How is IoC useful over conventional object instantiation ?

[Schandha Ravi]

Hi All,

I'm relatively new to Spring and espicially to IoC. Unrelated to Spring, can some one explain me with an example, how IoC is useful over conventional object instantiation (using new keyword). What is the benefit of this re-direction, where in creation and wiring of the objects is delegated to IoC container instead of handling it in code. I'm pretty sure that, there might be something apart from freeing the code from object creation, which does not seem to be lot of problematic.

[David Newton]

IoC allows us to decouple the interface from the implementation, and can allow external forces to determine the implementation.

Consider the implications for testing method foo:

public String foo() {
    Bar bar = new Bar();
    return bar.baz() == null ? "Kaaaaahhhhnnnnnnn!" : "OK";
}

We can only test foo() with an instance of a Bar. If we need to test the return value of foo(), we now have to re-implement our Bar.baz() to return each of the possible conditions foo() could encounter. If, on the other hand, we allow the Bar to be set, our unit test can decide which condition we're testing:

Bar bar;
public void setBar(Bar bar) { this.bar = bar; }
public String foo() { return bar.baz() == null ? "Kaaaaahhhhnnnnnnn!" : "OK"; }

IoC also allows for strong configurability. Consider a program intended to allow easy extension or modification of key components. Instead of going in and changing the original source code, we can just tell the program at startup which implementation to use. This might allow us to customize how HTTP requests are handled, provide a plugin mechanism, and so on.

[Schandha Ravi]

Thank you for explaining with an example. I have started reading "Spring in action" to understand more on this framework and I'm sure that I would come up with more questions to ask.