@AutoConfiguration

读取所有jar包下的 /META-INF/spring.factories 并追加到一个 LinkedMultiValueMap 中。每一个url中记录的文件路径如下：

file:/C:/Users/wangchao/apache-maven-3.5.0/repo/com/baomidou/mybatis-plus-boot-starter/3.5.1/mybatis-plus-boot-starter-3.5.1.jar!/META-INF/spring.factories

按照如下路径查看

// 1. @EnableAutoConfiguration

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@AutoConfigurationPackage

@Import(AutoConfigurationImportSelector.class)

public @interface EnableAutoConfiguration {

}

// 2. AutoConfigurationImportSelector.class#selectImports()

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,

		ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {

	@Override

	public String[] selectImports(AnnotationMetadata annotationMetadata) {

		if (!isEnabled(annotationMetadata)) {

			return NO_IMPORTS;

		}

		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);

		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());

	}

}

// 3. AutoConfigurationImportSelector.class#getAutoConfigurationEntry()

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {

	if (!isEnabled(annotationMetadata)) {

		return EMPTY_ENTRY;

	}

	AnnotationAttributes attributes = getAttributes(annotationMetadata);

	List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);

	configurations = removeDuplicates(configurations);

	Set<String> exclusions = getExclusions(annotationMetadata, attributes);

	checkExcludedClasses(configurations, exclusions);

	configurations.removeAll(exclusions);

	configurations = getConfigurationClassFilter().filter(configurations);

	fireAutoConfigurationImportEvents(configurations, exclusions);

	return new AutoConfigurationEntry(configurations, exclusions);

}

// 4. AutoConfigurationImportSelector.class#getCandidateConfigurations()

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {

	List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),

			getBeanClassLoader());

	Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "

			+ "are using a custom packaging, make sure that file is correct.");

	return configurations;

}

// 5. org.springframework.core.io.support.SpringFactoriesLoader#loadFactoryNames()

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {

	String factoryTypeName = factoryType.getName();

	return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());

}

最终使用 loadSpringFactories(@Nullable ClassLoader classLoader) 方法读取所有配置文件。

// 6. org.springframework.core.io.support.SpringFactoriesLoader#loadSpringFactories()

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {

		MultiValueMap<String, String> result = cache.get(classLoader);

		if (result != null) {

			return result;

		}

		try {

			Enumeration<URL> urls = (classLoader != null ?

					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :

					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));

			result = new LinkedMultiValueMap<>();

			while (urls.hasMoreElements()) {

			    // 读取所有jar包下的 /META-INF/spring.factories

				URL url = urls.nextElement();

				UrlResource resource = new UrlResource(url);

				Properties properties = PropertiesLoaderUtils.loadProperties(resource);

				for (Map.Entry<?, ?> entry : properties.entrySet()) {

					String factoryTypeName = ((String) entry.getKey()).trim();

					for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {

						result.add(factoryTypeName, factoryImplementationName.trim());

					}

				}

			}

			cache.put(classLoader, result);

			return result;

		}

		catch (IOException ex) {

			throw new IllegalArgumentException("Unable to load factories from location [" +

					FACTORIES_RESOURCE_LOCATION + "]", ex);

		}

	}

tomcat的自动配置内置于springboot的autoconfiguration中。参考tomcat的自动配置 https://www.cnblogs.com/zhaokejin/p/15626392.html

mybatis-plus的配置没有被springboot包括。因此mybatis-stater中包含一个包mybatis-spring-boot-autoconfigure，这其中配置了需要自动配置的类。

因此我们也可以在自己的项目下新建 /META-INF/spring.factories ，并配置自动配置类。

@Import

@Import 用于导入配置类或需要前置加载的类。被导入的类会注册为Bean，可直接作为Bean被引用。它的 value 属性可以支持三种类型：

被 @Configuration 修饰的配置类、或普通类（4.2版本之后可以）。
ImportSelector 接口的实现。
ImportBeanDefinitionRegistrar 接口的实现。

@Import 的配置

@Configuration

@Import(value = {TestA.class, TestB.class, MyImportSelector.class, MyImportBeanDefinitionRegistrar.class})

public class ConfigurationTest {

}

导入一个普通类

package com.example.ssmpdemo.entity;

public class TestA {

    public void fun(){

        System.out.println("testA");

    }

}

导入一个配置类

package com.example.ssmpdemo.entity;

import org.springframework.context.annotation.Configuration;

@Configuration

public class TestB {

    public void fun(){

        System.out.println("testB");

    }

}

通过实现 ImportSelector 接口

import org.springframework.context.annotation.ImportSelector;

import org.springframework.core.type.AnnotationMetadata;

public class MyImportSelector implements ImportSelector {

    @Override

    public String[] selectImports(AnnotationMetadata importingClassMetadata) {

        return new String[]{"com.example.ssmpdemo.entity.TestC"};

    }

}

通过重写 ImportBeanDefinitionRegistrar 的 registerBeanDefinitions 方法。

import com.example.ssmpdemo.entity.TestD;

import org.springframework.beans.factory.support.BeanDefinitionRegistry;

import org.springframework.beans.factory.support.RootBeanDefinition;

import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;

import org.springframework.core.type.AnnotationMetadata;

public class MyImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {

    @Override

    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {

        RootBeanDefinition root = new RootBeanDefinition(TestD.class);

        registry.registerBeanDefinition("testD", root);

    }

}

@ConfigurationProperties

支持常见的下划线、中划线和驼峰的转换。支持对象引导。比如：user.friend.name 代表的是user对象中的friend对象中的name
需要有set()方法
只添加 @ConfigurationProperties(prefix = "xxx") 并不会生效，需要配合 @Configuration 让容器识别到。
@EnableConfigurationProperties(value = ConfigData.class ) 会将value中指定的类注册为Bean，可直接用 @AutoWired 引用。

@ConfigurationProperties(prefix = "xxx")

ConfigData

package com.example.ssmpdemo.entity;

import lombok.Data;

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

/**

 * 用来记录Configuration的数据

 * @author wangc

 */

@Data

@ConfigurationProperties(value = "spring.datasource.druid")

public class ConfigData {

    private String driverClassName;

    private String url;

    private String username;

    private String password;

}

# 对应的yml文件

spring:

  datasource:

    druid:

      driver-class-name: com.mysql.cj.jdbc.Driver

      url: jdbc:mysql://localhost:5506/ssmpdemo?serverTimezone=UTC

      username: root

      password: xxxx

使用@EnableConfigurationProperties(JDBCProperties.class) 将 ConfigData 注册为Bean，并提供给ConfigurationTest使用。可将ConfigData作为参数注入到构造函数和普通函数中。

可以用以下方式引用被@ConfigurationProperties(value = "spring.datasource.druid")修饰的ConfigData

可以直接把 ConfigData 当成Bean使用

    /**

     * 可直接被注入

     */

    @Autowired

    private ConfigData configData;

可以用构造函数传入进来

@Data

@Configuration

@EnableConfigurationProperties(value = ConfigData.class )

public class ConfigurationTest {

    private ConfigData configData2;

    /**

     * 作为构造函数的参数注入

     * @param data

     */

    ConfigurationTest(ConfigData data){

        this.configData2 = data;

    }

也可以作为@Bean的方法函数的参数。只有当前类（ConfigurationTest）才可

    /**

     * 直接作为函数的参数

     * @param data

     * @return

     */

    @Bean(name = "configData2")

    HashMap<String, String> getBean(ConfigData data){

        return new HashMap<>(0);

    }

可以省略ConfigData直接将字段注入到返回结果中。

@Bean

@ConfigurationProperties(value = "spring.datasource.druid")

HashMap<String, String> getBean2(ConfigData data){

    // 会自动为hashMap赋值，或使用set方法为对象赋值

    return new HashMap<>();

}

EnableConfigurationProperties注解的内部如下，它导入了一个实现了 ImportBeanDefinitionRegistrar 接口的类。

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Import(EnableConfigurationPropertiesRegistrar.class)

class EnableConfigurationPropertiesRegistrar implements ImportBeanDefinitionRegistrar {

	@Override

	public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {

		registerInfrastructureBeans(registry);

		ConfigurationPropertiesBeanRegistrar beanRegistrar = new ConfigurationPropertiesBeanRegistrar(registry);

		// 获得@EnableConfigurationProperties的value指向的对象，并注册。

		getTypes(metadata).forEach(beanRegistrar::register);

	}