Constructor injection (preferred) in Spring Boot - Step-by-Step Execution

Concept Flow - Constructor injection (preferred)

Start Application

↓

Spring creates bean

↓

Identify constructor with dependencies

↓

Inject dependencies via constructor

↓

Bean ready with dependencies

↓

Bean used in application

Spring Boot creates a bean by calling its constructor and passing required dependencies automatically.

Execution Sample

Spring Boot

public class Service {
  private final Repo repo;
  public Service(Repo repo) {
    this.repo = repo;
  }
}

Defines a Service class that receives its Repo dependency through its constructor.

Execution Table

Step	Action	Constructor Called	Dependency Injected	Bean State
1	Spring starts and scans components	No	No	No bean created
2	Spring finds Service bean to create	No	No	Preparing to create bean
3	Spring identifies constructor with Repo parameter	Yes	Repo instance	Service bean created with repo
4	Service bean ready for use	Yes	Repo instance	Bean fully initialized

💡 Bean created after constructor injection completes with all dependencies

Variable Tracker

Variable	Start	After Step 3	Final
repo	null	Repo instance injected	Repo instance injected
Service bean	not created	created with repo	ready to use

Key Moments - 2 Insights

Why does Spring call the constructor with parameters instead of using setters?

Spring uses constructor injection to ensure dependencies are provided when the bean is created, as shown in execution_table step 3 where the constructor is called with the Repo instance.

What happens if the required dependency is missing?

Spring will fail to create the bean because it cannot call the constructor without the required dependency, stopping before step 3 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Spring inject the dependency?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Check the 'Dependency Injected' column in the execution_table

According to variable_tracker, what is the value of 'repo' after step 3?

A Repo instance injected

B not created

C null

D Service bean

💡 Hint

Look at the 'repo' row under 'After Step 3' in variable_tracker

If the constructor had no parameters, how would the execution_table change?

A Bean would be created with dependencies injected later

B Spring would fail to create the bean

C Dependency Injected would be 'No', constructor called with no params

D No change, same steps

💡 Hint

Think about what happens when constructor has no parameters in execution_table step 3

Concept Snapshot

Constructor injection means passing dependencies via the class constructor.
Spring calls the constructor with required beans automatically.
This ensures dependencies are set when the bean is created.
Preferred over setter injection for immutability and clarity.
If dependencies are missing, bean creation fails.
Use final fields to hold injected dependencies.

Full Transcript

Constructor injection in Spring Boot means that when Spring creates a bean, it calls the class constructor and passes in the required dependencies. This happens during application startup. Spring scans for components, finds the bean to create, identifies the constructor with parameters, and injects the dependencies by calling that constructor. The bean is then ready to use with all dependencies set. This method is preferred because it guarantees that dependencies are provided and the bean is immutable. If a required dependency is missing, Spring cannot create the bean and will throw an error. The example shows a Service class with a constructor that takes a Repo object. Spring injects the Repo instance when creating the Service bean.