SpringBoot为什么要新建MessageSource Bean?

本篇我们一起来看一下SpringBoot为什么要新建MessageSource Bean?这里我总结了以下内容：

* AbstractApplicationContext 的实现决定了 MessageSource 內建实现
* Spring Boot 通过外部化配置简化 MessageSource Bean 构建
* Spring Boot 基于 Bean Validation 校验非常普遍

接下来我们就通过实战的方式一起实践一下。

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package org.eleven.thinking.in.spring.i18n;

import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.WebApplicationType;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.MessageSource;
import org.springframework.context.annotation.Bean;
import org.springframework.context.support.AbstractApplicationContext;
import org.springframework.context.support.ReloadableResourceBundleMessageSource;

/**
 * Spring Boot 场景下自定义 {@link MessageSource} Bean
 *
 * @author <a href="mailto: eleven.hm@vip.163.com">eleven</a>
 * @see MessageSource
 * @see MessageSourceAutoConfiguration
 * @see ReloadableResourceBundleMessageSource
 * @since
 */
@EnableAutoConfiguration
public class CustomizedMessageSourceBeanDemo { // @Configuration Class


    /**
     * 在 Spring Boot 场景中，Primary Configuration Sources(Classes) 高于 *AutoConfiguration
     * @return
     */
    @Bean(AbstractApplicationContext.MESSAGE_SOURCE_BEAN_NAME)
    public MessageSource messageSource() {
        return new ReloadableResourceBundleMessageSource();
    }

    public static void main(String[] args) {

        ConfigurableApplicationContext applicationContext =
                // Primary Configuration Class
                new SpringApplicationBuilder(CustomizedMessageSourceBeanDemo.class)
                        .web(WebApplicationType.NONE)
                        .run(args);

        ConfigurableListableBeanFactory beanFactory = applicationContext.getBeanFactory();

        if (beanFactory.containsBean(AbstractApplicationContext.MESSAGE_SOURCE_BEAN_NAME)) {
            // 查找 MessageSource 的 BeanDefinition
            System.out.println(beanFactory.getBeanDefinition(AbstractApplicationContext.MESSAGE_SOURCE_BEAN_NAME));
            // 查找 MessageSource Bean
            MessageSource messageSource = applicationContext.getBean(AbstractApplicationContext.MESSAGE_SOURCE_BEAN_NAME, MessageSource.class);
            System.out.println(messageSource);
        }

        // 关闭应用上下文
        applicationContext.close();
    }
}