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How to inject a scoped-proxy bean into a singleton bean by using constructor injection

Question

I'm developing a web application using Spring MVC, and want to have a request scoped Date bean which indicates when each request happens. To define such Date bean, I have written a following bean definition into application context xml.

<bean id="now"
      class="java.util.Date"
      scope="request">
    <aop:scoped-proxy/>
</bean>

Injecting this bean into a singleton bean by using field injection works fine.

public class ASingletonBean {
    @Autowired
    private Date now;
    ...
}

But I don't want to use field injection because it's not recommended. My IDE suggests to use constructor injection instead.

public class ASingletonBean{
    private final Date now;

    @Autowired
    public ASingletonBean(Date now) {
        this.now = now;
    }
}

Now the code above throws a following exception when the application launches.

org.springframework.beans.factory.BeanCreationException:
Error creating bean with name 'scopedTarget.java.util.Date#0':
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;
nested exception is java.lang.IllegalStateException:
No thread-bound request found: Are you referring to request attributes outside of an actual web request, or processing a request outside of the originally receiving thread?
If you are actually operating within a web request and still receive this message, your code is probably running outside of DispatcherServlet/DispatcherPortlet:
In this case, use RequestContextListener or RequestContextFilter to expose the current request.

How can I avoid this error?

Answer

Please, don't do that. Creating a new bean (you don't need) on every request is an overhead that can be easily avoided. Instead create a class + bean that implements java.util.function.Supplier for example:

@Component
class DateTimeProvider implements java.util.function.Supplier<Date> {
...
}

and then inject this java.util.function.Supplier<Date> into your `ASingletonBean'. This way, you will be able to get a current date/time when processing a request.

And few additional notes:

Use JodaTime or JDK8 JavaTime API instead of java.util.Date,
If you cannot use JDK8 (java.util.function.Supplier was added in JDK8) then you can either create your own interface or use one provided by Guava,
If you need a very precise timing for the received request then you should consider creating some sort of a "time stamping" filter, most likely by extending org.springframework.web.filter.OncePerRequestFilter class.

Edited - to answer a questions from comment:

"why it is preferable to using request-scoped bean" - A bean would always have to be injected to any business component you would create. Seems a bit much for just having a 'request timestamp'.
The request object you receive from the outside world should be translated into some sort of a 'domain request' (that contains the timestamp) and then handled internally only in its domain form. (read more on: Hexagonal Architecture a.k.a Ports and Adapters, Domain Driver Design). Why so? Because one can easily imagine that a request that now enters the system only by the means of HTTP request could enter the system in the form of JMS message or batch import operation - then you would only need to provide a new adapter to the system and entire logic inside core domain would not change.
If you are using Spring Boot then it is enough to create a bean that extends org.springframework.web.filter.OncePerRequestFilter#OncePerRequestFilter and implement your logic inside the only method that needs to be implemented. Spring will automatically use your filter.

cc by-sa 3.0