How do you implement custom property sources in Spring?

Table of Contents

• Introduction
• Implementing Custom Property Sources in Spring
  • 1. Using **PropertySource** for Custom Property Sources
    • Steps to Implement a Custom Property Source:
    • Example:
    • Example:
    • Example:
  • 2. Using **@PropertySource** to Add Custom Property Files
    • Example:
  • 3. Using External Configuration Sources with **@ConfigurationProperties**
    • Example:
  • 4. Using **@EnableConfigurationProperties** for Custom Property Binding
    • Example:
• Conclusion

Introduction

In Spring, managing application properties and configurations is typically handled using application.properties or application.yml files. However, there are scenarios where you might want to load properties from custom sources, such as external files, databases, or APIs. To handle such cases, Spring provides a powerful feature called Custom Property Sources. These allow you to define and load properties from non-standard locations or dynamically generated sources.

This guide will walk you through how to implement custom property sources in Spring, enabling you to manage configuration values from different environments or external systems.

Implementing Custom Property Sources in Spring

1. Using **PropertySource** for Custom Property Sources

In Spring, the PropertySource class is used to represent a source of properties. You can create custom property sources that retrieve properties from external locations, such as custom files, databases, or even remote services.

Steps to Implement a Custom Property Source:

1. Create a Custom PropertySource Implementation

   The PropertySource class can be extended to implement custom behavior for loading properties from external sources. This involves overriding the getProperty(String name) method, where you can define how the properties are retrieved.

Example:

In this example, the CustomPropertySource class retrieves a property called custom.property and returns its value ("Custom Value").

2. Add the Custom PropertySource to the Environment

   Once the custom PropertySource is implemented, you can add it to the Spring Environment. This allows you to use it alongside the default property sources like application.properties.

   You can inject the Environment and add your custom property source programmatically using getPropertySources().

Example:

In this example, the CustomPropertySourceConfig class injects the AbstractEnvironment and adds the CustomPropertySource to the PropertySources. This ensures that your custom property source is available to the Spring application context.

3. Access the Custom Property Source

   Now that the custom property source has been added, you can retrieve the custom properties in your Spring beans using @Value or Environment.

Example:

In this example, the CustomPropertyService retrieves the value of custom.property from the environment and prints it to the console.

2. Using **@PropertySource** to Add Custom Property Files

In addition to implementing custom PropertySource classes, you can use the @PropertySource annotation to load properties from custom external property files (e.g., custom-config.properties) into the Spring environment.

Example:

1. Create a custom property file (**custom-config.properties**):

2. Use **@PropertySource** to Load the Property File:

3. Access the Properties from the Custom Property File:

In this example, the PropertySourceConfig class loads properties from custom-config.properties using @PropertySource. The CustomPropertyService class uses @Value to inject the custom.property from the file and print it.

3. Using External Configuration Sources with **@ConfigurationProperties**

If you want to bind custom properties to a bean, you can use @ConfigurationProperties. This is useful for grouping related properties and injecting them into a Spring-managed bean.

Example:

1. Define a custom property file (**external-config.properties**):

2. Create a POJO to Bind Properties:

3. Load the Properties Using **@PropertySource**:

4. Use the Bound Properties in Your Service:

In this example, the CustomAppConfig class binds properties from external-config.properties to its fields. The CustomAppService class prints the values of the custom.app.name and custom.app.version properties.

4. Using **@EnableConfigurationProperties** for Custom Property Binding

If you want to enable the binding of @ConfigurationProperties beans, you can use the @EnableConfigurationProperties annotation.

Example:

By enabling @EnableConfigurationProperties, you ensure that the properties defined in external-config.properties are automatically injected into the CustomAppConfig bean.

Conclusion

Implementing custom property sources in Spring allows you to load properties from external sources, such as custom files, databases, or dynamic systems. Using the PropertySource abstraction, you can extend Spring’s environment to include properties from any location, enabling greater flexibility in managing application configurations. Whether you use custom PropertySource implementations, @PropertySource annotations, or @ConfigurationProperties for property binding, Spring makes it easy to manage your application's configuration dynamically. Custom property sources allow your application to scale and adapt to different environments, external systems, and changing requirements.