Setter Injection With Objects, Spring Dependency In The Form Of Objects

In previous example, we have seen that our spring bean class object depends on the string primitive, and now will see what if our class is depends on other class object, i mean dependency in the form of object.

• While constructing spring beans, if one spring bean is depends on another spring bean class for performing some logic, this process of dependency is called object dependency
• If object dependency is there then in spring framework, the spring IOC container is responsible for creating that required object and injecting into the dependent class

For spring configuration xml, we have 2 ways to inform to the spring container about this object dependency

• By using inner beans
• Using <ref /> element

Actually with inner beans we have some disadvantages and its not the way to use in the real time projects so am not going to explain about it.

So we will see about <ref  /> element, and i forgot to tell you actually in previous example we have one spring configuration file right [ spconfig.xml ], in spring we can write multiple configuration xmls, i will tell you how its going to work in the following example

Using <ref /> element

Syntax will be

<ref local/parent/bean=”id of collaborator bean”>

Actually we used to write either local or parent or bean, means as i told you earlier, we can write any number of spring configuration xmls for the spring application.  Our collaborator bean may be in same xml or other xml so spring has given these 3 options, we will see one by one.

<ref local=”id value” />

If we use the local attribute in the <ref /> element then the spring IOC container will verify for the collaborator bean with in same container [ i mean in same xml ]

In general we try to configure all spring beans into a single spring configuration xml only.  But its not mandatory we can create multiple configure files too right.

Exmple:

public DemoBean
{
   public SampleBean sb;
   public void setSb(SampleBean sb)
   {
      this.sb = sb;
   }

   public void m1()
   {
      sb.m2();
   }
}

Note: See am calling m2() method in SamepleBean so now let us see how the xml file will be

<beans>

  <bean id="id1">
    <property name="sb" class="DemoBean">
      <ref local="id2" />
    </property>
  </bean>

  <bean id="id2" class="SampleBean">

</beans>

So our DemoBean class depends on other class object SampleBean,  see in the xml line number 5 i have specified ref tag with local attribute and given that required class id, why local…? because that required (collaborator) class also i have configured in the same xml file  [ line number 9 ], so spring container will check only in this xml only

<ref parent=”id value” />

But we can also configure the collaborator class in other xml like…

spconfig1.xml

<beans>

  <bean id="id1">
    <property name="sb" class="DemoBean">
      <ref parent="id2" />
    </property>
  </bean>

</beans>

spconfig2.xml

<beans>

  <bean id="id2" class="SampleBean">

</beans>

In this case, we have to write parent as attribute, see spconfig1.xml  line number 5.  As we specified parent, spring container will only checks in the parent i mean spconfig2.xml only

If we give attribute as bean, then first it will checks at local xml file, then parent if its not available at local.  Hey you will be able to understand once you got through the example…, let us see the program on this in the next session.

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::. About the Author .::
		Sivateja Kandula - Java/J2EE Full Stack Developer Founder of Java4s - Get It Yourself, A popular Java/J2EE Programming Blog, Love Java and UI frameworks. You can sign-up for the Email Newsletter for your daily dose of Java tutorials.