@Value for property injection in Spring Boot - Mini Project: Build & Apply

Using @Value for Property Injection in Spring Boot

📖 Scenario: You are building a simple Spring Boot application that needs to read configuration values from the application.properties file. These values will be injected into your Java class using the @Value annotation.

🎯 Goal: Learn how to use the @Value annotation to inject property values from application.properties into a Spring Boot component.

📋 What You'll Learn

Create an application.properties file with specific properties

Create a Spring Boot component class

Use @Value annotation to inject property values

Add a method to return the injected values

💡 Why This Matters

🌍 Real World

Injecting configuration values like app name, version, or API keys into Spring Boot components is common in real applications.

💼 Career

Understanding @Value is essential for configuring Spring Boot apps and managing environment-specific settings.

1

Create application.properties with properties

Create an application.properties file with these exact entries:
app.name=MySpringApp
app.version=1.0.3

Spring Boot

# Create application.properties file with the properties
# Your code here

Hint

Properties files use key=value format. Make sure to write exactly app.name=MySpringApp and app.version=1.0.3.

2

Create a Spring Boot component class

Create a Java class named AppInfo annotated with @Component inside package com.example.demo.

Spring Boot

app.name=MySpringApp
app.version=1.0.3

// Create class AppInfo annotated with @Component
# Your code here

Hint

Use @Component annotation above the class declaration to make it a Spring bean.

3

Inject properties using @Value

Inside the AppInfo class, create two private String fields named name and version. Use @Value to inject ${app.name} into name and ${app.version} into version.

Spring Boot

app.name=MySpringApp
app.version=1.0.3

package com.example.demo;

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

@Component
public class AppInfo {
    // Add private String fields name and version with @Value injection
    # Your code here
}

Hint

Use @Value("${property.key}") above each field to inject the property value.

4

Add method to return injected values

Add a public method getAppDetails() in AppInfo that returns a String combining name and version in this format: "App: MySpringApp, Version: 1.0.3".

Spring Boot

app.name=MySpringApp
app.version=1.0.3

package com.example.demo;

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

@Component
public class AppInfo {

    @Value("${app.name}")
    private String name;

    @Value("${app.version}")
    private String version;

    // Add public method getAppDetails() that returns formatted string
    # Your code here
}

Hint

Concatenate the fields name and version with the text exactly as shown.

Practice

1. What is the main purpose of the @Value annotation in Spring Boot?

easy

A. To create database connections

B. To define a new Spring bean

C. To handle HTTP requests

D. To inject values from configuration files into Spring beans

Solution

1. Step 1: Understand the role of @Value

   @Value is used to inject configuration values like those from application.properties into Spring beans.

2. Step 2: Compare with other options

   Defining beans, handling HTTP requests, and database connections are done by other annotations or classes, not @Value.

3. Final Answer:

   To inject values from configuration files into Spring beans -> Option D

4. Quick Check:

   @Value injects config values = A [OK]

Hint: Remember: @Value pulls config values into your code [OK]

Common Mistakes:

• Confusing @Value with @Bean or @Autowired
• Thinking @Value handles HTTP or DB tasks
• Ignoring that @Value reads from properties

2. Which of the following is the correct syntax to inject a property named app.name using @Value?

easy

A. @Value(${app.name}) private String appName;

B. @Value("${app.name}") private String appName;

C. @Value("app.name") private String appName;

D. @Value(app.name) private String appName;

Solution

1. Step 1: Recall correct syntax for property injection

   The property key must be inside ${} and the whole expression inside quotes: @Value("${property.key}").

2. Step 2: Check each option

   @Value("${app.name}") private String appName; matches the correct syntax. The other options miss either the quotes around the expression, the ${} delimiters, or use quotes around just the property name.

3. Final Answer:

   @Value("${app.name}") private String appName; -> Option B

4. Quick Check:

   Correct syntax uses ${} inside quotes = D [OK]

Hint: Use @Value("${property.key}") with quotes and ${} [OK]

Common Mistakes:

• Omitting quotes around the expression
• Forgetting the ${} around the property key
• Using plain property name without ${}

3. Given the property server.port=8080 in application.properties, what will be the output of this Spring component?

public class ServerConfig {
  @Value("${server.port}")
  private int port;

  public int getPort() {
    return port;
  }
}

// In main method:
ServerConfig config = new ServerConfig();
System.out.println(config.getPort());

medium

A. 0

B. 8080

C. null

D. Compilation error

Solution

1. Step 1: Understand Spring bean lifecycle and injection

   Property injection with @Value happens only when Spring creates and manages the bean. Here, ServerConfig is created manually with new, so injection does not happen.

2. Step 2: Check default value of int field

   Since port is a primitive int and not injected, it defaults to 0.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Manual new means no injection, int defaults to 0 [OK]

Hint: Only Spring-managed beans get @Value injection [OK]

Common Mistakes:

• Assuming @Value works on manually created objects
• Expecting null for primitive int fields
• Ignoring Spring bean lifecycle

4. What is wrong with this code snippet?

@Value("${missing.property}:defaultValue")
private String value;

When missing.property is not defined in properties, the application fails to start with an error.

medium

A. The field must be public for @Value to work

B. The property key must be defined; default values are not supported

C. The default value syntax is incorrect; colon should be inside quotes

D. The @Value annotation cannot be used on String fields

Solution

1. Step 1: Check default value syntax in @Value

   The default value must be inside the quotes as part of the expression: @Value("${property:default}") is correct.

2. Step 2: Identify the error cause

   If the colon is outside quotes or expression is malformed, Spring cannot parse it and throws an error.

3. Final Answer:

   The default value syntax is incorrect; colon should be inside quotes -> Option C

4. Quick Check:

   Default values must be inside ${} quotes [OK]

Hint: Put default after colon inside quotes in @Value [OK]

Common Mistakes:

• Placing colon outside the quotes
• Assuming default values are not supported
• Making fields public unnecessarily

5. You want to inject a list of strings from a property app.modes=dev,prod,test into a Spring bean using @Value. Which approach correctly converts the property into a List<String>?

hard

A. @Value("#{T(java.util.Arrays).asList('${app.modes}'.split(','))}") private List<String> modes;

B. @Value("${app.modes}") private List<String> modes; // Spring auto-converts comma-separated string

C. @Value("${app.modes}") private String[] modes; // Use array instead of List

D. @Value("${app.modes}") private String modes; // Parse manually later

Solution

1. Step 1: Understand property injection for collections

   Spring does not automatically convert comma-separated strings to List<String> with plain @Value.

2. Step 2: Use SpEL expression for conversion

   @Value("#{T(java.util.Arrays).asList('${app.modes}'.split(','))}") private List<String> modes; uses Spring Expression Language (SpEL) to split the string and convert it to a List<String>, which is the correct approach.

3. Final Answer:

   @Value("#{T(java.util.Arrays).asList('${app.modes}'.split(','))}") private List<String> modes; -> Option A

4. Quick Check:

   Use SpEL split and asList for List injection [OK]

Hint: Use SpEL with split and asList to inject List<String> [OK]

Common Mistakes:

• Expecting automatic conversion from String to List
• Using array when List is required without conversion
• Injecting as String and forgetting to parse