Why Does @Value Work in Constructor Injection but Not in Field Injection Immediately?

1️⃣ Constructor Injection Happens During Bean Instantiation (Earlier)

When you use @Value inside a constructor, Spring resolves the value before the Bean is fully created. This ensures that by the time your object exists, it has all the required values.

✅ Example: Constructor Injection Works

@Component
public class MyService {

    private final String apiKey;

    public MyService(@Value("${api.key}") String apiKey) { // Injected immediately
        this.apiKey = apiKey;
    }

    public void printApiKey() {
        System.out.println("API Key: " + apiKey);
    }
}

✔ Works perfectly because @Value is injected before the object is created.

✔ No risk of null values since the constructor ensures it gets the required value.

2️⃣ Field Injection Happens After Bean Instantiation (Later)

When using @Value directly on a field, Spring first creates the object (calls the constructor) but injects values later in a separate step.

🚨 This means if you try to access the field in the constructor, it will still be null!

❌ Example: Field Injection Can Cause Problems

java
CopyEdit
@Component
public class MyService {

    @Value("${api.key}")
    private String apiKey; // Injected after constructor runs

    public MyService() {
        System.out.println("Constructor: API Key = " + apiKey); // ❌ Will print null
    }

    @PostConstruct
    public void init() {
        System.out.println("PostConstruct: API Key = " + apiKey); // ✅ Now it's injected
    }
}

⚠ Problem: apiKey is null inside the constructor because Spring injects it only after the object is created.

✅ Solution: @PostConstruct ensures it is used only after Spring injects values.

🛠️ When to Use Constructor vs. Field Injection

📌 TL;DR