD})
#Retention(RetentionPolicy.RUNTIME)
#Conditional(OnEnvironmentPropertyCondition.class)
public #interface ConditionalOnEnvProperty
{
public String value();
public boolean exists() default true;
}
You can add featuretoggles.properties to the environment with the #PropertySource annotation.