Before discussing other things lets take a look at few important points -

1. IOC & DI is done using Object Factory for construction & management of objects, so that it is easily configurable and loosely coupled.
2. Two ways to inject dependencies - Setter injection & constructor injection
3. Framework that provides IOC is called IOC Container. Spring has 2 implementation of IOC container - ApplicationContext(has more features like i18n, AOP), BeanFactory(Memory efficient)

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Inversion Of Control & Dependency Injection

IOC : Creation and management of objects.
DI : Injecting object dependencies.

We let take Spring control of creating objects and injecting as we require them.

Why do we need DI ?

Suppose we want to implement Binary Search which requires a Sorting algorithm. So we will have to create object of a Sorting algortihm inside Binary Search.

class BinarySearch{
          // Creating object of Sort Interface
          Sort obj = new BubbleSort();
}

If in a few days we realize we need to use some other sorting algorithm to make our binary search fast, we will have to find this file and change implementation.

class BinarySearch{
         // Changed the sorting implementation being used
         // Sort obj = new BubbleSort();
         Sort obj = new QuickSort();
}

This was just 1 file, now imagine doing it with hundreads of files, with thousands of lines of code. That's where IOC kicks in, and Spring takes care of creating objects at runtime.

If we want to do same thing with Spring, all we need is make use @Autowired annotation. Spring will automatically create object of a class implementing Sort interface and inject it.

@Component
class BinarySearch{
         @Autowired
          Sort quickSort; 
          // Autowiring by name i.e, quickSort is a class and its object will be injected here
}

Autowiring

Autowiring is a way to inject the object dependencies. It can be done in 2 popular ways.

1. byType : Done using a constructor injection
2. byName : Done using setter injection. Variable name is kept same as class name to specify particular class

Autowiring Resolution

If we have 2 implementations for sort interface, we will get a error because Spring does'nt know which object to inject. In this case we case we have 3 options-

• @Primary annotation on the sorting class we want to use.
• @Qualifier annotation to tag classes with a particular qualifier and then using the same name on injecting class. @Qualifier("bubble") on injecting and injected class.
• By Type : In Autowiring byType we keep variable name same as class name.

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Example-1

In this example we make use of Autowiring byType. We have only 1 implemenation of sort interface.

DiExampleAppln.java

@SpringBootApplication
public class DiExampleApplication {
    public static void main(String[] args) {
    ApplicationContext apx=SpringApplication.run(DiExampleApplication.class, args);
    binarySearch b = apx.getBean(binarySearch.class);
    b.binarySearchMethod();
    }
}

binarySearch.java

@Component
public class binarySearch {
    @Autowired
    sort s;
    binarySearch(sort s){ //autowiring by type
        this.s=s;
    }
    public void binarySearchMethod() {
    s.sortmethod();
    System.out.println("Binary searched");
    }    
}

sort.java

public interface sort {
    public void sortmethod();
}

bubbleSort.java

@Component
public class bubbleSort{
    @Override
    public void sortmethod() {
        System.out.println("Bubble sorted");
    }
}

OUTPUT

Bubble sorted
Binary searched

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Example-2

In this example, we have 2 implementations of sort interface, and we make use of @Qualifier annotation to do autowiring resolution

DIExampleAppln remains same.

bubbleSort.java

@Component
@Qualifier("bubble")
public class bubbleSort implements sort{
    @Override
    public void sortmethod() {
        System.out.println("Bubble sorted");
    }
}

quickSort.java

@Component
@Qualifier("quick")
public class quickSort implements sort{
    @Override
    public void sortmethod() {
        System.out.println("Quick sorted");
    }
}

binarySearch.java

@Component
public class binarySearch {
    @Autowired
    @Qualifier("bubble")
    private sort s;     
    public void binarySearchMethod() {
    s.sortmethod();
    System.out.println("Binary searched");
    }    
}

OUTPUT

Bubble sorted
Binary searched

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Code link
References - Spring Master Class
Article by - Arjit Sharma