How do you create a conditional bean in Spring?

Table of Contents

Introduction

In Spring Framework, beans are typically registered in the Spring ApplicationContext during the application startup. However, there may be cases where you want to register or create a bean conditionally, based on certain criteria such as the environment, application properties, or specific conditions that are met at runtime.

This is where Spring's conditional bean registration comes into play. Using the **@Conditional** annotation and related classes, you can define conditions under which a bean should be registered, providing more control and flexibility over the application's behavior.

In this guide, we'll explain how to create conditional beans in Spring and show different ways to implement them using the **@Conditional** annotation.

1. Understanding the **@Conditional** Annotation

The **@Conditional** annotation in Spring allows you to create beans only when certain conditions are met. You can define custom conditions or use pre-built conditions provided by the Spring framework.

The **@Conditional** annotation can be applied to configuration classes or methods annotated with **@Bean**. The condition logic is implemented using a **Condition** interface, which allows you to specify whether or not a bean should be created.

Basic Syntax of @Conditional:

Here, **MyCondition.class** is a class that implements the **Condition** interface, where the logic to decide whether the bean should be created or not is written.

2. Creating a Custom **Condition** Implementation

To use the **@Conditional** annotation, you must create a custom condition by implementing the **Condition** interface. This interface has a single method, **matches()**, that returns a boolean indicating whether the condition is met.

Example: Custom Condition Implementation

Let's create a condition that only registers a bean if a specific system property is set.

In this example:

  • The **matches()** method checks if the system property **my.custom.property** is set to "true".
  • If this condition is met, the bean will be registered; otherwise, it won't.

Using the Condition in Configuration:

Now, the **MyService** bean will only be registered if the **my.custom.property** system property is set to "true".

3. Using Spring’s Predefined Conditions

Spring provides several built-in conditions that you can use directly, so you don't have to create a custom **Condition** class. Some commonly used predefined conditions include:

  • **@Profile**: Registers beans only when a certain profile is active.
  • **@ConditionalOnProperty**: Registers beans when a specific property is present or has a certain value.
  • **@ConditionalOnClass**: Registers beans when a specific class is available on the classpath.
  • **@ConditionalOnMissingBean**: Registers beans only if a specific bean is not already registered.

Let's explore some of these built-in conditions in detail.

4. Using **@ConditionalOnProperty**

The **@ConditionalOnProperty** annotation registers a bean if a specific property is present and matches a defined value. This is especially useful for enabling or disabling beans based on application properties.

Example: Conditional Bean with Property

In this example:

  • The **myFeatureService()** bean will only be registered if the **myapp.featureEnabled** property is present in the application’s **application.properties** file and has a value of "true".

This can be useful in enabling/disabling features dynamically by changing the configuration without modifying code.

5. Using **@ConditionalOnClass**

The **@ConditionalOnClass** annotation allows beans to be registered only if a certain class is present on the classpath. This can be useful when creating beans that depend on optional libraries.

Example: Conditional Bean Based on Class Presence

In this example:

  • The **externalLibraryService()** bean will only be registered if the **com.example.ExternalLibrary** class is available on the classpath. This ensures that the bean is only created when the external library is present.

6. Using **@ConditionalOnMissingBean**

The **@ConditionalOnMissingBean** annotation allows you to define a bean that will only be registered if a specific bean of the same type is not already defined in the application context. This can be helpful when you want to provide a default implementation that can be overridden by a user-defined bean.

Example: Conditional Bean Based on Missing Bean

In this example:

  • The **myService()** bean will only be registered if no other **MyService** bean is already defined in the application context.

7. Combining Multiple Conditions

You can also combine multiple conditions using **@Conditional** with custom logic to create more complex conditional scenarios. However, Spring doesn't provide a built-in annotation to combine conditions directly, so you would need to implement a custom **Condition** that evaluates multiple conditions.

Example: Combining Multiple Conditions

In this case, both conditions must be true for the bean to be created: the system property must be set, and the class must be available on the classpath.

8. Conclusion

Creating conditional beans in Spring provides a powerful way to control which beans are registered in the Spring ApplicationContext based on various runtime conditions, configuration properties, or the environment. The **@Conditional** annotation and its variants like **@ConditionalOnProperty**, **@ConditionalOnClass**, and **@ConditionalOnMissingBean** offer great flexibility for enabling or disabling beans depending on specific conditions.

By using conditional bean registration, you can create more modular, flexible, and environment-aware Spring applications that can adapt based on the configuration or classpath available at runtime.

This approach improves the maintainability of your Spring applications, especially when dealing with different environments, feature toggles, or optional dependencies.

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