Lab 2: Autowired

Goal

Understand how Spring does Dependency Injection via its Autowiring mechanism

Note

Make sure to stop the application before working on this section.

a. Autowire Dependencies

Inside the CoffeeOrderController class, add the following import:

import org.springframework.beans.factory.annotation.Autowired;

and replace the constructor with the following:

@Autowired
public CoffeeOrderController(CoffeeOrderService coffeeOrderService) {
  System.out.println();
  System.out.println(this.getClass().getName() + " has been instantiated.");
  System.out.println("  --> Was passed a reference to a dependency: " + coffeeOrderService);
  System.out.println();
}

Now re-run the CoffeeKioskApplication and notice the difference.

Now stop the application.

b. Dependencies on Dependencies

Open up the CoffeeOrderService and add the following import:

import org.springframework.beans.factory.annotation.Autowired;

and replace the constructor with:

@Autowired
public CoffeeOrderService(CoffeeOrderComponent coffeeOrderComponent, AnotherCoffeeOrderComponent anotherCoffeeOrderComponent) {
  System.out.println();
  System.out.println(this.getClass().getName() + " has been instantiated. ");
  System.out.println("  --> Was passed a reference to two dependencies: " + coffeeOrderComponent);
  System.out.println("  -->                                        and: " + anotherCoffeeOrderComponent);
  System.out.println();
}

Re-run the CoffeeKioskApplication and notice the difference in terms of when the classes are instantiated.

Stop the application.

c. Dependencies As Singletons

Open up the CoffeeOrderConfiguration, add the import and replace the constructor as follows:

import org.springframework.beans.factory.annotation.Autowired;

@Autowired
public CoffeeOrderConfiguration(CoffeeOrderService coffeeOrderService) {
  System.out.println();
  System.out.println(this.getClass().getName() + " has been instantiated.");
  System.out.println("  --> Was passed a reference to a dependency: " + coffeeOrderService);
  System.out.println();
}

Re-run the application and notice how the Controller and Configuration classes share the same instance of the CoffeeOrderService class.

d. Explore

Try these experiments out:

• What happens if you try to inject (autowire) a dependency that's not annotated with a Spring annotation? Create a new Java class, without any annotations, and try to autowire it to one of the existing @Component annotated classes. Run the application and look at the output.

• What happens if you create a circular dependency? Try modifying the constructor of CoffeeOrderComponent to autowire CoffeeOrderController as a dependency. Run the application and look at the output.

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Dependency Injection

Martin Fowler's tutorial on Inversion of Control containers and Dependency Injection: https://martinfowler.com/articles/injection.html