1. Introduction
The @ConditionalOnWebApplication annotation is used to conditionally apply configurations only when the application is a web application. This is useful when you have configurations that should only be activated in a web context, such as controllers, view resolvers, or web-specific beans. Let's explore this annotation through a practical example.
Key Points:
1. @ConditionalOnWebApplication is used to conditionally enable beans only when the application is running in a web environment.
2. It helps in creating auto-configuration that's specific to web applications without impacting standalone applications.
3. The annotation can specify whether the condition should apply to a servlet-based web application or a reactive web application.
4. It's typically used in @Configuration classes inside auto-configure modules.
5. This conditional is evaluated in the auto-configuration phase of Spring Boot's startup process.
2. Implementation Steps
1. Define a bean that should only be loaded in a web application context.
2. Create a configuration class and use the @ConditionalOnWebApplication annotation on the bean.
3. Develop a Spring Boot application with web capabilities to demonstrate the configuration.
4. Run the application and observe that the bean is only created in a web application context.
3. Implementation Example
// Step 1: Create a web-specific bean, for instance, a controller or a configuration for web MVC
@RestController
class HelloController {
@GetMapping("/")
public String hello() {
return "Hello, Web World!";
}
}
// Step 2: Create a configuration class that declares the web-specific beans
@Configuration
public class WebAppConfig {
// This bean will only be created if the application is a web application
@Bean
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
public HelloController helloController() {
return new HelloController();
}
}
// Step 3: Build the main application class with web capabilities
@SpringBootApplication
public class WebApplicationExample {
public static void main(String[] args) {
SpringApplication.run(WebApplicationExample.class, args);
}
}
Output:
// When accessing the root URL of the web application, the output will be: Hello, Web World!
Explanation:
1. HelloController: A simple REST controller that is part of the web layer and should only be available in a web application context.
2. @ConditionalOnWebApplication: This annotation ensures that the helloController bean is only created if the application is running as a web application, specifically a servlet-based one.
3. type = ConditionalOnWebApplication.Type.SERVLET: Specifies that the condition should only apply to servlet-based web applications.
4. WebAppConfig: A configuration class that declares a bean with the @ConditionalOnWebApplication annotation.
5. @Configuration: Marks the class as a source of bean definitions.
6. @SpringBootApplication: Used to mark the main application class, which triggers auto-configuration, component scanning, and, in this case, starts up a web application context.
7. SpringApplication.run(): Boots the Spring Boot application, which in this case will start up with web capabilities due to the presence of embedded servlet container dependencies (like Tomcat).