SpringBoot关于自动配置类加载的学习笔记

本文记录一下学习SpringBoot是如何加载自动配置类的
上文写到，SpringBoot会根据pom.xml的配置，自动加载需要的自动配置类。但是，自动配置类里面，都有什么？？？
随意点开一个来看看 例如RedisAutoConfiguration

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.springframework.boot.autoconfigure.data.redis;

import java.net.UnknownHostException;
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;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import org.springframework.data.redis.connection.RedisConnectionFactory;
import org.springframework.data.redis.core.RedisOperations;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.data.redis.core.StringRedisTemplate;

@Configuration
@ConditionalOnClass({RedisOperations.class})
@EnableConfigurationProperties({RedisProperties.class})
@Import({LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class})
public class RedisAutoConfiguration {
    public RedisAutoConfiguration() {
    }

    @Bean
    @ConditionalOnMissingBean(
        name = {"redisTemplate"}
    )
    public RedisTemplate redisTemplate(RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
        RedisTemplate template = new RedisTemplate();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }

    @Bean
    @ConditionalOnMissingBean
    public StringRedisTemplate stringRedisTemplate(RedisConnectionFactory redisConnectionFactory) throws UnknownHostException {
        StringRedisTemplate template = new StringRedisTemplate();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }
}

可以看到，类头文件有四个注解
@Configuration表明为配置类。。见得太多了
@Import 加载其他配置类，这个也不是本次学习的重点
@ConditionalOnClass 检测类是否存在，若存在才加载配置，若不存在则不加载。。（所以正常SpringBoot是不会有ClassNotFound的 学习了）
@EnableConfigurationProperties({RedisProperties.class}) 这个是本文学习的目标。此注解会加载配置的默认属性。从名称上看，RedisProperties.class 表明Redis的参数类。我们进入此类看看

@ConfigurationProperties(
    prefix = "spring.redis"
)
public class RedisProperties {
    private int database = 0;
    private String url;
    private String host = "localhost";
    private String password;
    private int port = 6379;
    private boolean ssl;
    private Duration timeout;
    private RedisProperties.Sentinel sentinel;
    private RedisProperties.Cluster cluster;
    private final RedisProperties.Jedis jedis = new RedisProperties.Jedis();
    private final RedisProperties.Lettuce lettuce = new RedisProperties.Lettuce();
    .......

可以看到，这些都是我们在application文件配置的属性，而prefix则是配置的前缀。像host，database，port等，都已经有了默认的值，所以SpringBoot是拥有默认配置的。
@ConfigurationProperties也可以看出是配置对应application的相应前缀
再打开一个类进行查看
例如HttpencodingAutoConfiguration

@Configuration
@EnableConfigurationProperties({HttpProperties.class})
@ConditionalOnWebApplication(
    type = Type.SERVLET
)
@ConditionalOnClass({CharacterEncodingFilter.class})
@ConditionalOnProperty(
    prefix = "spring.http.encoding",
    value = {"enabled"},
    matchIfMissing = true
)

发现头部又多了几个注解
@Codition开头的注解，基本都是验证注解。
例如
@ConditionalOnWebApplication:判断是否Web应用
@ConditionalOnClass：判断是否存在这个类
@ConditionalOnProperty：判断文件中是否存在某些配置

理解他是如何自动装配的，需要使用的时候便会方便许多

视频学习地址：https://ke.qq.com/webcourse/index.html#cid=179440&term_id=100211540&taid=2691127723605232&vid=h1428ktjxpn