What is bean in Spring ? How beans created and injected ? - JavaStuff.in

What is bean in Spring ? How beans created and injected ?

Posted in: Spring Framework

What is a bean in Spring ?

Typical java bean with a unique id
Typically defined in an XML file
In spring there are basically two types

Singleton : One instance of the bean created and referenced each time it is requested
Prototype (non-singleton) : New bean created each time. Same as new ClassName()

Beans are normally created by Spring as late as possible

Example:

<bean id="exampleBean" class=”org.example.ExampleBean">

   <property name="beanOne">
      <ref bean="anotherExampleBean"/>
   </property>

   <property name="beanTwo">
     <ref bean="yetAnotherBean"/>
   </property>

   <property name="integerProperty">
     <value>1</value>
   </property>

</bean>

How are beans created ?

Beans are created in order based on the dependency graph

Often they are created when the factory loads the definitions
Can override this behavior in bean <bean class=“className” lazy-init=“true” />
You can also override this in the factory or context but this is not recommended

Spring will instantiate beans in the order required by their dependencies

app scope singleton - eagerly instantiated at container startup
lazy dependency - created when dependent bean created
VERY lazy dependency - created when accessed in code

How are beans injected ?

A dependency graph is constructed based on the various bean definitions
Beans are created using constructors (mostly no-arg) or factory methods
Dependencies that were not injected via constructor are then injected using setters
Any dependency that has not been created is created as needed