Spring Dependency Injection

 What is Inversion of control (IOC)?

    💜Where control of object is transferred to container or framework.
    💜IOC allows container to take control on flow of program.
    💜To implement IoC, the dependency injection is used.

Spring Container

    

    💜 The interface "ApplicationContext" represents the spring container.

    💜 Spring container instantiate, configuring and assemble spring beans and manage their life cycle.

Spring bean

    💜 An object which is managed by spring container is known as spring bean.

What is Dependency in spring framework?

public class Invoice {

    String name;

    Double amount;

    List<String> items;

    Vendor vendor;

    TaxDetails taxDetails;

}

class Vendor {

}

interface TaxDetails {

}

   💜 Invoice class has various instance variables (name, amount, items, vendor, and taxDetails).
   💜 If we want to create object of Invoice then we need all instance variables.
   💜 So instance of class depends on all instance variables inside the class.

Dependent bean

    💜 We define Invoice class as bean in spring and it has dependencies as vendor and taxDetails.
    💜 Invoice is target bean and Vendor is dependent bean.

Spring container injects dependencies, if dependent variables data type is

1. Primitive Type: (byte, short, int, long, float, double, boolean, char, String)
2. Collection Type Dependency: (List, Set, Map, Properties)
3. Reference Type: (variables created using interface or class)

Injection

    

    💜 Inserting something into another thing.

    💜 Process of inserting one object into another object is called injection.

Spring Dependency Injection?

    

    💜 Spring container injects other objects into an object.

    💜 Dependencies which are declared in the spring bean.

Implementing DI

1. XML based.
2. Java annotation based.

Autowiring

    💜 Spring Container detects relationship between the beans at the time of booting up application.

    💜 It will automatically create objects and wire the dependencies.

    💜 Only provide @Autowired on variable

Autowire conflict

    💜 If more than one same type of bean available in container, then spring throw NoUniqueBeanDefinitionException. (Conflict beans)

    💜 To resolve this conflict, need to tell spring that which bean we want to inject using @Primary or @Qualifier annotations along with @Autowired.

Type of Spring Dependency Injection

    

    💚 Setter Based Injection:

            

                Spring container inject value to variable using setter method.

@Component

public class Vendor{

    int code;

    String name;

}

public class Invoice{

    private Vendor vendor;

    @Autowired

    public void setVendor(Vendor vendor){

        this.vendor=vendor;

    }

}

    💚Constructor Based Injection:

        

            Spring container inject value to variable using parameterized constructor.

            

@Component

public class Vendor{

    int code;

    String name;

}

public class Invoice{

    private Vendor vendor;

    @Autowired

    public Invoice(Vendor vendor){

        this.vendor=vendor;

    }

}

    💚 Field or property-based Injection

        

            When the annotation @Autowired is used on top of the field or property in the class, it is known as Field-based Dependency Injection

@Component

public class Vendor{

    int code;

    String name;

}

public class Invoice{

    @Autowired

    private Vendor vendor;

}

@Autowired(reuired=false)

    💜 When bean is not registered with container and we apply autowire then it will throw exception for missing dependency.

    💜 In this case we use @Autowired(reuired=false) to tell container that this dependency is optional.

When To Use Setter Injection and When to Use Constructor Injection?

    💜 Constructor injection is preferred when all the properties of bean class are required to participate in DI.
    💜 Setter Injection is preferred when bean properties of spring bean class are optional to participate in DI.