We autowire class in spring framework so that we can call the methods of autowired class to performe some action. My question is when spring creates object of that class internally.
The Spring Context is initialized directly when you start your application by the Spring Framework.
THen maybe you can refine your question if you want to ask something specific about ordering or availability of your beans.
Related
I know what is application Context. It is an interface which provides spring beans.
All beans get initialized in Spring IOC containers.
But How all three are connected. Could you please explain.
Spring beans are the instances of classes that spring manages. You create classes and "mark" them to be spring managed (putting #Component, using #Bean in java configuration and so forth - there are many ways to tell spring that the instance of some particular class should be managed by spring)
When spring starts it creates an application context with is a registry of all beans it resolves.
Spring can inject one bean into another and its the way of spring to instantiate beans.
The principle of "providing dependencies by external container" as opposed to maintaining dependencies by class itself called Inversion of control, and spring implements this concept.
Update 1
There is no such a thing as "spring context" technically speaking
Spring IOC container is a framework that manages the beans (your classes) by means of providing a technical abstraction called application context (its a real interface in java with implementation inside the spring code).
In order to benefit from spring your Beans should have dependencies between them. In this case spring framework that implements IOC principle can "inject" (provide, resolve) dependencies between beans.
Here is an example:
#Component
class A {
}
#Component
class B {
#Autowired
private A a;
}
When spring container instantiates class B (creates the object : new B()) it "understands" that this instance (we call it bean because it's managed by spring) has a "dependency" on class A and since A is also managed by spring, it can "inject" (read put a value) into the property a of class B.
This is called an Inversion of Control. You, as a programmer do not have to instantiate property b by yourself, spring does it for you.
In my sample spring boot application, i have added a dependency of a custom jar. My sample application has a support for web and jpa.
The jar which i've created contains a Spring MVC controller. Below is the sample code
#Controller
public class StartStopDefaultMessageListenerContainerController {
#Autowired(required=false)
private Map<String, DefaultMessageListenerContainer> messageListeners;
I haven't manually created a bean instance of this controller anywhere in my code.
Problem - When i start my spring boot application by running the main class, i get an error in console that prob while autowiring DefaultMessageListenerContainer.
My question here is, even though this class StartStopDefaultMessageListenerContainerController is just present in the classpath, it's bean shouldn't be created and autowiring should not happen. But spring boot is scanning the class automatically and then it tries to autowire the fields.
Is this the normal behavior of spring and is there anyway i can avoid this?
If the StartStopDefaultMessageListenerContainerController class is part of component scanning by spring container, Yes spring tries to instantiate and resolve all dependencies.
Here your problem is #Autowired on collection. Spring docs says,
Beans that are themselves defined as a collection or map type cannot be injected through #Autowired, because type matching is not properly applicable to them. Use #Resource for such beans, referring to the specific collection or map bean by unique name.
And also Refer inject-empty-map-via-spring
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
I have written a camel component by extending DefaultComponent and also have the associative classes implemetation for endpoint, consumer, producer. My producer is extending the camel DefaultProducer and I want to inject a spring bean inside this class, so that whenever a route will be executed like
<route id="myRoute"><from uri="file://inbox"/><to uri="myComp://outbox"/>
I will be able to get the file from the file system and store it into database. For storing the file into the DB I have a service class instantiated by the spring container, but whenever I inject that bean into MyProducer we are getting null.
I recongnized the problem was not about Camel, it is related to the spring and I was injecting the bean in a wrong way. I resolved the problem by implementing the ApplicationContextAware interface into my helper class and storing the spring context as static variable and with the help of this helper class I am able to get spring bean inside MyProducer class. Thanks for spring ApplicationContextAware interface.
In XML file in spring we have two bean with different id but same class. They have the same properties offcourse. Now I have InitializingBean interface and in afterPropertySet() I am just printing the value of properties.
Its printing the values two times for me?
According Spring Documentation:
afterPropertySet()
Invoked by a BeanFactory after it has set all bean properties supplied (and satisfied BeanFactoryAware and ApplicationContextAware).
So the short answer on your question is: yes
Spring doesn't manipulate classes or object. Spring manipulates Bean Entity. It is the simplest object manipulated by Spring IOC. Bean has additional behaivior rules introduced by Spring.
If you create two beans for example with Singleton scope and not Lazy initializated Spring creates two instances of your class.
Probably you are calling this Class also invoking a Test or by launching a Integration test like this . check the breakpoints , if you are using SpringRunner, try to mock the component