How do I post-process beans of @Configuration classes that define more @Beans in JavaConfig?

Question

In Spring XML, I can define a bean that instantiates a class annotated with @Configuration. When I do, that bean is post-processed. Any methods inside that class with @Bean are also added to the container. How do I perform a similar post-processing in JavaConfig?

Here's the XML version:

<bean id="test" class="com.so.Test">
    <property name="prop" value="set before instantiating @Beans defined in Test"/>
</bean>

The associated Test class:

@Configuration
class Test {
    private String prop;

    void setProp(final String prop) {
        this.prop = prop;
    }

    @Bean
    NeedThisBean needThisBeanToo() {
        return new NeedThisBean(prop);
    }
}

If I use Spring XML Config, both test and needThisBeanToo are available in the container. needThisBeanToo is added via a BeanPostProcessor, though I can't recall which one. If I use JavaConfig, only test is available in the container. How do I make needThisBeanToo available to the container? @Import would work, except that prop being set is required for needThisBeanToo to be initialized correctly.

The part that makes all of this complicated is that Test is vended from a library I'm consuming. I don't control Test, nor can I change it. If I drive it from JavaConfig, it would look like this:

@Configuration
class MyConfiguration
{
    @Bean
    Test test() {
        Test test = new Test();
        test.setProp("needed to init `needThisBeanToo` and others");
        return test;
    }
}

The JavaConfig example does not instantiate needThisBeanToo despite it being defined in Test. I need to get needThisBeanToo defined, preferably without doing it myself, since I don't want to copy code I don't own. Delegation isn't attractive, since there are a number of subsequent annotations/scopes defined on needThisBeanToo (and others defined inside Test).

Answer 1

Your problem is is that you're ignoring the @Configuration annotation completely. Why is that?

When code reaches this line Test test = new Test(); it just doesn't do anything with @Configuration. Why? Because annotation is not something that a constructor is aware of. Annotation only marks some meta-data for the class. When spring loads classes it searches for annotations, when you call a constructor of a class, you don't. So the @Configuration is just ignored because you instantiate Test with new Test() and not through spring.

What you need to do is to import Test as a spring bean. Either via XML as you showed in your question OR using @Import. You problem with prop is that the setter isn't called because that's just not the way to do it. What you need to be doing is either do something like that:

@Configuration
class Test {
  private String prop = "set before instantiating @Beans defined in Test";

  @Bean
  NeedThisBean needThisBeanToo() {
      return new NeedThisBean(prop);
  }
}

Or to create a property in spring (this is a different subject) and inject the value:

@Configuration
class Test {
  @Autowired
  @Value("${some.property.to.inject}") // You can also use SPeL syntax with #{... expression ...}
  private String prop;

  @Bean
  NeedThisBean needThisBeanToo() {
      return new NeedThisBean(prop);
  }
}

You can also create a bean of type String and inject it as follows:

@Configuration
class Test {
  @Autowired
  @Qualifer("nameOfBeanToInject")
  private String prop;

  @Bean
  NeedThisBean needThisBeanToo() {
      return new NeedThisBean(prop);
  }
}

In the last case you can define your original MyConfiguration with this bean:

@Configuration
@Import(Test.class)
class MyConfiguration
{
    @Bean(name = "nameOfBeanToInject")
    String test() {
        return "needed to init `needThisBeanToo` and others";
    }
}

In any case you have to import Test either using @Import or as a normal XML bean. It won't work by calling the constructor explicitly.

Answer 2

Here's a way to handle vended @Configuration classes that require some properties to be set prior to creating their @Beans:

Vended @Configuration class:

@Configuration
class Test {

    private String property;

    public setProperty(final String property) {
        this.property = property;
    }

    @Bean
    PropertyUser propertyUser() {
        return new PropertyUser(property);
    }

    @Bean
    SomeBean someBean() {
        // other instantiation logic
        return new SomeBeanImpl();
    }
}

Here's the consuming @Configuration class:

@Configuration
class MyConfig {

    @Bean
    static String myProperty() {
        // Create myProperty
    }

    /**
     * Extending Test allows Spring JavaConfig to create
     * the beans provided by Test.  Declaring
     * Test as a @Bean does not provide the @Beans defined
     * within it.
     */
    @Configuration
    static class ModifiedTest extends Test {

        ModifiedTest() {
            this.setProperty(myProperty());
        }

        @Override
        @Bean
        SomeBean someBean() {
            return new SomeBeanCustomImpl(this.propertyUser());
    }
}