How to inject objects of the same class with different scopes?

问题

In terms of simplicity and correctness, what is the best way to inject objects of the same class with different scopes?

In a servlet I want to have injected objects of the same class with different scopes. Still don't know if going to use jsf.

• Simplicity: Making a Qualifier and a producer method for each scope is too much; making an interface, two classes and adding and alternative in beans.xml is also too much; having an Address#isCurrent() method doesn't make sense.
• Correctness: JSR299, 3.11 says: The use of @Named as an injection point qualifier is not recommended. Still don't know why.
  Though using @Named at injection point works with @ApplicationScoped and @RequestScoped but not with @SessionScoped. See named snippet below.

In spring it is very easy:
Spring snippet

<bean id="currentAddress" class="xxx.Address" scope="session" />
<bean id="newAddress" class="xxx.Address" scope="request" />
<bean id="servlet" class="xxx.MyServlet">
 <property name="currentAddress" ref="currentAddress" />
 <property name="newAddress" ref="newAddress" />
</bean>

named snippet

/* Address class */
@Produces @RequestScoped @Named(value="request")
 public Address getNewAddress(){
 return new Address();
}

@Produces @SessionScoped @Named(value="application")
 public Address getCurrentAddress(){
 return new Address();
}
/* Servlet */
@Inject @RequestScoped @Named("request")  private Address newAddress;
@Inject @ApplicationScoped @Named("application") private Address currentAddress;

回答1:

The reason for the inclusion of this recommendation is the same reason one would prefer Enumeration over arbitrary strings for constants and that is because its not type-safe. You can easily mistype the name of the class and it would compile fine and fail at runtime. The recommendation is included because in most cases @named makes your application unnecessarily fragile when you have the ability force these constraints at compile time.

Here is a good article that outlines the reasons:

The preferred way to handle this situation is using @Qualifiers with enumerated values. Checkout the section entitled "String Qualifiers are Legacy" and "The Right Way" for the steps to handle this.

回答2:

Thanks to @nsfyn55 for pointing out that good article, after reading the section "The Right Way", I came up with what I think is the best way to achieve it in terms of simplicity and correctness.

So I am using only one interface for the qualifier annotation.

/* Qualifier annotation */
@Qualifier
@Retention(RUNTIME)
@Target({FIELD,METHOD})
public @interface Scope {

 Type value();

 enum Type { REQUEST, SESSION, APPLICATION };
}


/* Address class */
@Produces @Scope(REQUEST) @RequestScoped
 public Address request() {
 return new Address();
}

@Produces @Scope(SESSION) @SessionScoped
 public Address session() {
 return new Address();
}

/* Servlet */
@Inject @Scope(REQUEST)
private Address newAddress;

@Inject @Scope(SESSION)
private Address currentAddress;

来源：https://stackoverflow.com/questions/10373617/how-to-inject-objects-of-the-same-class-with-different-scopes