自动配置文件如下：

org.springframework.cloud.bootstrap.BootstrapConfiguration=\
com.demo.test.AlgConfigAutoConfiguration
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.demo.test.AlgConfigRefresherAutoConfiguration

1. Springboot 的自动配置

可以看到，springboot 采用的是org.springframework.boot.autoconfigure.EnableAutoConfiguration

1. org.springframework.boot.autoconfigure.EnableAutoConfiguration 源码

package org.springframework.boot.autoconfigure;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Import;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

2. 加载地方：

org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#getAutoConfigurationEntry

    protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) {
        if (!this.isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        } else {
            AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
            List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
            configurations = this.removeDuplicates(configurations);
            Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
            this.checkExcludedClasses(configurations, exclusions);
            configurations.removeAll(exclusions);
            configurations = this.filter(configurations, autoConfigurationMetadata);
            this.fireAutoConfigurationImportEvents(configurations, exclusions);
            return new AutoConfigurationEntry(configurations, exclusions);
        }
    }

    protected AnnotationAttributes getAttributes(AnnotationMetadata metadata) {
        String name = this.getAnnotationClass().getName();
        AnnotationAttributes attributes = AnnotationAttributes.fromMap(metadata.getAnnotationAttributes(name, true));
        Assert.notNull(attributes, () -> {
            return "No auto-configuration attributes found. Is " + metadata.getClassName() + " annotated with " + ClassUtils.getShortName(name) + "?";
        });
        return attributes;
    }

    protected Class<?> getAnnotationClass() {
        return EnableAutoConfiguration.class;
    }

    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
    }

    protected Class<?> getSpringFactoriesLoaderFactoryClass() {
        return EnableAutoConfiguration.class;
    }

继续调用到：org.springframework.core.io.support.SpringFactoriesLoader#loadFactoryNames

    public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        String factoryTypeName = factoryType.getName();
        return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
    }

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {
            return result;
        } else {
            try {
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                MultiValueMap<String, String> result = new LinkedMultiValueMap();

                while(urls.hasMoreElements()) {
                    URL url = (URL)urls.nextElement();
                    UrlResource resource = new UrlResource(url);
                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                    Iterator var6 = properties.entrySet().iterator();

                    while(var6.hasNext()) {
                        Map.Entry<?, ?> entry = (Map.Entry)var6.next();
                        String factoryTypeName = ((String)entry.getKey()).trim();
                        String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                        int var10 = var9.length;

                        for(int var11 = 0; var11 < var10; ++var11) {
                            String factoryImplementationName = var9[var11];
                            result.add(factoryTypeName, factoryImplementationName.trim());
                        }
                    }
                }

                cache.put(classLoader, result);
                return result;
            } catch (IOException var13) {
                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
            }
        }
    }

3. 调用时机

2. Springcloud的自动配置

可以看到，springboot 采用的是org.springframework.cloud.bootstrap.BootstrapConfiguration

1. BootstrapConfiguration

package org.springframework.cloud.bootstrap;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface BootstrapConfiguration {
    Class<?>[] exclude() default {};
}

2. 加载地方

(1).org.springframework.cloud.bootstrap.BootstrapImportSelectorConfiguration

package org.springframework.cloud.bootstrap;

import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;

@Configuration(
    proxyBeanMethods = false
)
@Import({BootstrapImportSelector.class})
public class BootstrapImportSelectorConfiguration {
    public BootstrapImportSelectorConfiguration() {
    }
}

(2). org.springframework.cloud.bootstrap.BootstrapImportSelector#selectImports

    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        List<String> names = new ArrayList(SpringFactoriesLoader.loadFactoryNames(BootstrapConfiguration.class, classLoader));
        names.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(this.environment.getProperty("spring.cloud.bootstrap.sources", ""))));
        List<OrderedAnnotatedElement> elements = new ArrayList();
        Iterator var5 = names.iterator();

        while(var5.hasNext()) {
            String name = (String)var5.next();

            try {
                elements.add(new OrderedAnnotatedElement(this.metadataReaderFactory, name));
            } catch (IOException var8) {
            }
        }

        AnnotationAwareOrderComparator.sort(elements);
        String[] classNames = (String[])elements.stream().map((e) -> {
            return e.name;
        }).toArray((x$0) -> {
            return new String[x$0];
        });
        return classNames;
    }

3. 调用时机

3. 加载顺序

程序启动调用到：

org.springframework.boot.SpringApplication#run(java.lang.String...)

    public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
        this.configureHeadlessProperty();
        SpringApplicationRunListeners listeners = this.getRunListeners(args);
        listeners.starting();

        Collection exceptionReporters;
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
            this.configureIgnoreBeanInfo(environment);
            Banner printedBanner = this.printBanner(environment);
            context = this.createApplicationContext();
            exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
            this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            this.refreshContext(context);
            this.afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
            }

            listeners.started(context);
            this.callRunners(context, applicationArguments);
        } catch (Throwable var10) {
            this.handleRunFailure(context, var10, exceptionReporters, listeners);
            throw new IllegalStateException(var10);
        }

        try {
            listeners.running(context);
            return context;
        } catch (Throwable var9) {
            this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null);
            throw new IllegalStateException(var9);
        }
    }

可以看到cloud 是在该方法的prepareEnvironment 过程中执行， boot 的自动配置是在refreshContext。 所以先cloud后boot。

cloud在prepareEnvironment过程中，cloud 注入了一些自己的bean以及listener等。

(1). BootstrapApplicationListener 完成bootstrap.yml 加载
(2). BootstrapImportSelector 加载org.springframework.cloud.bootstrap.BootstrapConfiguration 指定的配置类

(3). /spring-cloud-context/2.2.2.RELEASE/spring-cloud-context-2.2.2.RELEASE.jar!/META-INF/spring.factories

# AutoConfiguration
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.autoconfigure.ConfigurationPropertiesRebinderAutoConfiguration,\
org.springframework.cloud.autoconfigure.LifecycleMvcEndpointAutoConfiguration,\
org.springframework.cloud.autoconfigure.RefreshAutoConfiguration,\
org.springframework.cloud.autoconfigure.RefreshEndpointAutoConfiguration,\
org.springframework.cloud.autoconfigure.WritableEnvironmentEndpointAutoConfiguration
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.cloud.bootstrap.BootstrapApplicationListener,\
org.springframework.cloud.bootstrap.LoggingSystemShutdownListener,\
org.springframework.cloud.context.restart.RestartListener
# Bootstrap components
org.springframework.cloud.bootstrap.BootstrapConfiguration=\
org.springframework.cloud.bootstrap.config.PropertySourceBootstrapConfiguration,\
org.springframework.cloud.bootstrap.encrypt.EncryptionBootstrapConfiguration,\
org.springframework.cloud.autoconfigure.ConfigurationPropertiesRebinderAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration

4. 总结

1. boot和cloud 都是通过SpringFactoriesLoader.loadFactoryNames(XXXConfiguration.class, classLoader) 找META-INF/spring.factories 文件中对应的配置类
2. springcloud 环境的切入是在org.springframework.boot.SpringApplication#run(java.lang.String...) 方法内部调用this.prepareEnvironment(listeners, applicationArguments); 的时候。 也就是时机早于boot的相关自动配置。（可以理解，cloud是增强了一些组件）

包括springcloud 支持配置中心等，也是在 this.prepareEnvironment(listeners, applicationArguments); 做的扩展。