Spring 3: Inject Default Bean Unless Another Bean Present

Question

I would like to configure Spring via XML such that if a particular bean exists, it will be injected into the target bean. If it does not exist, a different, default bean, will be injected.

For example if I have a file like this

<bean id="carDriver" class="Driver">
  <property name="car" value="SOME EXPRESSION GOES HERE, SEE ATTEMPT BELOW"/>
</bean>

<bead id="defaultCar" class="Car">
  <property name="name" value="Honda Accord"/>
</bean>

And load it, I would like the defaultCar injected into the driver. However, if I also load the following file:

<bean id="customCar" class="FlyingCar">
  <property name="name" value="Rocket Car"/>
  <property name="maxAltitude" value="80000"/>
</bean>

I would want the customCar bean to be used instead of the defaultCar bean. My initial attempt does not work, but I think illustrates what I'm trying to achieve:

<bean id="carDriver" class="Driver">
  <property name="car" value="#{ @customCar eq null ? 'defaultCar' : 'customCar' }"/>
</bean>

I know how to do this with a PropertyPlaceholderConfigurer, but I don't want to have to provide a property file / VM property / environment variable / etc. in addition to the file that contains the custom bean. Thanks!

---

Update:

Based on the "use a factory bean" comments, I looked into this and came up with the following solution. First, I created a generic factory bean that allows you to specify a default bean name and an override bean name:

public class DefaultOverrideFactoryBean implements FactoryBean, BeanFactoryAware {

    public Object getObject() throws Exception {
        return beanFactory.containsBean(overrideBeanName) ?
               beanFactory.getBean(overrideBeanName)      :
               beanFactory.getBean(defaultBeanName);
    }

    public Class<?> getObjectType() {
        return Object.class;
    }

    public boolean isSingleton() {
        return true;
    }

    public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
        this.beanFactory = beanFactory;
    }

    public void setDefaultBeanName(String defaultBeanName) {
        this.defaultBeanName = defaultBeanName;
    }

    public void setOverrideBeanName(String overrideBeanName) {
        this.overrideBeanName = overrideBeanName;
    }

    private String defaultBeanName;
    private String overrideBeanName;
    private BeanFactory beanFactory;
}

To configure my example car driver, you would do this:

<bean id="carDriver" class="Driver">
  <property name="car">
    <bean class="DefaultOverrideFactoryBean">
      <property name="defaultBeanName" value="defaultCar"/>
      <property name="overrideBeanName" value="customCar"/>
    </bean>
  </property>
</bean>

I would have preferred to use SpEL, but this works. Perhaps adding a custom schema element woud make this cleaner.

Additional comments appreciated.

Answer 1

You may used @Qualifier to choose one version of Car (custom or default), but you shall know the specific name of what you gonna use, and you may want to use just:

 @Autowired
 private Car car;

You may also use @Primary to solve this, but it just gives a preference to avoid ambiguity and it will be created the unwanted versions. So i would recomend to use the annotation

org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean

So you will only instantate one bean if another is not created. Its specially usefull when the beans are declared in differents modules.

//Core module creates a default Car
@Bean()
@ConditionalOnMissingBean(Car.class)
Car car()
{
  return new DefaultCar();
}

and

//Car module creates the wanted prototype car
@Bean()
Car car()
{
  return new Toyota();
}

Answer 2

Using FactoryBean is the simplest solution - you can describe any algorithm you want. More information is at

http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/beans/factory/FactoryBean.html

http://docs.spring.io/spring/docs/current/spring-framework-reference/html/beans.html#beans-factory-extension-factorybean