java - Please explain the @Produces annotation in CDI -

i have read @produces annotation in cdi, don't understand usage.

public class resources {  // expose entity manager using resource producer pattern @suppresswarnings("unused") @persistencecontext @produces private entitymanager em;                                        //   @produces logger getlogger(injectionpoint ip) {                            //      string category = ip.getmember()                         .getdeclaringclass()                         .getname();     return logger.getlogger(category); }  @produces facescontext getfacescontext() {                                 //      return facescontext.getcurrentinstance(); } 

}

taken from: http://www.jboss.org/jdf/quickstarts/jboss-as-quickstart/guide/greeterquickstart/#greeterquickstart-

how container know call producer method? if inject entitymanager, how container call @produces entitymanager? , how getlogger producer method called?

i don't see reason go through of trouble.

section 3.3 of cdi specification gives pretty high level overview of use of @produces annotation:

a producer method acts source of objects injected, where:

• objects injected not required instances of beans, or
• concrete type of objects injected may vary @ runtime, or
• objects require custom initialization not performed bean constructor.

let's say, example, wanted bridge between java ee managed component entity manager , other cdi components, utilize @produces annotation. benefit being avoid having duplicate @persistencecontext annotations throughout data domain layer.

class {     @persistencecontext       // jpa annotation     @produces                 // cdi 'hook'     private entitymanager em;  }  class b {    @inject                    // can inject entity manager    private entitymanager em; } 

another handy use getting around libraries not have cdi friendly beans (for example, no default constructors):

class sometplclass {     public sometplclass(string id) {     } }  class sometplclassproducer {     @produces     public sometplclass getinstance() {         return new sometplclass("");     } } 

the javadoc produces shows interesting (but rare case) of producing named collection can later on injected other managed beans (very cool):

public class shop {      @produces @applicationscoped      @catalog @named("catalog")      private list<product> products = new linkedlist<product>(8);      //... }  public class orderprocessor {     @inject     @catalog     private list<product> products; } 

the container responsible processing methods , fields marked @produces annotation, , when application deployed. processed methods , fields used part of injection point resolution managed beans, needed.