Spring Boot @ConditionalOnClass Example

1. Introduction

Spring Boot provides a powerful mechanism to conditionally enable features through its @ConditionalOnClass annotation. This is particularly useful when dealing with optional dependencies. In this real-time example, we'll see how we can use @ConditionalOnClass to optionally set up a cache manager only when the EhCache library is present in the classpath.

Key Points:

1. @ConditionalOnClass ensures bean creation is dependent on the presence of specific classes.

2. It helps in avoiding ClassNotFoundException by not attempting to load beans that require unavailable classes.

3. This annotation is essential for creating auto-configuration classes that adapt to the environment.

4. It improves application startup time by not loading unnecessary configurations.

5. @ConditionalOnClass can be used with third-party libraries to conditionally enable certain features if that library is included.

2. Implementation Steps

1. Add the EhCache dependency in your project's build configuration file (optional).

2. Create a configuration class with a cache manager bean.

3. Annotate the cache manager bean method with @ConditionalOnClass.

4. Test the application with and without the EhCache library in the classpath.

3. Implementation Example

// Step 1: Create a configuration class that declares a cache manager bean
@Configuration
public class CacheConfiguration {

    // Step 2: Conditionally define a bean for EhCache's CacheManager
    // This bean will only be loaded if EhCache is present on the classpath
    @Bean
    @ConditionalOnClass(net.sf.ehcache.CacheManager.class)
    public net.sf.ehcache.CacheManager ehCacheManager() {
        // Step 3: Configure and return the EhCache CacheManager
        return net.sf.ehcache.CacheManager.newInstance();
    }
}

// Step 4: Create a Spring Boot application class
@SpringBootApplication
public class CacheApplication {

    public static void main(String[] args) {
        SpringApplication.run(CacheApplication.class, args);
        // The application will start with or without EhCache in the classpath
    }
}

Output:

// No explicit output will be displayed. The presence of the CacheManager bean in the Spring context
// is conditional on the EhCache class being available in the classpath.

Explanation:

1. @Configuration: Marks the class as a source of bean definitions.

2. @Bean: Indicates that a method produces a bean to be managed by the Spring container.

3. @ConditionalOnClass: The ehCacheManager bean will only be instantiated if the net.sf.ehcache.CacheManager class is found in the classpath.

4. CacheManager.newInstance(): This is a method from EhCache that creates a new instance of a cache manager.

5. @SpringBootApplication: Marks the main application class, which also serves as a configuration class in this context.