#autowired field and also #autowired constructor - spring

Question

I have a spring MVC application. I have a bean which is wired as a singleton and also a field which is also autowired but is of request scope.
For example:
class Hello {
#Autowired
BDepend b; // this is defined as a request scope bean
#Autowired
public Hello(ADepend a){
}
}
Here my class Hello is a singleton but BDepend is a request scope object.
How does spring handle right instance of Bdepend for each of the request since Hello is a singleton.

Answer 1

By default you will have runtime exception at application sturtup. Something like this:
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'b': Scope 'request' is not active for the current thread; consider defining a scoped proxy for this bean if you intend to refer to it from a singleton;
To use request scope objects inside of some singleton you need to obtain a fresh instace each time from application context. You can do it:
using application context directly
class Hello {
#Autowired
private ApplicationContext ctx;
#Autowired
public Hello(ADepend a){
}
public useB(){
BDepend instance = ctx.getBean(BDepend.class);
instance.doSomething();
}
}
or get a fresh instance via javax.inject.Provider.
class Hello {
#Autowired
private Provider<BDepend> provider;
#Autowired
public Hello(ADepend a){
}
public useB(){
BDepend instance = this.provider.get();
instance.doSomething();
}
}
or declare scoped proxy for this bean:
#Component
#Scope(value = "request", proxyMode = ScopedProxyMode.TARGET_CLASS)
class BDepend {
}
class Hello {
#Autowired
private BDepend b;
#Autowired
public Hello(ADepend a){
}
public useB(){
b.doSomething();
}
}
or use scoped proxies system wide
<context:component-scan base-package="com.mycompany.beans" scoped-proxy="targetClass" />
Personally I prefer second option (javax.inject.Provider): it is clear from the code that your bean has different scope.