@Value for property injection in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Spring Boot component using @Value?

Consider this Spring Boot component:

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class Greeting {
    @Value("${app.greeting}")
    private String message;

    public String getMessage() {
        return message;
    }
}

If the application.properties contains app.greeting=Hello, Spring!, what will getMessage() return?

A null

B ${app.greeting}

C Compilation error due to missing setter

D Hello, Spring!

💡 Hint

Think about how @Value injects property values into fields.

The @Value annotation injects the property value from application.properties into the field. Since app.greeting=Hello, Spring! is defined, message will hold that string.

📝 Syntax

intermediate

Which option correctly injects a default value with @Value?

You want to inject a property app.name but provide a default value "MyApp" if the property is missing. Which @Value syntax is correct?

A @Value("${app.name:MyApp}")

B @Value("${app.name|MyApp}")

C @Value("${app.name?MyApp}")

D @Value("${app.name?default=MyApp}")

💡 Hint

Spring uses colon ':' to specify default values in @Value expressions.

The correct syntax to provide a default value in @Value is ${property:defaultValue}. So @Value("${app.name:MyApp}") injects the property or "MyApp" if missing.

🔧 Debug

advanced

Why does this @Value injection fail with a NullPointerException?

Given this code:

@Component
public class Config {
    @Value("${app.timeout}")
    private Integer timeout;

    public int getTimeout() {
        return timeout;
    }
}

and app.timeout is not set in properties, what causes a NullPointerException when calling getTimeout()?

A Missing @Autowired causes field not to be injected

B Spring cannot inject Integer fields, must use int primitive

C timeout is null because property is missing, unboxing to int causes NullPointerException

D The @Value annotation syntax is invalid for Integer fields

💡 Hint

Consider what happens when a null Integer is converted to int.

If the property is missing, Spring injects null into the Integer field. When getTimeout() returns int (primitive), Java tries to unbox null to int, causing NullPointerException.

🧠 Conceptual

advanced

What happens if you use @Value on a static field?

Consider this code snippet:

@Component
public class StaticConfig {
    @Value("${app.version}")
    private static String version;

    public static String getVersion() {
        return version;
    }
}

What will be the behavior of the version field?

A version is injected but only after calling getVersion() once

B version remains null because Spring does not inject @Value into static fields

C Compilation error because @Value cannot be used on static fields

D version is injected correctly with the property value

💡 Hint

Think about how Spring manages instance fields versus static fields.

Spring injects values into instance fields during bean creation. Static fields belong to the class, not the instance, so @Value does not inject into static fields. The field remains null.

❓ state_output

expert

What is the output of this Spring Boot component with SpEL in @Value?

Given this component:

@Component
public class Calculator {
    @Value("#{2 * 3 + 4}")
    private int result;

    public int getResult() {
        return result;
    }
}

What will getResult() return?

A 10

B 14

C 6

D Runtime error due to invalid expression

💡 Hint

Evaluate the expression 2 * 3 + 4 using normal math rules.

The expression inside SpEL is 2 * 3 + 4. Multiplication happens first: 2*3=6, then add 4 = 10. So result is 10.