What are the ways to configure an application in Spring?

There are several ways:

1. Using XML files: Traditional method where configuration is described in XML files.

   <!-- applicationContext.xml -->
   <beans xmlns="http://www.springframework.org/schema/beans"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://www.springframework.org/schema/beans
          http://www.springframework.org/schema/beans/spring-beans.xsd">
   
       <bean id="myService" class="com.example.MyService"/>
   
   </beans>
   

2. JavaConfig: Configuration is described in Java classes using annotations.

   // AppConfig.java
   package com.example.config;
   
   import com.example.MyService;
   import org.springframework.context.annotation.Bean;
   import org.springframework.context.annotation.Configuration;
   
   @Configuration // Indicates this class is a configuration class
   public class AppConfig {
   
       @Bean // Declares a bean
       public MyService myService() {
           return new MyService();
       }
   }
   

3. Annotations: Bean configuration is done directly in classes using annotations like @Component, @Service, @Repository, @Controller.

   // MyService.java
   package com.example;
   
   import org.springframework.stereotype.Service; // Annotation indicating Spring should create a bean for this class
   
   @Service
   public class MyService {
       // Service implementation
   }
   

4. @PropertySource and Environment: For reading properties from external files or system variables.

   // AppConfig.java
   package com.example.config;
   
   import org.springframework.beans.factory.annotation.Value;
   import org.springframework.context.annotation.Configuration;
   import org.springframework.context.annotation.PropertySource; // Indicates properties file
   import org.springframework.core.env.Environment; // Interface for environment variables
   
   @Configuration
   @PropertySource("classpath:application.properties") // Load properties from application.properties
   public class AppConfig {
   
       private final Environment env;
   
       public AppConfig(Environment env) {
           this.env = env;
       }
   
       @Value("${my.property}") // Inject property value
       private String myProperty;
   
       public void printProperty() {
           System.out.println("Value from Environment: " + env.getProperty("another.property"));
           System.out.println("Value from @Value: " + myProperty);
       }
   }
   

   Content of application.properties:

   # application.properties
   my.property=someValue
   another.property=anotherValue
   

5. Spring Boot Autoconfiguration: Automatic setup of most components based on dependencies present in classpath.

The most common and recommended approach in modern Spring and Spring Boot applications is to use JavaConfig and annotations, often combined with @PropertySource for external settings. XML is still used in legacy systems.