Not sure about exact difference between Constructor Injection and Setter Injection

[Maan Suraj]

I was going through this link below
http://blog.springsource.com/2007/07/11/setter-injection-versus-constructor-injection-and-the-use-of-required/
Above, author gives couple of reasons why people use setter injection more than constructor injection. I was ok with first reason, but couldnt understand completely the second reason.

The second reason why setter injection is used a lot more often than you would expect, is the fact that frameworks like Spring in general, are much more suited to be configured by setter injection than by constructor injection. This is mostly because frameworks that need to be configured often contain lots of optional values. Making optional values configurable using constructor injection would lead to needless clutter and proliferating constructors, especially when used in combination with class inheritance.

Could anyone help me to understand above reason(I mean what is that optional values etc etc).

[Mark Spritzler]

A setter method does not have to be called for your app to create instances. But with Constructor, you cannot create that object without its dependencies. With a setter method I can create that object without its dependencies. That makes them optional.

public class Mark {

    public Mark(OtherObject oob) {}
}

try creating a Mark without an OtherObject? You can't.

but with

public class Mark {

    public void setOtherObject(OtherObject oob) {}
}

You can with just new Mark();

Mark

[Maan Suraj]

hmm..got it..thanks

[Bill Gorder]

jinesh parikh,
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