Custom configuration properties in Spring Boot

Introduction

Custom configuration properties let you organize your app settings in a neat way. They help keep your code clean and easy to change without touching the code itself.

You want to group related settings like database info or API keys in one place.

You need to change app behavior without rebuilding the app.

You want to avoid hardcoding values inside your Java classes.

You want to validate configuration values automatically.

You want to keep configuration readable and maintainable.

Syntax

Spring Boot

1. Create a POJO class with fields for your properties.
2. Annotate the class with @ConfigurationProperties(prefix = "your.prefix")
3. Add @Component or @EnableConfigurationProperties to register it.
4. Use getters and setters for the fields.
5. Define the properties in application.properties or application.yml using the prefix.

The prefix groups your properties under a common name.

Spring Boot automatically binds the properties to your class fields.

Examples

This class binds properties starting with app.mail like app.mail.host and app.mail.port.

Spring Boot

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;

@Component
@ConfigurationProperties(prefix = "app.mail")
public class MailProperties {
    private String host;
    private int port;

    public String getHost() {
        return host;
    }

    public void setHost(String host) {
        this.host = host;
    }

    public int getPort() {
        return port;
    }

    public void setPort(int port) {
        this.port = port;
    }
}

These are the property values that will be injected into the MailProperties class.

Spring Boot

application.properties
app.mail.host=smtp.example.com
app.mail.port=587

Alternative way to register your properties class without using @Component.

Spring Boot

import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Configuration;

@Configuration
@EnableConfigurationProperties(MailProperties.class)
public class AppConfig {
    // This enables the binding if you don't use @Component on the properties class
}

Sample Program

This Spring Boot app defines a custom properties class MailProperties bound to app.mail prefix. The controller shows these values at /mailconfig.

Set these in application.properties:

app.mail.host=smtp.example.com
app.mail.port=587

Spring Boot

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

@Component
@ConfigurationProperties(prefix = "app.mail")
class MailProperties {
    private String host;
    private int port;

    public String getHost() {
        return host;
    }

    public void setHost(String host) {
        this.host = host;
    }

    public int getPort() {
        return port;
    }

    public void setPort(int port) {
        this.port = port;
    }
}

@RestController
class MailController {
    private final MailProperties mailProperties;

    public MailController(MailProperties mailProperties) {
        this.mailProperties = mailProperties;
    }

    @GetMapping("/mailconfig")
    public String getMailConfig() {
        return "Mail Host: " + mailProperties.getHost() + ", Port: " + mailProperties.getPort();
    }
}

Important Notes

Make sure to add @Component or use @EnableConfigurationProperties to register your properties class.

Use camelCase in Java fields and kebab-case or dot notation in properties files.

You can add validation annotations like @NotNull to check values automatically.

Summary

Custom configuration properties help organize app settings cleanly.

Use @ConfigurationProperties with a prefix to bind properties to a class.

Define values in application.properties or application.yml and inject the class where needed.

Practice

1. What is the main purpose of using @ConfigurationProperties in Spring Boot?

easy

A. To create REST endpoints automatically

B. To enable security features in the application

C. To define database connection pools

D. To bind external configuration properties to a Java class

Solution

1. Step 1: Understand the role of @ConfigurationProperties

   This annotation is used to map external configuration values (from files like application.properties or application.yml) to fields in a Java class.

2. Step 2: Compare with other options

   Options A, C, and D relate to other Spring Boot features but not to property binding.

3. Final Answer:

   To bind external configuration properties to a Java class -> Option D

4. Quick Check:

   Property binding = B [OK]

Hint: Remember: @ConfigurationProperties binds config files to classes [OK]

Common Mistakes:

• Confusing @ConfigurationProperties with @RestController
• Thinking it manages database connections
• Assuming it enables security automatically

2. Which of the following is the correct way to enable @ConfigurationProperties support in a Spring Boot application?

easy

A. Add @SpringBootApplication on the properties class

B. Add @EnableConfigurationProperties on a configuration class or main class

C. Add @RestController on the properties class

D. Add @ComponentScan on the properties class

Solution

1. Step 1: Identify how to activate configuration properties

   Spring Boot requires @EnableConfigurationProperties on a configuration or main class to enable binding of @ConfigurationProperties classes.

2. Step 2: Review other annotations

   @ComponentScan scans components but does not enable properties binding. @RestController is for web controllers. @SpringBootApplication is for main app class but does not alone enable properties binding.

3. Final Answer:

   Add @EnableConfigurationProperties on a configuration class or main class -> Option B

4. Quick Check:

   Enable config properties = D [OK]

Hint: Use @EnableConfigurationProperties to activate property binding [OK]

Common Mistakes:

• Confusing @ComponentScan with enabling properties
• Adding @RestController on properties class
• Assuming @SpringBootApplication alone enables properties

3. Given the following properties in application.yml and class, what will be the output of System.out.println(appProperties.getTimeout())?

application.yml:
app:
  timeout: 5000

Java class:
@ConfigurationProperties(prefix = "app")
public class AppProperties {
    private int timeout;
    public int getTimeout() { return timeout; }
    public void setTimeout(int timeout) { this.timeout = timeout; }
}

medium

A. null

B. 0

C. 5000

D. Compilation error

Solution

1. Step 1: Understand property binding from YAML to class

   The app.timeout property in YAML is set to 5000. The class with @ConfigurationProperties(prefix = "app") binds this value to the timeout field.

2. Step 2: Check getter method output

   Calling getTimeout() returns the bound value 5000, so the printed output is 5000.

3. Final Answer:

   5000 -> Option C

4. Quick Check:

   Bound value printed = 5000 [OK]

Hint: Property prefix matches class prefix to bind values correctly [OK]

Common Mistakes:

• Forgetting to add setter method causes binding failure
• Using wrong prefix in annotation
• Expecting null or default instead of bound value

4. Identify the error in this custom properties class that prevents property binding:

@ConfigurationProperties(prefix = "server")
public class ServerProperties {
    private String port;
    public String getPort() { return port; }
    // Missing setter method
}

medium

A. Missing setter method for 'port' prevents binding

B. Prefix 'server' is invalid and causes error

C. Class must be annotated with @Component

D. Field 'port' should be public for binding

Solution

1. Step 1: Check property binding requirements

   Spring Boot binds properties using setters. Without a setter for 'port', the value cannot be set.

2. Step 2: Validate other options

   The prefix 'server' is valid. The class does not require @Component if registered via @EnableConfigurationProperties. Fields should be private with getters/setters, not public.

3. Final Answer:

   Missing setter method for 'port' prevents binding -> Option A

4. Quick Check:

   Setter missing = binding fails [OK]

Hint: Always add setters for fields to enable binding [OK]

Common Mistakes:

• Assuming prefix must be custom or different
• Thinking @Component is always required
• Making fields public instead of using setters

5. You want to create a nested configuration property for a mail server with host and port inside application.properties like this:

mail.server.host=smtp.example.com
mail.server.port=587

Which of the following Java classes correctly models this nested property using @ConfigurationProperties?

hard

A. @ConfigurationProperties(prefix = "mail")
public class MailProperties {
private Server server;
public static class Server {
private String host;
private int port;
// getters and setters
}
// getters and setters for server
}

B. @ConfigurationProperties(prefix = "mail.server")
public class MailProperties {
private String host;
private int port;
// getters and setters
}

C. @ConfigurationProperties(prefix = "mail")
public class MailProperties {
private String host;
private int port;
// getters and setters
}

D. @ConfigurationProperties(prefix = "mail.server")
public class MailProperties {
private Server server;
public static class Server {
private String host;
private int port;
// getters and setters
}
// getters and setters for server
}

Solution

1. Step 1: Understand nested property binding

   Properties like mail.server.host require a class with prefix mail and a nested class Server to hold host and port.

2. Step 2: Analyze options

   @ConfigurationProperties(prefix = "mail") public class MailProperties { private Server server; public static class Server { private String host; private int port; // getters and setters } // getters and setters for server } correctly uses @ConfigurationProperties(prefix = "mail") with a nested Server class. @ConfigurationProperties(prefix = "mail.server") public class MailProperties { private String host; private int port; // getters and setters } flattens properties incorrectly. @ConfigurationProperties(prefix = "mail") public class MailProperties { private String host; private int port; // getters and setters } misses nesting. @ConfigurationProperties(prefix = "mail.server") public class MailProperties { private Server server; public static class Server { private String host; private int port; // getters and setters } // getters and setters for server } uses prefix mail.server but still nests, which is inconsistent.

3. Final Answer:

   @ConfigurationProperties(prefix="mail") with nested Server class -> Option A

4. Quick Check:

   Nested class with prefix mail = A [OK]

Hint: Use nested classes for nested property groups [OK]

Common Mistakes:

• Using wrong prefix for nested properties
• Flattening nested properties into one class
• Missing getters/setters in nested class