Spring boot 官网学习笔记 - Configuration Class（@import）

1. 推荐使用 Java-based configuration ，也可以使用xml
2. we generally recommend that your primary source be a single @Configuration class. Usually the class that defines the main method is a good candidate as the primary @Configuration
3. Importing Additional Configuration Classes

   1. You need not put all your @Configuration into a single class. 
      The @Import annotation can be used to import additional configuration classes. 
      Alternatively, you can use @ComponentScan to automatically pick up 
      all Spring components, including @Configuration classes.

   2. 示例

      1. Car.java
         package javabeat.net.basic;
         public interface Car {
             public void print();
         }
         
         Toyota.java
         package javabeat.net.basic;
         import org.springframework.stereotype.Component;
         @Component
         public class Toyota implements Car{
             public void print(){
                 System.out.println("I am Toyota");
             }
         }
         
         Volkswagen.java
         package javabeat.net.basic;
         import org.springframework.stereotype.Component;
         @Component
         public class Volkswagen implements Car{
             public void print(){
                 System.out.println("I am Volkswagen");
             }
         }
         
         JavaConfigA.java
         package javabeat.net.basic;
         
         import org.springframework.context.annotation.Bean;
         import org.springframework.context.annotation.Configuration;
         @Configuration
         public class JavaConfigA {
             @Bean(name="volkswagen")
             public Car getVolkswagen(){
                 return new Volkswagen();
             }
         }
         
         JavaConfigB.java
         package javabeat.net.basic;
         import org.springframework.context.annotation.Bean;
         import org.springframework.context.annotation.Configuration;
         @Configuration
         public class JavaConfigB {
             @Bean(name="toyota")
             public Car getToyota(){
                 return new Toyota();
             }
         }
         
         ParentConfig.java
         package javabeat.net.basic;
         import org.springframework.context.annotation.Configuration;
         import org.springframework.context.annotation.Import;
         @Configuration
         @Import({JavaConfigA.class,JavaConfigB.class})
         public class ParentConfig {
             //Any other bean definitions
         }
         
         ContextLoader.java
          package javabeat.net.basic;
         import org.springframework.context.annotation.AnnotationConfigApplicationContext;
         public class ContextLoader {
             public static void main (String args[]){
                 AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(ParentConfig.class);
                 Car car = (Toyota)context.getBean("toyota");
                 car.print();
                 car = (Volkswagen)context.getBean("volkswagen");
                 car.print();
                 context.close();
             }
         }

      2. If you run the above example,the printed output will be,
         
4. Importing XML Configuration

   1. If you absolutely must use XML based configuration,
       we recommend that you still start with a @Configuration class. 
      You can then use an @ImportResource annotation to load XML configuration files.