Determine FactoryBean object type via generics
For the particular use case detailed in SPR-8514, with this change we now attempt to determine the object type of a FactoryBean through its generic type parameter if possible. For (a contrived) example: @Configuration public MyConfig { @Bean public FactoryBean<String> fb() { return new StringFactoryBean("foo"); } } The implementation will now look at the <String> generic parameter instead of attempting to instantiate the FactoryBean in order to call its #getObjectType() method. This is important in order to avoid the autowiring lifecycle issues detailed in SPR-8514. For example, prior to this change, the following code would fail: @Configuration public MyConfig { @Autowired Foo foo; @Bean public FactoryBean<String> fb() { Assert.notNull(foo); return new StringFactoryBean("foo"); } } The reason for this failure is that in order to perform autowiring, the container must first determine the object type of all configured FactoryBeans. Clearly a chicken-and-egg issue, now fixed by this change. And lest this be thought of as an obscure bug, keep in mind the use case of our own JPA support: in order to configure and return a LocalContainerEntityManagerFactoryBean from a @Bean method, one will need access to a DataSource, etc -- resources that are likely to be @Autowired across @Configuration classes for modularity purposes. Note that while the examples above feature methods with return types dealing directly with the FactoryBean interface, of course the implementation deals with subclasses/subinterfaces of FactoryBean equally as well. See ConfigurationWithFactoryBeanAndAutowiringTests for complete examples. There is at least a slight risk here, in that the signature of a FactoryBean-returing @Bean method may advertise a generic type for the FactoryBean less specific than the actual object returned (or than advertised by #getObjectType for that matter). This could mean that an autowiring target may be missed, that we end up with a kind of autowiring 'false negative' where FactoryBeans are concerned. This is probably a less common scenario than the need to work with an autowired field within a FactoryBean-returning @Bean method, and also has a clear workaround of making the generic return type more specific. Issue: SPR-8514
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