Constructor injection (preferred) in Spring Boot - Performance & Optimization

Performance: Constructor injection (preferred)

MEDIUM IMPACT

This affects application startup time and memory usage by managing dependency creation efficiently.

Injecting dependencies into Spring components

Spring Boot

public class Service {
    private final Repository repo;

    public Service(Repository repo) {
        this.repo = repo;
    }
}

Constructor injection creates dependencies at object creation, avoiding reflection and improving startup speed.

📈 Performance Gainreduces reflection overhead, improves startup time by up to 10-20% in complex apps

Injecting dependencies into Spring components

Spring Boot

public class Service {
    @Autowired
    private Repository repo;

    public Service() {}
}

Field injection uses reflection and can delay dependency resolution, increasing startup time and making testing harder.

📉 Performance Costadds reflection overhead during startup, can increase memory usage due to proxy creation

Performance Comparison

Pattern	Reflection Usage	Startup Delay	Memory Overhead	Verdict
Field Injection (@Autowired on fields)	High	Medium	Higher due to proxies	[X] Bad
Constructor Injection (preferred)	Low	Low	Lower	[OK] Good

Rendering Pipeline

Constructor injection affects the backend initialization phase before the web layer renders content. It optimizes how dependencies are wired, reducing delays before the application can serve requests.

→Application Startup

→Dependency Resolution

⚠️ BottleneckReflection and proxy creation during dependency injection

Optimization Tips

1Use constructor injection to reduce reflection and improve startup speed.

2Avoid field injection to prevent proxy overhead and improve testability.

3Constructor injection enables immutable dependencies, enhancing performance and code clarity.

Performance Quiz - 3 Questions

Why is constructor injection preferred over field injection in Spring Boot?

A It allows dependencies to be changed at runtime

B It requires less code to write

C It reduces reflection overhead and improves startup performance

D It automatically lazy loads dependencies

DevTools: Spring Boot Actuator and JVM Profilers

How to check: Enable Spring Boot Actuator metrics and use a JVM profiler to measure startup time and memory usage during application boot.

What to look for: Look for reduced reflection calls and faster bean creation times indicating efficient constructor injection.

Practice

1. What is the main advantage of using constructor injection in Spring Boot?

easy

A. It automatically creates objects without any code.

B. It requires less code than field injection.

C. It allows changing dependencies after the object is created.

D. It makes dependencies explicit and helps with testing.

Solution

1. Step 1: Understand constructor injection purpose

   Constructor injection passes dependencies when creating the object, making them clear and fixed.

2. Step 2: Benefits of explicit dependencies

   This clarity helps testing because dependencies can be easily replaced with mocks.

3. Final Answer:

   It makes dependencies explicit and helps with testing. -> Option D

4. Quick Check:

   Constructor injection = explicit dependencies [OK]

Hint: Constructor injection shows dependencies clearly [OK]

Common Mistakes:

• Thinking constructor injection creates objects automatically
• Confusing constructor injection with field injection
• Believing dependencies can be changed after creation

2. Which of the following is the correct syntax for constructor injection in a Spring Boot component?

easy

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

B. public class Service { @Autowired private Repo repo; }

C. public class Service { private Repo repo; public void setRepo(Repo repo) { this.repo = repo; } }

D. public class Service { private Repo repo = new Repo(); }

Solution

1. Step 1: Identify constructor injection pattern

   Constructor injection requires a constructor that takes dependencies as parameters and assigns them.

2. Step 2: Check each option

   public class Service { private final Repo repo; public Service(Repo repo) { this.repo = repo; } } shows a constructor with a parameter and assigns it to a final field, which is correct.

3. Final Answer:

   public class Service { private final Repo repo; public Service(Repo repo) { this.repo = repo; } } -> Option A

4. Quick Check:

   Constructor with parameters = constructor injection [OK]

Hint: Constructor injection needs constructor with parameters [OK]

Common Mistakes:

• Using @Autowired on fields instead of constructor
• Using setter methods instead of constructor
• Creating new dependency instances inside the class

3. Given the code below, what will be the output when the printMessage method is called?

\@Component
public class Printer {
  private final MessageService service;
  public Printer(MessageService service) {
    this.service = service;
  }
  public void printMessage() {
    System.out.println(service.getMessage());
  }
}

\@Component
public class MessageService {
  public String getMessage() {
    return "Hello Spring!";
  }
}

medium

A. null

B. Hello Spring!

C. Compilation error

D. Runtime NullPointerException

Solution

1. Step 1: Understand constructor injection wiring

   Spring injects MessageService into Printer via constructor, so service is not null.

2. Step 2: Check method output

   Calling service.getMessage() returns "Hello Spring!", which is printed.

3. Final Answer:

   Hello Spring! -> Option B

4. Quick Check:

   Injected service returns message [OK]

Hint: Constructor injection ensures dependencies are not null [OK]

Common Mistakes:

• Assuming service is null without injection
• Confusing field injection with constructor injection
• Expecting compilation error due to missing annotations

4. What is wrong with the following Spring Boot component using constructor injection?

\@Component
public class UserService {
  private final UserRepository repo;
  public UserService() {
    this.repo = null;
  }
}

medium

A. Constructor does not accept UserRepository, so dependency is not injected.

B. Field repo should not be final.

C. Missing @Autowired annotation on the constructor.

D. UserRepository should be instantiated inside the constructor.

Solution

1. Step 1: Check constructor parameters for injection

   The constructor has no parameters, so Spring cannot inject UserRepository.

2. Step 2: Understand consequence of no injection

   repo is assigned null manually, so dependency is missing and will cause errors later.

3. Final Answer:

   Constructor does not accept UserRepository, so dependency is not injected. -> Option A

4. Quick Check:

   Constructor must have dependency parameters [OK]

Hint: Constructor must accept dependencies as parameters [OK]

Common Mistakes:

• Forgetting to add parameters to constructor
• Thinking @Autowired is required on constructor in Spring Boot
• Instantiating dependencies manually inside constructor

5. You have a Spring Boot service that depends on two other services: PaymentService and NotificationService. How should you write the constructor to follow best practices for constructor injection?

hard

A. public OrderService(PaymentService paymentService) { this.paymentService = paymentService; }

B. public OrderService() { this.paymentService = new PaymentService(); this.notificationService = new NotificationService(); }

C. public OrderService(PaymentService paymentService, NotificationService notificationService) { this.paymentService = paymentService; this.notificationService = notificationService; }

D. public OrderService(@Autowired PaymentService paymentService, NotificationService notificationService) { this.paymentService = paymentService; this.notificationService = notificationService; }

Solution

1. Step 1: Identify correct constructor injection with multiple dependencies

   The constructor should accept all required dependencies as parameters and assign them.

2. Step 2: Check options for best practice

   public OrderService(PaymentService paymentService, NotificationService notificationService) { this.paymentService = paymentService; this.notificationService = notificationService; } correctly accepts both services and assigns them, following best practice.

3. Final Answer:

   public OrderService(PaymentService paymentService, NotificationService notificationService) { this.paymentService = paymentService; this.notificationService = notificationService; } -> Option C

4. Quick Check:

   Constructor with all dependencies = best practice [OK]

Hint: Include all dependencies as constructor parameters [OK]

Common Mistakes:

• Instantiating dependencies inside constructor
• Missing one of the dependencies in constructor
• Using @Autowired on constructor parameters unnecessarily