Spring generic bean factory

Question

Is there a mechanism in Spring which provides a way for automation of generic beans construction?

For, example if I have a class definition like:

class Foo<T> {
    private final T type;
    ...
}

and dependency like:

@Autowired
private Foo<String> foo;

I'd like to use some mechanism in Spring which provides, in some form, T from dependency definition (in above example, String) and provides a way for automatic instance creation?

Answer 1

By defining this in your Spring config :

@Bean
public Foo<String> foo() {
    return new Foo<>("bar");
}

Or instead of the value, you can specify the type :

@Bean
public Foo<String> foo() {
    return new Foo<>(String.class);
}

If you use Spring Boot, you can add @ConditionalOnProperty or @ConditionalOnBean to dynamize the bean instantiation.

(update) If you want to avoid declaration, you should extend DefaultListableBeanFactory and ApplicationContext . Here is a working example with Spring Boot :

@Controller
@EnableAutoConfiguration
public class BeanFactoryTest {
    public static void main(String[] args) throws Exception {
        SpringApplication app = new SpringApplication(BeanFactoryTest.class);
        app.setApplicationContextClass(CustomAppContext.class);
        app.run(args);
    }

    @Autowired
    private Foo<String> foo1;

    @Autowired
    private Foo<String> foo2;

    @Autowired
    private Foo<Integer> foo3;

    @PostConstruct
    public void initialize() {
        System.out.println(foo1); // prints BeanFactoryTest$Foo@344b8190
        System.out.println(foo2); // prints BeanFactoryTest$Foo@344b8190
        System.out.println(foo3); // prints BeanFactoryTest$Foo@5b69d40d
    }

    public static class CustomAppContext extends AnnotationConfigApplicationContext {
        public CustomAppContext() {
            super(new CustomBeanFactory());
        }
    }

    public static class CustomBeanFactory extends DefaultListableBeanFactory {
        @Override
        protected Map<String, Object> findAutowireCandidates(String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {
            Map<String, Object> map = super.findAutowireCandidates(beanName, requiredType, descriptor);

            if (Foo.class.isAssignableFrom(requiredType)) {
                ResolvableType type = ResolvableType.forField(descriptor.getField());
                ResolvableType genericType = type.getGeneric(0);
                Class<?> genericTypeRaw = genericType.getRawClass();

                boolean hasInstance =
                        map.values()
                            .parallelStream()
                            .map(Foo.class::cast)
                            .map(Foo::getType)
                            .filter(genericTypeRaw::isAssignableFrom)
                            .findFirst()
                            .isPresent();

                if (!hasInstance) {
                    super.registerResolvableDependency(requiredType, new Foo<>(genericTypeRaw));

                    map = super.findAutowireCandidates(beanName, requiredType, descriptor);
                }
            }

            return map;
        }
    }

    public static class Foo<T> {
        private final Class<T> type;

        public Foo(Class<T> type) {
            this.type = type;
        }

        public Class<T> getType() {
            return type;
        }
    }
}

Answer 2

It seems you can use the ResolvableType in Spring 4. http://docs.spring.io/spring/docs/4.0.0.RC2/javadoc-api/org/springframework/core/ResolvableType.html There is also a detail sample https://spring.io/blog/2013/12/03/spring-framework-4-0-and-java-generics

Hope this help.

Answer 3

The question is a little old, but doesn't contain an answer with example, so...

Using ResolvableType :

public <E> Foo<E> produceFoo(final InjectionPoint ip) {
    ResolvableType resolved = ResolvableType.forField(ip.getField());
    Foo<E> fooInstance = new FooImpl<>();

    Class<E> parameterClass = (Class<E>) resolved.getGeneric(0).resolve();
    fooInstance.doSomethingWithParametrized(parameterClass);

    return fooInstance;
}

InjectionPoint cames from: import org.springframework.beans.factory.InjectionPoint;