@Autowired for dependency injection in Spring Boot - Step-by-Step Execution

Concept Flow - @Autowired for dependency injection

Start Application

↓

Spring scans for @Autowired

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Find dependency to inject

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Create or find bean instance

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Inject bean into target class

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Use injected bean in code

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Application runs with dependencies ready

Spring scans for @Autowired annotations, finds the needed beans, creates or retrieves them, and injects them into the target class before running the application.

Execution Sample

Spring Boot

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

@Service
public class Service {
  @Autowired
  private Repository repo;

  public void doWork() {
    repo.save();
  }
}

This code shows Spring injecting a Repository bean into Service using @Autowired, so Service can call repo.save() without manual setup.

Execution Table

Step	Action	Evaluation	Result
1	Spring starts and scans classes	Find @Autowired on 'repo' field	Marks 'repo' for injection
2	Spring looks for Repository bean	Repository bean found or created	Repository instance ready
3	Spring injects Repository instance into Service.repo	Field 'repo' set with Repository instance	Service.repo now references Repository bean
4	Service.doWork() called	Calls repo.save()	Repository.save() executes
5	Application runs with dependencies injected	No manual new Repository() needed	Dependency injection successful

💡 All dependencies injected, application runs with ready-to-use beans

Variable Tracker

Variable	Start	After Injection	After doWork() call
repo	null	Repository instance	Repository instance (used)

Key Moments - 3 Insights

Why is 'repo' null before injection but not after?

Before injection, Spring has not assigned the Repository instance, so 'repo' is null. After step 3 in execution_table, Spring injects the Repository bean, so 'repo' references the instance.

What happens if Spring cannot find a Repository bean to inject?

Spring will throw an error during startup because it cannot fulfill the @Autowired dependency, stopping the application from running.

Does @Autowired create a new instance every time?

No, by default Spring injects a singleton bean instance, so the same Repository object is reused unless configured otherwise.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'repo' after step 2?

A null

B Repository instance

C New Service instance

D Undefined

💡 Hint

Check variable_tracker column 'After Injection' and execution_table step 2 where bean is found but not yet injected

At which step does Spring inject the Repository bean into Service?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table step 3 where injection is described

If the Repository bean is missing, what will happen according to the key moments?

A Spring creates a new Repository automatically

B Spring throws an error and stops startup

C Service runs with repo as null

D Application runs but repo.save() does nothing

💡 Hint

Refer to key_moments about missing bean behavior

Concept Snapshot

@Autowired injects dependencies automatically.
Spring scans for @Autowired fields.
Finds or creates matching beans.
Injects bean instance into target.
No manual new needed.
Injection happens before use.

Full Transcript

When a Spring Boot application starts, it scans classes for the @Autowired annotation. When it finds a field marked with @Autowired, like 'repo' in the Service class, it looks for a matching bean to inject. If a Repository bean exists, Spring creates or retrieves it and injects it into the Service's 'repo' field. Before injection, 'repo' is null. After injection, 'repo' references the Repository instance. When Service calls doWork(), it uses the injected Repository to perform actions like save(). If Spring cannot find a required bean, it throws an error and stops the application from starting. This automatic wiring saves you from manually creating and passing dependencies.