自动配置原理：谁在你一行没写时替你做了配置

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-redis</artifactId>
</dependency>

spring:
  data:
    redis:
      host: localhost
      port: 6379

package com.example.firstapp.controller;

import org.springframework.data.redis.core.StringRedisTemplate;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class PingController {

    private final StringRedisTemplate redis;

    public PingController(StringRedisTemplate redis) {
        this.redis = redis;
    }

    @GetMapping("/ping")
    public String ping(String key) {
        redis.opsForValue().set(key, "pong");
        return redis.opsForValue().get(key);
    }
}

@SpringBootApplication
  └─ @EnableAutoConfiguration
       └─ @Import(AutoConfigurationImportSelector.class)
            └─ 选择器读取 META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports
                 └─ 得到一百多个候选自动配置类（每行一个全限定类名）
                      └─ 条件注解逐个裁决，幸存者以 @Configuration 身份注册进容器
                           └─ RedisAutoConfiguration 提供了 redisTemplate 与 stringRedisTemplate

org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration
……（一百多个候选，数量随版本浮动）

@AutoConfiguration
@ConditionalOnClass(RedisOperations.class)          // 没引 redis 客户端就整个跳过
public class RedisAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean(name = "redisTemplate")
    public RedisTemplate<Object, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory) {
        RedisTemplate<Object, Object> template = new RedisTemplate<>();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }

    @Bean
    @ConditionalOnMissingBean
    public StringRedisTemplate stringRedisTemplate(RedisConnectionFactory redisConnectionFactory) {
        return new StringRedisTemplate(redisConnectionFactory);
    }
}

mvn spring-boot:run -Dspring-boot.run.arguments=--debug
# 等价姿势：java -jar app.jar --debug，或 application.yml 里写 debug: true

============================
CONDITIONS EVALUATION REPORT
============================

Positive matches:
-----------------

   RedisAutoConfiguration matched:
      - @ConditionalOnClass found required class
        'org.springframework.data.redis.core.RedisOperations' (OnClassCondition)

Negative matches:
-----------------

   RabbitAutoConfiguration:
      Did not match:
         - @ConditionalOnClass did not find required class
           'com.rabbitmq.client.Channel' (OnClassCondition)

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

management:
  endpoints:
    web:
      exposure:
        include: conditions,health

curl http://localhost:8080/actuator/conditions

package com.example.greeting;

public interface GreetingService {

    String greet(String name);
}

package com.example.greeting;

public class DefaultGreetingService implements GreetingService {

    private final String prefix;

    public DefaultGreetingService(String prefix) {
        this.prefix = prefix;
    }

    @Override
    public String greet(String name) {
        return prefix + ", " + name;
    }
}

package com.example.greeting;

import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties("greeting")
public class GreetingProperties {

    private String prefix = "hello";

    public String getPrefix() { return this.prefix; }
    public void setPrefix(String prefix) { this.prefix = prefix; }
}

package com.example.greeting;

import org.springframework.boot.autoconfigure.AutoConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;

@AutoConfiguration
@ConditionalOnClass(GreetingService.class)
@EnableConfigurationProperties(GreetingProperties.class)
public class GreetingAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean(GreetingService.class)
    public GreetingService greetingService(GreetingProperties properties) {
        return new DefaultGreetingService(properties.getPrefix());
    }
}

META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports

com.example.greeting.GreetingAutoConfiguration

@Bean
public GreetingService greetingService(GreetingProperties properties) {
    return name -> "hi, " + name;    // 你一声明，自动配置里的默认实现自动让位
}