目标

让不同日志实现（比如 Logback、Log4j2）像插件一样自动加载

具体实现

1. 定义日志接口（服务提供者）

   public interface LogService {
       void log(String message);
   }

2. 实现两个日志类（模拟不同日志框架）同时结合 @ConditionalOnProperty 选择实现类(如果在下面的自动装配类的构造方法上已经有了

   @ConditionalOnProperty,则这里就不需要了)

   @Service
   @ConditionalOnProperty(name = "logging.plugin", havingValue = "logback")
   public class LogbackService implements LogService {
       public void log(String msg) {
           System.out.println("[Logback] " + msg);
       }
   }
   
   @Service
   @ConditionalOnProperty(name = "logging.plugin", havingValue = "log4j2")
   public class Log4jService implements LogService {
       public void log(String msg) {
           System.out.println("[Log4j2] " + msg);
       }
   }
   

3. 自定义 Starter 模块,结构如下：

   logging-starter/
   ├── META-INF/spring.factories
   ├── src/main/java/com/example/logging/
   │   ├── LogService.java
   │   ├── LogbackService.java
   │   └── LogAutoConfiguration.java
   

4. 配置自动装配类：

   @Configuration
   public class LogAutoConfiguration {
   
       @Bean
       @ConditionalOnProperty(name = "logging.plugin", havingValue = "logback")
       public LogService logback() {
           return new LogbackService();
       }
   
       @Bean
       @ConditionalOnProperty(name = "logging.plugin", havingValue = "log4j2")
       public LogService log4j2() {
           return new Log4jService();
       }
   }
   

5. 在 spring.factories 文件中注册：

   org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
   com.example.logging.LogAutoConfiguration
   

6. 提供一个 SPI 插件接口：

   public interface LogPluginProvider {
       LogService getLogService();
   }
   

7. 第三方只需实现 LogPluginProvider，并在 META-INF/services 中注册，就能注入自己的日志框架，你通过 ServiceLoader 把它加载进来：(这一段逻辑可以加到自动装配类的构造方法中)

   ServiceLoader<LogPluginProvider> loader = ServiceLoader.load(LogPluginProvider.class);
   for (LogPluginProvider provider : loader) {
       LogService service = provider.getLogService();
       service.log("来自插件的日志");
   }
   

8. 支持配置文件动态启用某个日志实现,在 application.yml 中添加：

   logging.plugin: logback