Singleton beans by design pattern should be of single instance.
Hence, if a thread's execution is on the way of execution blocking a series of beans (say #Controller then #Service then simple beans) one after another
What will happen if many other simultaneous threads having different functionality but using the same series of beans runs - I mean any conflict situation with data/value may arise or Spring Singleton is thread safe ?
How to resolve the same if not ?
Controllers, as singleton, aren't thread safety.
I think your singleton beans should be stateless.
But you can inject other request or session scoped beans using proxyMode, for instance http://healthycoder.in/session-scoped-bean-in-a-controller-spring-mvc/
Related
I'm having 1 #Controller bean in my project
and 2 #Service beans that this controller calls.
the services using 2 different #Repository beans for persisting.
My question is :
my server is about to get hundreds of calls simultaniously, isnt it "awkward" that all my beans i described above are of scope="singleton"? should i use "prototype" instead? or maybe spring does it automatically ?
I think in both cases you wil have the same number of objects.
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 (that is, it is injected into another bean or it is
requested via a programmatic getBean() method call on the
container). As a rule of thumb, you should use the prototype scope
for all beans that are stateful, while the singleton scope should be
used for stateless beans.
in Spring by default all beans it is "singleton" and should be in most cases.
Could someone give an overview or a summary of what the purpose of beans in a Spring framework context?
I understand the standard Java bean (no arg constructor, getters/setters, often serialized), but the Spring bean purpose seems to be different.
Is it a way of implementing the Singleton design pattern (one instance, for like factory classes) in a simple, reusable fashion?
I've mainly used Spring with annotations, but I feel I need to grasp this in order to understand Spring.
Thanks!
Beans are objects that form the backbone of the application.
A bean is simply an object that is instantiated, assembled and otherwise managed by a Spring IoC container; other than that, there is nothing special about a bean.It is in all other respects one of probably many objects in your application.
Spring beans are defined in a spring configuration file or by using annotations, instantiated by the Spring container, and then injected into your application.
Spring beans will not be singleton design pattern until you explicitly make them to be.The singleton design pattern and the spring scope 'singleton' are different things.You can define different bean scopes depending on your requirements.
The scopes could be :
singleton – Return a single bean instance per Spring IoC container
prototype – Return a new bean instance each time when requested
request – Return a single bean instance per HTTP request.
session – Return a single bean instance per HTTP session.
globalSession – Return a single bean instance per global HTTP
session.
The default scope is singleton.
I understand the standard Java bean (no arg constructor,
getters/setters, often serialized), but the Spring bean purpose seems
to be different.
You mean always serialized. Why do you think the purpose seems different?
In the end, you write classes. A lot of time these are POJOs, Plain Old Java Objects. Sometimes you implement an interface or extend a class, but its all just classes.
Beans are just classes. Don't overcomplicate it.
Now Spring might take your beans (classes) and manage them for you via any of a number of policies (prototype, singleton) but that doesn't change what a bean is, it speaks to how Spring manages the bean.
To understand best, you should get familiar with dependency injection. In a few words dependency injection allows you to use objects, or services without explicitly creating them (of course, it gives other benefits, but let's focus on the question). This is achieved by maintaining a dependency container that is - roughly said - a collection of beans.
A bean is a service/component you use in your application. Unlike the EJB, with Spring the bean is not constrained to constructor arguments or specific annotations (especially if you use xml contexts). You register a bean with a container (by defining a context), and when you require it, the container will provide you with an instance of that bean. In order to create the bean, the container examines its class and constructors, and uses any other registered beans within that context, to call the appropriate constructor or property setter.
You can configure a bean to be a singleton - this is not a singleton as in the design pattern term. Singleton beans are created once within the container, and the same instance is used whenever the bean is requested from that container. You can also use the prototype scope to force the container to create a new instance each time.
Spring beans have 2 types of scopes (if you do not take into account scopes for web): singleton, which is default and prototype. Roughly they implement singleton and prototype design patterns within context.
So if the bean has prototype scope it can hold a unique state like the stateful bean in EJB. When the scope of the bean is singleton the container will create only one instance of it. So we can say that singleton beans in EJB 3.1 is the analogue of singleton beans in spring.
But how I can get the features of stateless beans in Spring (I'm referring to pooling of stateless beans in EJB containers and about that each thread has a unique instance of the stateless bean)?
Either you can make the bean thread-safe, and a singleton bean is OK (that's the majority of the cases).
Or you can't, and you'll have to use a prototype. The difference I see between Spring prototype beans and stateless EJB session beans is that stateless session beans are pooled. But in these ages, pooling them or creating a new instance each time won't make much difference. Creating a new instance each time might even help the GC.
I am trying to better understand Spring instantiation of beans. To illustrate my doubts, let's assume we have a Service class being wired in a Controller, here are the questions:
How will Spring manage the lifecycle
of the Controller? Will a new object
be created per request?
Once a Service is instantiaded and
wired to a Controller, will Spring
re-use that object reference to wire
it in to other beans?
Like Servlets, Controllers' lifecycle spans beyond requests. All of controllers in the application are instantiated only once when application is started; afterwards those objects are re-used to service all requests.
As Bozho pointed out, by default all beans are in singleton scope, therefore they will be re-used everywhere, unless specified otherwise.
The default scope is singleton, which means beans will be re-used (i.e. 1) no, a new object will not be created per request, and 2) yes, the object reference will be reused).
This can all be configured. Have a look at http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#beans-factory-scopes.
It all depends on the bean scope. By default all beans are in singleton scope - that is, they are instantiated by the container only once.
If you specify #Scope("request") (or the xml equivalent) then the same service object (singleton) will be injected in all instances of the request-scoped controller. (But you rarely need request-scoped controllers)
I am reading spring through its official documentation and at one place I came to a line that uses prototype scope for all stateful beans while singleton for stateless beans.
I know there is something as stateful as well as stateless beans in EJB but this is not what they have mentioned in the documents.
Can anyone explain to me what exactly this means of stateful as well stateless beans in Spring
Thanks in advance
From spring perspective
stateless beans: beans that are singleton and are initialized only once. The only state they have is a shared state. These beans are created while the ApplicationContext is being initialized. The SAME bean instance will be returned/injected during the lifetime of this ApplicationContext.
stateful beans: beans that can carry state (instance variables). These are created EVERY time an object is required (like using the "new" operator in java).
These are not EJB statfull/stateless session beans.
There is no point in storing specific information like client data, per request data in Singleton bean as they are created only once by Spring IOC container. That's why singleton beans are stateless. They are shared resources. They can only be used for storing global information.
When a request is made for creating a prototype bean, a new request is created every time. So, they can be used to store some specific information for each request. So they are stateful.
It totally depends on the implementation. See tomee for example http://tomee.apache.org/statelesscontainer-config.html . You'll have to check in your server docs