If we create beans of two class one having singleton scope and other having prototype scope. If singleton bean refers to prototype bean how many instances will be created? What will happen internally?
Spring was born as IoC (Inversion Of Control) framework. From documentation:
IoC is also known as dependency injection (DI). It is a process
whereby objects define their dependencies, that is, the other objects
they work with, only through constructor arguments, arguments to a
factory method, or properties that are set on the object instance
after it is constructed or returned from a factory method. The
container then injects those dependencies when it creates the bean.
This process is fundamentally the inverse, hence the name Inversion of
Control (IoC), of the bean itself controlling the instantiation or
location of its dependencies by using a direct construction of
classes, or a mechanism such as the Service Locator pattern.
In your case: if A is singleton object that referrers a B object scoped prototyped, B will be instantiated once, because A will invoke it only a time (during its creation).
Related
The instances passed to my producer does represent effective beans?
#Qualif1
#Prodcues
B qualif1(#Any Instance<B> instances){
return instances.select(B1.class).get(); // Select instance of B1 which extends B
}
I am wondering if instances are already beans or only the selected one will be a bean managed by the container?
Thanks in advance
This does not depend on using instances, but on the scopes of the beans. A call to get works like any other injection point: if you get a prototype bean, a new instance of the bean gets created. If you get an eager singleton - it is already created before you call get.
It seems like your question stems from mixing up beans and instances of beans: the former are definitions (and are all, in a sense "managed" by the container), the latter are instances (which are in the direct sense "managed").
The ladder of abstraction goes like this:
class -> bean -> instance
So the "bean" is neither a class (although in some projects every bean is also a class) nor an instance (although in some projects every bean is also an instance).
Personally, I blame Spring for blurring the boundaries between the three, by encouraging usage of eager singletons for practically everything.
I'm studying spring beans and came across #Lookup, it says:
If we happen to decide to have a prototype Spring bean, then we are
almost immediately faced with the problem of how will our singleton
Spring beans access these prototype Spring beans?
hmm, I don't get it, because when I studied scope=prototype it says:
4.4.2 The prototype scope
The non-singleton, prototype scope of bean deployment results in the creation of a new bean instance every time a
request for that specific bean is made
so it seems i misinterpreted the words:
a request for that specific bean is made
actually programming in spring framework every line of the code is inside of some bean (i.e. #controller, #Service, etc), isn't it?
And almost all of them are singletons, isn't it?
So if I need prototype I just make scope=prototype and almost everytime it's injected to another bean (i.e. #controller, #Service, etc) isn't it?
So please give a real world scenarios, 1) when one should use #Lookup and 2) when it's not needed
Ok for the 1) the scenario:
#Component
#Scope("prototype")
public class SchoolNotification {
// ... prototype-scoped state
}
#Component
public class StudentServices {
// ... member variables, etc.
#Lookup
public SchoolNotification getNotification() {
return null;
}
// ... getters and setters
}
Please, show me scenario for the 2) case, and explain please the difference
Thank u
The implicit Bean scope in Spring is Singleton.
That means for a JVM instance, only a single instance of a Bean exists in memory (theoretically).
When you #Autowire a Prototype-scoped Bean inside a Singleton-scoped Bean, that Prototype one becomes a sort-of-singleton. Just think about it; a Singleton gets created, its injectable fields get Autowired, and that is it, the instance lives forever along with all its fields (keep in mind those Prototype-scoped fields are "pure" instances, they're not proxied).
#Lookup
is a proxy-driven annotation. What that means is Spring will extend your class using JDK proxies or CGLIB proxies, and it will override/implement the #Lookup-annotated method, providing its own version which uses a BeanFactory#getBean each time it is invoked.
The documentation is clear on this point
An annotation that indicates 'lookup' methods, to be overridden by the
container to redirect them back to the BeanFactory for a getBean call.
Thus, that means a fresh Bean instance is returned every time.
Just for your knowledge, another approach for working with Prototype-scoped Beans inside "other"-scoped Beans is using ProxyFactoryBean. The only difference is that the proxy is created at configuration-time, and then made available for direct #Autowireing, thus not requiring the definition of a #Lookup method, which sometimes is not wanted (usually by folks that are obsessed with clean code, like me).
Need clarification on spring singleton .I have three objects ,these are readonly so never changed values.
I am planning to create these objects with singleton bean as properties.
My question: When spring creates these objects,everytime it will create new object when bean calls or once in life time it will call and create those objects?. What are the possible threading issues if any?
It depends on what you say to Spring to do. Spring has an Inversion Of Control container that manages the instances of the objects. Depending on the scope you give to your objects, it will make them singletons or not.
More info:
http://static.springsource.org/spring/docs/3.0.0.M3/reference/html/ch04s04.html
I have a class as follows.
public class MyClass {
public void doSomething(){
//B b = //some how get new instance of B each time when doSomething is called
//do it now
}
}
Does spring provides something has something inbuilt like this?
OR i will have to create a factory myself which will create and return B each time get method of factory is called?
This is possible, and documented in the manual:
In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean, or a non-singleton bean needs to collaborate with another non-singleton bean, you typically handle the dependency by defining one bean as a property of the other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use non-singleton (prototype) bean B, perhaps on each method invocation on A. The container only creates the singleton bean A once, and thus only gets one opportunity to set the properties. The container cannot provide bean A with a new instance of bean B every time one is needed.
A solution is to forego some inversion of control. You can make bean A aware of the container by implementing the ApplicationContextAware interface, and by making a getBean("B") call to the container ask for (a typically new) bean B instance every time bean A needs it.
[...]
Lookup method injection is the ability of the container to override methods on container managed beans, to return the lookup result for another named bean in the container. The lookup typically involves a prototype bean as in the scenario described in the preceding section. The Spring Framework implements this method injection by using bytecode generation from the CGLIB library to generate dynamically a subclass that overrides the method.
I have a stateless session bean, but I want to add a reference to another bean in order to do some process. So if I add reference to another bean it shows up at instance level variable like this,
#Stateless
public class AccountFacade extends AbstractFacade<Account> implements AccountFacadeRemote {
#EJB
private BillerFacadeRemote billerFacade;
public AccountFacade() {
super(Account.class);
}
... all other methods...
}
Question
Now, by definition stateless bean should not have any instance level variables. So I am quite confuse about where to put this private BillerFacadeRemote billerFacade;?
Your code is fine.
The #EJB annotation is injecting the bean into your class, and the application server manages its lifetime.
I recommend reading, or skimming the pretty long Java EE tutorial.
"The EJB container typically creates and maintains a pool of stateless session beans, beginning the stateless session bean’s lifecycle. The container performs any dependency injection and then invokes the method annotated #PostConstruct, if it exists. The bean is now ready to have its business methods invoked by a client."
It's okay for a stateless beans to have instance variables that represent dependencies.
In fact, this is even encouraged. Without instance variables you could in many situations just use static method in a utility class instead.
What however is NOT encouraged, is have instance variables representing client observeable state. That is wrong, but dependencies like the entity manager, jms queues, JDBC connections, and thus other services that your stateless service delegates (part of) its work to, is absolutely okay.
Notice that the injections are true instance injections in instance variables. It are not class level (static) injections.