Constructor Injection In Spring [ Full Concept ]

In this type of injection spring container uses constructor of the bean class for assigning the dependencies. In spring config xml, we need to inform to the spring IOC container about constructor injection by using <constructor –arg />

In spring bean class, if both constructor and setter injection applied for same property then constructor injection will be overridden by setter injection, because constructor injection will happen at the object creation time, and setter after objection right…, so setter will overrides

For Example:

public class DemoBean
{
    public string message;

      public DemoBean (String message)
      {
         This.message = message;
      }

      public void setMessage(String message)
      {
         This.message = message;
      }

    public void show()
    {
       System.out.println("some logic here");
    }

}

In Config XML

<bean id="id1" class="DemoBean">

    <constructor-arg value="Welcome to java4s" />
    <property name="message" value="Welcome to spring" />

</bean>

Output

In client application, when we call factory.getBean(“id1”), then internally spring framework executes following statements

     DemoBean ob = new DemoBean("Welcome to java4s");
              Ob.setMessage("Welcome to spring");

finally value in message will be  Welcome to spring, not Welcome to java4s, as setter will over rides constructor, the reason being see i have taken primitive value [string] as dependency,  we have written injection for this property in constructor and setter method.  We all know constructor will be executed at object creation, so at the time of object creation only some value will be assigned into message property, then setter will be called so previous value will be overrides

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In constructor injection,  if argument types  are different, then at the time of configuring of xml file we can use type attribute

Example..

public class DemoBean
{
  public int id;
  public String sname;

     public DemoBean(int id, string sname)
     {
         This.id  = id;
         This.sname = sname;
     }

}

In Config XML

<bean id="id1" class="DemoBean">

  <constructor-arg type="java.lang.string" value="1000" />
  <constructor-arg  value="10" />

</bean>

According to above xml, DemoBean object will be created with 10 as id and 1000 as string [sname]

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Let if we have 2 properties of same type, like user name and password

Example..

public class DemoBean
{
   public String uname, password;

    public DemoBean (String uname, String password)
    {
         This.uname = uname;
         This.password = password;
    }
}

In our xml

<bean id="id1" class="DemoBean">

<constructor-arg value="myuserName"  index="0" />
<constructor-arg value="mypassword"  index="1" />

</bean>

Now bean object will be created with myuserName as username, and mypassword as password

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Dependency in the form of object

Let us see how to work with dependency in the form of objects in this constructor injection…

public class DemoBean
{
   public SampleBean sb;
     public DemoBean(SampleBean sb)
     {
          This.sb = sb;
     }
}

In the xml

<bean id="id1" class="DemoBean">
<constructor-arg ref="sb" />
</bean>
<bean id="sb" class="SampleBean" />

Note: Here see, directly i given ref as an attribute, so internally it meas ref-bean only not ref-local or ref-parent

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If we have multiple constructors then..

public class DemoBean
{
public String uname, password;

public DemoBean (int id, String uname)
{
This.id = id;
This.uname = uname;

}

public DemoBean (string uname, int id)
{
This.id = id;
This.uname = uname;

}

}

XML file will be

<beans>
  <bean id="id1" class="DemoBean">

     <constructor-arg value="10" />
     <constructor-arg value="MyUserName" type="java.lang.string"/>

  </bean>

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

      <constructor-arg value="MyUserName" />
      <constructor-arg value="10" />

  </bean>

</beans>

In the above example, when we class factory.getBean(“id1“) from client application then spring framework creates an object of DemoBean by calling 1st constructure

If we call factory.getBean(“id2“) then spring framework crates the object of DemoBean by calling 2nd constructor automatically

This is total about this constructor injection in Spring, nothing more than that.

So mates, i don’t think you need any example program on this 🙂

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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.