I am new to springs and have the following queries on ApplicationContext.
1.What does it mean to declare two instances of classPathXmlApplicationContext on a single beans.xml file?
2.How is a beanPostProcessor programmatically associated with a single ApplicationContext?
1.What does it mean to declare two instances of classPathXmlApplicationContext on a single beans.xml file?
This would result in two different Spring application context unaware of each other. If there are any beans that are defined as singleton, each application context will now have its own instances of singleton beans, which means two bean instances one for each application context.
2.How is a beanPostProcessor programmatically associated with a single ApplicationContext?
To register a BeanPostProcessor you can add that to the spring configuration(xml/annotation) as a normal bean and spring will detect that automatically during the container startup and will invoke its callback methods during bean creation.
If you want to do this programatically, you can use BeanFactoryPostProcessor and addBeanPostProcessor method
Related
I need to scan new beans for the presence of an annotation when they are added to the application context. I need something like BeanPostProcessor from Spring.
I have read that there are several types of bean scopes in Spring framework when declaring them. However, I do not understand why we need several types. Can anyone explain what is the need of having Bean scopes in a Spring application with some examples?
Thank you in Advance!
Spring Inversion of Control Container (Ioc container) creates and manages the beans in a Spring Application.With Each declared Spring Bean, we can provide metadata which specifies that number of instances of particular bean should get created and how long they should live i.e. life time of the bean.
Basically with plain Java we our-self creates object. Position where we create object decides it's lifetime for example object created within method is destroyed as soon as method is returned. But in case of Spring, v creates bean for us and to manage the life cycle of the bean Spring uses scope of the bean. It also provide flexibility to the developer to override default scope which is "Singleton".
Now why do we need several types ?
Simply because each bean could have its own lifetime. Depending upon the lifetime it has scope defined as below
singleton (Default) Scopes a single bean definition to a single object instance per Spring IoC container.
prototype Scopes a single bean definition to any number of object instances.
request Scopes a single bean definition to the lifecycle of a single HTTP request; that is, each HTTP request has its own instance of a bean created off the back of a single bean definition. Only valid in the context of a web-aware Spring ApplicationContext.
session Scopes a single bean definition to the lifecycle of an HTTP Session. Only valid in the context of a web-aware Spring ApplicationContext.
application Scopes a single bean definition to the lifecycle of a ServletContext. Only valid in the context of a web-aware Spring ApplicationContext.
websocket Scopes a single bean definition to the lifecycle of a WebSocket. Only valid in the context of a web-aware Spring ApplicationContext.
For example :
Singleton : objects like controller, service, repository required only one instance to be existed in the application.
Prototype : objects like Student, Product which might need to be created everytime you wanted to create new resource.
Remaining are WebAware scopes and self explanatory.
Could anyone explain the difference between these two Spring bean scopes?
I'm familiar with the Singleton pattern.
Would this be the only difference?
You can have a list of beans in the Spring container using application scope.
Also, are you able to run multiple web servers in one Spring container? If yes, that would be a reason to use the application scope over the singleton scope since otherwise the bean would get shared over the two servers.
The documentation explains it:
This is somewhat similar to a Spring singleton bean but differs in two important ways: It is a singleton per ServletContext, not per Spring 'ApplicationContext' (or which there may be several in any given web application), and it is actually exposed and therefore visible as a ServletContext attribute
In application scope, the container creates one instance per web application runtime.
The application scope is almost similar to singleton scope. So, the difference is
Application scoped bean is singleton per ServletContext however singleton scoped bean is singleton per ApplicationContext. It means that there can be multiple application contexts for single application.
SINGLETON SCOPED BEAN
//load the spring configuration file
ClassPathXmlApplicationContext context =
new ClassPathXmlApplicationContext("context.xml");
// retrieve bean from spring container
MyBean myBean = context.getBean("myBean", MyBean.class);
MyBean myBean2 = context.getBean("myBean", MyBean.class);
// myBean == myBean2 - output is true.
I'm new to Spring and a little confused about how it works. I get that I can use the application context to instantiate beans and have them populated. However, is the idea that I should be able to just write Bean b = new Bean() and then have Spring to somehow automagically populate that Bean?
I'm experimenting with Spring in a web application, and as far as I can see I need to inject the ApplicationContext into, say, the servlets to be able to instantiate other beans (services, daos etc.) from there. It's a bit cumbersome, but probably works.
However, is Spring meant to be able to hook into any object instantiation which happens on classes defined as beans in applicationContext.xml?
Spring is an Inversion of Control container. A bean is an object whose life cycle is managed by Spring. If you want Spring to populate an object, it needs to go through Spring, ie. it needs to be bean.
is Spring meant to be able to hook into any object instantiation
which happens on classes defined as beans in applicationContext.xml?
Spring doesn't hook into anything. You configure your beans and the relationships between them with Spring and Spring handles creating the instances and linking them up.
For domain objects, Spring provides a solution via the #Configurable annotation: http://docs.spring.io/spring/docs/4.0.0.RELEASE/spring-framework-reference/htmlsingle/#aop-atconfigurable
It requires compile- or load-time-weaving and, thus, introduces some additional complexity but having the convenience of using the standard new Bean() syntax plus Spring's autowiring is worth it in my opinion.
Alternatively, you could define your domain objects as beans with prototype scope and use some factory to create them using the Spring ApplicationContext.getBean() method. With a scope of prototype a new instance will be returned every time and since you go through the ApplicationContext, Spring will do all the dependency injection magic as usual.
As for services and other beans with singleton scope, you would typically NOT retrieve them by first injecting the ApplicationContext and using it but instead you would inject them via either a constructor, setter or annotation-based strategy. The documentation covers that in detail: http://docs.spring.io/spring/docs/4.0.0.RELEASE/spring-framework-reference/htmlsingle/#beans-factory-collaborators
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.