SpringBoot实现自动配置的方式详解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { /* ... */ })
public @interface SpringBootApplication {
    // 组合了 @Configuration、@EnableAutoConfiguration、@ComponentScan
}

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ DataSource.class, EmbeddedDatabaseType.class })
@ConditionalOnMissingBean(type = "io.r2dbc.spi.ConnectionFactory")
@EnableConfigurationProperties(DataSourceProperties.class)
@Import({ DataSourcePoolMetadataProvidersConfiguration.class, DataSourceInitializationConfiguration.class })
public class DataSourceAutoConfiguration {
    // 当类路径存在 DataSource 且未配置 R2DBC 时生效
}

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(ServerProperties.class)
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
@ConditionalOnClass(CharacterEncodingFilter.class)
@ConditionalOnProperty(prefix = "server.servlet.encoding", value = "enabled", matchIfMissing = true)
public class HttpEncodingAutoConfiguration {

    private final Encoding properties;

    public HttpEncodingAutoConfiguration(ServerProperties properties) {
        this.properties = properties.getServlet().getEncoding();
    }

    @Bean
    @ConditionalOnMissingBean
    public CharacterEncodingFilter characterEncodingFilter() {
        // 基于配置创建过滤器
        CharacterEncodingFilter filter = new CharacterEncodingFilter();
        filter.setEncoding(this.properties.getCharset().name());
        filter.setForceRequestEncoding(this.properties.shouldForce(Encoding.Type.REQUEST));
        filter.setForceResponseEncoding(this.properties.shouldForce(Encoding.Type.RESPONSE));
        return filter;
    }
}

server:
  port: 8080
  servlet:
    encoding:
      charset: UTF-8
      enabled: true

@ConfigurationProperties(prefix = "server")
public class ServerProperties {
    private Integer port;
    private Servlet servlet;
    
    public static class Servlet {
        private final Encoding encoding = new Encoding();
        // getters/setters
    }
}

debug=true

============================
CONDITION EVALUATION REPORT
============================
Positive matches:
-----------------
   DataSourceAutoConfiguration matched:
      - @ConditionalOnClass found required classes 'javax.sql.DataSource', 'org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType' (OnClassCondition)

Negative matches:
-----------------
   ActiveMQAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition)

@SpringBootApplication(exclude = {DataSourceAutoConfiguration.class})

spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration

@Configuration
@ConditionalOnClass(MyService.class)
@EnableConfigurationProperties(MyServiceProperties.class)
public class MyServiceAutoConfiguration {
    @Bean
    @ConditionalOnMissingBean
    public MyService myService(MyServiceProperties properties) {
        return new MyService(properties.getEndpoint());
    }
}

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.example.MyServiceAutoConfiguration

public class OnProductionEnvironmentCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return "prod".equals(context.getEnvironment().getProperty("app.env"));
    }
}

@Configuration
@Conditional(OnProductionEnvironmentCondition.class)
public class ProductionConfig {
    // 生产环境专用配置
}