@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

}

public class SpringApplication {
    public static ConfigurableApplicationContext run(Class<?> primarySource,
			String... args) {
		return run(new Class<?>[] { primarySource }, args);
	}

    public static ConfigurableApplicationContext run(Class<?>[] primarySources,
			String[] args) {
		return new SpringApplication(primarySources).run(args);
	}

	public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(
					args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners,
					applicationArguments);
			configureIgnoreBeanInfo(environment);
			Banner printedBanner = printBanner(environment);
			// 创建 context
			context = createApplicationContext();
			exceptionReporters = getSpringFactoriesInstances(
					SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			// 初始化 context
			prepareContext(context, environment, listeners, applicationArguments,
					printedBanner);
			// 刷新 context
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass)
						.logStarted(getApplicationLog(), stopWatch);
			}
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, listeners);
			throw new IllegalStateException(ex);
		}

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

public class SpringApplication {
    protected ConfigurableApplicationContext createApplicationContext() {
		Class<?> contextClass = this.applicationContextClass;
		if (contextClass == null) {
			try {
				switch (this.webApplicationType) {
				case SERVLET:
					contextClass = Class.forName(DEFAULT_WEB_CONTEXT_CLASS);
					break;
				case REACTIVE:
					contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
					break;
				default:
					contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
				}
			}
			catch (ClassNotFoundException ex) {
				throw new IllegalStateException(
						"Unable create a default ApplicationContext, "
								+ "please specify an ApplicationContextClass",
						ex);
			}
		}
		return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
	}
}

public class SpringApplication {
    private void prepareContext(ConfigurableApplicationContext context,
			ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
			ApplicationArguments applicationArguments, Banner printedBanner) {
		context.setEnvironment(environment);
		postProcessApplicationContext(context);
		applyInitializers(context);
		listeners.contextPrepared(context);
		if (this.logStartupInfo) {
			logStartupInfo(context.getParent() == null);
			logStartupProfileInfo(context);
		}

		// Add boot specific singleton beans
		context.getBeanFactory().registerSingleton("springApplicationArguments",
				applicationArguments);
		if (printedBanner != null) {
			context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
		}

		// Load the sources
		Set<Object> sources = getAllSources();
		Assert.notEmpty(sources, "Sources must not be empty");
		// 加载核心逻辑
		load(context, sources.toArray(new Object[0]));
		listeners.contextLoaded(context);
	}

	protected void load(ApplicationContext context, Object[] sources) {
		if (logger.isDebugEnabled()) {
			logger.debug(
					"Loading source " + StringUtils.arrayToCommaDelimitedString(sources));
		}
		BeanDefinitionLoader loader = createBeanDefinitionLoader(
				getBeanDefinitionRegistry(context), sources);
		if (this.beanNameGenerator != null) {
			loader.setBeanNameGenerator(this.beanNameGenerator);
		}
		if (this.resourceLoader != null) {
			loader.setResourceLoader(this.resourceLoader);
		}
		if (this.environment != null) {
			loader.setEnvironment(this.environment);
		}
		// bean 的加载，继续跟，里面调用的加载逻辑 2.5.1 配置文件封装就开始讲了
		loader.load();
	}
}

class BeanDefinitionLoader {
	private int load(Object source) {
		Assert.notNull(source, "Source must not be null");
		if (source instanceof Class<?>) {
			return load((Class<?>) source);
		}
		if (source instanceof Resource) {
			return load((Resource) source);
		}
		if (source instanceof Package) {
			return load((Package) source);
		}
		if (source instanceof CharSequence) {
			return load((CharSequence) source);
		}
		throw new IllegalArgumentException("Invalid source type " + source.getClass());
	}

	private int load(Resource source) {
		if (source.getFilename().endsWith(".groovy")) {
			if (this.groovyReader == null) {
				throw new BeanDefinitionStoreException(
						"Cannot load Groovy beans without Groovy on classpath");
			}
			return this.groovyReader.loadBeanDefinitions(source);
		}
		// 2.5.1 配置文件封装就开始讲了
		return this.xmlReader.loadBeanDefinitions(source);
	}
}

public class SpringApplication {
    private void refreshContext(ConfigurableApplicationContext context) {
		refresh(context);
		if (this.registerShutdownHook) {
			try {
				context.registerShutdownHook();
			}
			catch (AccessControlException ex) {
				// Not allowed in some environments.
			}
		}
	}

    protected void refresh(ApplicationContext applicationContext) {
		Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
		((AbstractApplicationContext) applicationContext).refresh();
	}
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
}

public class AutoConfigurationImportSelector
		implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
		BeanFactoryAware, EnvironmentAware, Ordered {
	protected boolean isEnabled(AnnotationMetadata metadata) {
		if (getClass() == AutoConfigurationImportSelector.class) {
			return getEnvironment().getProperty(
					EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
					true);
		}
		return true;
	}
}

public class AutoConfigurationImportSelector
		implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
		BeanFactoryAware, EnvironmentAware, Ordered {
	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		// 使用 isEnabled
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
		// 关注 getCandidateConfigurations 
		List<String> configurations = getCandidateConfigurations(annotationMetadata,
				attributes);
		configurations = removeDuplicates(configurations);
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
		configurations = filter(configurations, autoConfigurationMetadata);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return StringUtils.toStringArray(configurations);
	}

	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;
	}
}

public abstract class SpringFactoriesLoader {

	public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
	
    public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
		String factoryClassName = factoryClass.getName();
		return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
	}

	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()) {
				URL url = urls.nextElement();
				UrlResource resource = new UrlResource(url);
				Properties properties = PropertiesLoaderUtils.loadProperties(resource);
				for (Map.Entry<?, ?> entry : properties.entrySet()) {
					List<String> factoryClassNames = Arrays.asList(
							StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
					result.addAll((String) entry.getKey(), factoryClassNames);
				}
			}
			cache.put(classLoader, result);
			return result;
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
	}
}

class ConfigurationClassParser {
	private void processDeferredImportSelectors() {
		List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
		this.deferredImportSelectors = null;
		if (deferredImports == null) {
			return;
		}

		deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
		Map<Object, DeferredImportSelectorGrouping> groupings = new LinkedHashMap<>();
		Map<AnnotationMetadata, ConfigurationClass> configurationClasses = new HashMap<>();
		for (DeferredImportSelectorHolder deferredImport : deferredImports) {
			Class<? extends Group> group = deferredImport.getImportSelector().getImportGroup();
			DeferredImportSelectorGrouping grouping = groupings.computeIfAbsent(
					(group == null ? deferredImport : group),
					(key) -> new DeferredImportSelectorGrouping(createGroup(group)));
			grouping.add(deferredImport);
			configurationClasses.put(deferredImport.getConfigurationClass().getMetadata(),
					deferredImport.getConfigurationClass());
		}
		for (DeferredImportSelectorGrouping grouping : groupings.values()) {
			// 打个断点，调试下，可以看到有很多 XXAutoConfiguration,包括默认的和自定义的，如 MessageSourceAutoConfiguration
			grouping.getImports().forEach((entry) -> {
				ConfigurationClass configurationClass = configurationClasses.get(
						entry.getMetadata());
				try {
					// 核心业务逻辑：配置文件的处理逻辑
					processImports(configurationClass, asSourceClass(configurationClass),
							asSourceClasses(entry.getImportClassName()), false);
				}
				catch (BeanDefinitionStoreException ex) {
					throw ex;
				}
				catch (Throwable ex) {
					throw new BeanDefinitionStoreException(
							"Failed to process import candidates for configuration class [" +
									configurationClass.getMetadata().getClassName() + "]", ex);
				}
			});
		}
	}
}

class ConfigurationClassParser {
	
	// configClass 就是 spring.factories 中定义的配置类
	protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
			throws IOException {

		// Recursively process any member (nested) classes first
		processMemberClasses(configClass, sourceClass);

		// Process any @PropertySource annotations
		for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), PropertySources.class,
				org.springframework.context.annotation.PropertySource.class)) {
			if (this.environment instanceof ConfigurableEnvironment) {
				processPropertySource(propertySource);
			}
			else {
				logger.warn("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
						"]. Reason: Environment must implement ConfigurableEnvironment");
			}
		}

		// Process any @ComponentScan annotations
		// 获取对应的注解配置信息，也就是对应的 @ComponentScan 中最主要的扫描路径信息，然后委托给 parse 进一步扫描。  
		Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
		if (!componentScans.isEmpty() &&
				!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
			for (AnnotationAttributes componentScan : componentScans) {
				// The config class is annotated with @ComponentScan -> perform the scan immediately
				Set<BeanDefinitionHolder> scannedBeanDefinitions =
						this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
				// Check the set of scanned definitions for any further config classes and parse recursively if needed
				for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
					// 对扫描出来的类进行过滤
					if (ConfigurationClassUtils.checkConfigurationClassCandidate(
							holder.getBeanDefinition(), this.metadataReaderFactory)) {
						// 将所有扫描出来的类委托到 parse 方法中递归处理
						parse(holder.getBeanDefinition().getBeanClassName(), holder.getBeanName());
					}
				}
			}
		}

		// Process any @Import annotations
		processImports(configClass, sourceClass, getImports(sourceClass), true);

		// Process any @ImportResource annotations
		AnnotationAttributes importResource =
				AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
		if (importResource != null) {
			String[] resources = importResource.getStringArray("locations");
			Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
			for (String resource : resources) {
				String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
				configClass.addImportedResource(resolvedResource, readerClass);
			}
		}

		// Process individual @Bean methods
		Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
		for (MethodMetadata methodMetadata : beanMethods) {
			configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
		}

		// Process default methods on interfaces
		processInterfaces(configClass, sourceClass);

		// Process superclass, if any
		if (sourceClass.getMetadata().hasSuperClass()) {
			String superclass = sourceClass.getMetadata().getSuperClassName();
			if (superclass != null && !superclass.startsWith("java") &&
					!this.knownSuperclasses.containsKey(superclass)) {
				this.knownSuperclasses.put(superclass, configClass);
				// Superclass found, return its annotation metadata and recurse
				return sourceClass.getSuperClass();
			}
		}

		// No superclass -> processing is complete
		return null;
	}
}

public class AnnotationConfigUtils {
	static Set<AnnotationAttributes> attributesForRepeatable(AnnotationMetadata metadata,
			Class<?> containerClass, Class<?> annotationClass) {

		return attributesForRepeatable(metadata, containerClass.getName(), annotationClass.getName());
	}

	static Set<AnnotationAttributes> attributesForRepeatable(
			AnnotationMetadata metadata, String containerClassName, String annotationClassName) {

		Set<AnnotationAttributes> result = new LinkedHashSet<>();

		// Direct annotation present?
		addAttributesIfNotNull(result, metadata.getAnnotationAttributes(annotationClassName, false));

		// Container annotation present?
		Map<String, Object> container = metadata.getAnnotationAttributes(containerClassName, false);
		if (container != null && container.containsKey("value")) {
			for (Map<String, Object> containedAttributes : (Map<String, Object>[]) container.get("value")) {
				addAttributesIfNotNull(result, containedAttributes);
			}
		}

		// Return merged result
		return Collections.unmodifiableSet(result);
	}
}

class ComponentScanAnnotationParser {
	public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, final String declaringClass) {
		ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,
				componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);

		Class<? extends BeanNameGenerator> generatorClass = componentScan.getClass("nameGenerator");
		boolean useInheritedGenerator = (BeanNameGenerator.class == generatorClass);
		scanner.setBeanNameGenerator(useInheritedGenerator ? this.beanNameGenerator :
				BeanUtils.instantiateClass(generatorClass));

		// scopeProxy 属性构造
		ScopedProxyMode scopedProxyMode = componentScan.getEnum("scopedProxy");
		if (scopedProxyMode != ScopedProxyMode.DEFAULT) {
			scanner.setScopedProxyMode(scopedProxyMode);
		}
		else {
			Class<? extends ScopeMetadataResolver> resolverClass = componentScan.getClass("scopeResolver");
			scanner.setScopeMetadataResolver(BeanUtils.instantiateClass(resolverClass));
		}
		// resourcePattern 属性构造
		scanner.setResourcePattern(componentScan.getString("resourcePattern"));

		// includeFilters 设置
		for (AnnotationAttributes filter : componentScan.getAnnotationArray("includeFilters")) {
			for (TypeFilter typeFilter : typeFiltersFor(filter)) {
				scanner.addIncludeFilter(typeFilter);
			}
		}
		// excludeFilters 属性设置
		for (AnnotationAttributes filter : componentScan.getAnnotationArray("excludeFilters")) {
			for (TypeFilter typeFilter : typeFiltersFor(filter)) {
				scanner.addExcludeFilter(typeFilter);
			}
		}

		boolean lazyInit = componentScan.getBoolean("lazyInit");
		if (lazyInit) {
			scanner.getBeanDefinitionDefaults().setLazyInit(true);
		}

		// basePackages 设置
		Set<String> basePackages = new LinkedHashSet<>();
		String[] basePackagesArray = componentScan.getStringArray("basePackages");
		for (String pkg : basePackagesArray) {
			String[] tokenized = StringUtils.tokenizeToStringArray(this.environment.resolvePlaceholders(pkg),
					ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
			Collections.addAll(basePackages, tokenized);
		}
		for (Class<?> clazz : componentScan.getClassArray("basePackageClasses")) {
			basePackages.add(ClassUtils.getPackageName(clazz));
		}
		if (basePackages.isEmpty()) {
			basePackages.add(ClassUtils.getPackageName(declaringClass));
		}

		scanner.addExcludeFilter(new AbstractTypeHierarchyTraversingFilter(false, false) {
			@Override
			protected boolean matchClassName(String className) {
				return declaringClass.equals(className);
			}
		});
		return scanner.doScan(StringUtils.toStringArray(basePackages));
	}
}

class OnPropertyCondition extends SpringBootCondition {
	public ConditionOutcome getMatchOutcome(ConditionContext context,
			AnnotatedTypeMetadata metadata) {
		// 扫描配置的 ConditionalOnProperty 信息，可以 DEBUG 看看
		List<AnnotationAttributes> allAnnotationAttributes = annotationAttributesFromMultiValueMap(
				metadata.getAllAnnotationAttributes(
						ConditionalOnProperty.class.getName()));
		List<ConditionMessage> noMatch = new ArrayList<>();
		List<ConditionMessage> match = new ArrayList<>();
		for (AnnotationAttributes annotationAttributes : allAnnotationAttributes) {
			// 核心的验证逻辑
			ConditionOutcome outcome = determineOutcome(annotationAttributes,
					context.getEnvironment());
			(outcome.isMatch() ? match : noMatch).add(outcome.getConditionMessage());
		}
		if (!noMatch.isEmpty()) {
			return ConditionOutcome.noMatch(ConditionMessage.of(noMatch));
		}
		return ConditionOutcome.match(ConditionMessage.of(match));
	}

	private ConditionOutcome determineOutcome(AnnotationAttributes annotationAttributes,
			PropertyResolver resolver) {
		Spec spec = new Spec(annotationAttributes);
		List<String> missingProperties = new ArrayList<>();
		List<String> nonMatchingProperties = new ArrayList<>();
		// 初始化 missingProperties、nonMatchingProperties
		spec.collectProperties(resolver, missingProperties, nonMatchingProperties);
		// 不匹配情况 1，缺失 missingProperties
		if (!missingProperties.isEmpty()) {
			return ConditionOutcome.noMatch(
					ConditionMessage.forCondition(ConditionalOnProperty.class, spec)
							.didNotFind("property", "properties")
							.items(Style.QUOTE, missingProperties));
		}
		// 不匹配情况 2，对应不匹配逻辑
		if (!nonMatchingProperties.isEmpty()) {
			return ConditionOutcome.noMatch(
					ConditionMessage.forCondition(ConditionalOnProperty.class, spec)
							.found("different value in property",
									"different value in properties")
							.items(Style.QUOTE, nonMatchingProperties));
		}
		return ConditionOutcome.match(ConditionMessage
				.forCondition(ConditionalOnProperty.class, spec).because("matched"));
	}

	private void collectProperties(PropertyResolver resolver, List<String> missing,
				List<String> nonMatching) {
		for (String name : this.names) {
			String key = this.prefix + name;
			if (resolver.containsProperty(key)) {
				// 使用 PropertyResolver 验证对应的属性是否存在
				if (!isMatch(resolver.getProperty(key), this.havingValue)) {
					nonMatching.add(name);
				}
			}
			else {
				if (!this.matchIfMissing) {
					missing.add(name);
				}
			}
		}
	}
}

public class ConditionOutcome {
	private final boolean match;

	private final ConditionMessage message;

	public ConditionOutcome(boolean match, ConditionMessage message) {
		Assert.notNull(message, "ConditionMessage must not be null");
		this.match = match;
		this.message = message;
	}

	public static ConditionOutcome noMatch(ConditionMessage message) {
		return new ConditionOutcome(false, message);
	}

	public static ConditionOutcome match(ConditionMessage message) {
		return new ConditionOutcome(true, message);
	}
}

class ConfigurationClassParser {
	protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
		// 整个 condition 逻辑生效的切入点，如果验证不通过则会直接忽略掉后面的解析逻辑
		if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
			return;
		}

		ConfigurationClass existingClass = this.configurationClasses.get(configClass);
		if (existingClass != null) {
			if (configClass.isImported()) {
				if (existingClass.isImported()) {
					existingClass.mergeImportedBy(configClass);
				}
				// Otherwise ignore new imported config class; existing non-imported class overrides it.
				return;
			}
			else {
				// Explicit bean definition found, probably replacing an import.
				// Let's remove the old one and go with the new one.
				this.configurationClasses.remove(configClass);
				this.knownSuperclasses.values().removeIf(configClass::equals);
			}
		}

		// Recursively process the configuration class and its superclass hierarchy.
		SourceClass sourceClass = asSourceClass(configClass);
		do {
			sourceClass = doProcessConfigurationClass(configClass, sourceClass);
		}
		while (sourceClass != null);

		this.configurationClasses.put(configClass, configClass);
	}
}

class ConfigurationClassParser {
	public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
		if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
			return false;
		}

		if (phase == null) {
			if (metadata instanceof AnnotationMetadata &&
					ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
				return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
			}
			return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
		}

		List<Condition> conditions = new ArrayList<>();
		// condition 的获取，获取某一个特定类的解析，metadata 中包含了完整的配置类信息
		for (String[] conditionClasses : getConditionClasses(metadata)) {
			for (String conditionClass : conditionClasses) {
				Condition condition = getCondition(conditionClass, this.context.getClassLoader());
				conditions.add(condition);
			}
		}

		AnnotationAwareOrderComparator.sort(conditions);

		for (Condition condition : conditions) {
			ConfigurationPhase requiredPhase = null;
			if (condition instanceof ConfigurationCondition) {
				requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
			}
			// condition 的匹配，condition 对应的运行态类为 OnPropertyCondition 
			if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
				return true;
			}
		}

		return false;
	}
}

class ConditionEvaluator {
	private List<String[]> getConditionClasses(AnnotatedTypeMetadata metadata) {
		MultiValueMap<String, Object> attributes = metadata.getAllAnnotationAttributes(Conditional.class.getName(), true);
		Object values = (attributes != null ? attributes.get("value") : null);
		return (List<String[]>) (values != null ? values : Collections.emptyList());
	}
}

public class QualifierAnnotationAutowireCandidateResolver extends GenericTypeAwareAutowireCandidateResolver {
	private Class<? extends Annotation> valueAnnotationType = Value.class;

	protected Object findValue(Annotation[] annotationsToSearch) {
		AnnotationAttributes attr = AnnotatedElementUtils.getMergedAnnotationAttributes(
				AnnotatedElementUtils.forAnnotations(annotationsToSearch), this.valueAnnotationType);
		if (attr != null) {
			// 打个断点，DEBUG 下,可以看到在此停留
			return extractValue(attr);
		}
		return null;
	}

	protected Object extractValue(AnnotationAttributes attr) {
		Object value = attr.get(AnnotationUtils.VALUE);
		if (value == null) {
			throw new IllegalStateException("Value annotation must have a value attribute");
		}
		return value;
	}
}

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
		implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
	public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
			@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
			// ...
			// 里面对应上面 QualifierAnnotationAutowireCandidateResolver.findValue 解析逻辑
			Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
			// ...
			String strVal = resolveEmbeddedValue((String) value);
	}
}

public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
	public String resolveEmbeddedValue(@Nullable String value) {
		if (value == null) {
			return null;
		}
		String result = value;
		for (StringValueResolver resolver : this.embeddedValueResolvers) {
			// 属性的解析委托给了 StringValueResolver 类处理
			result = resolver.resolveStringValue(result);
			if (result == null) {
				return null;
			}
		}
		return result;
	}
}

public class PropertySourcesPlaceholderConfigurer extends PlaceholderConfigurerSupport implements EnvironmentAware {
	protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
			final ConfigurablePropertyResolver propertyResolver) throws BeansException {

		propertyResolver.setPlaceholderPrefix(this.placeholderPrefix);
		propertyResolver.setPlaceholderSuffix(this.placeholderSuffix);
		propertyResolver.setValueSeparator(this.valueSeparator);

	}
}

/**
 * 对应上面解析逻辑
 */
public abstract class PlaceholderConfigurerSupport extends PropertyResourceConfigurer
		implements BeanNameAware, BeanFactoryAware {
		
	/** Default placeholder prefix: {@value}. */
	public static final String DEFAULT_PLACEHOLDER_PREFIX = "${";

	/** Default placeholder suffix: {@value}. */
	public static final String DEFAULT_PLACEHOLDER_SUFFIX = "}";

	/** Default value separator: {@value}. */
	public static final String DEFAULT_VALUE_SEPARATOR = ":";

}

public class PropertySourcesPlaceholderConfigurer extends PlaceholderConfigurerSupport implements EnvironmentAware {
	protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
			final ConfigurablePropertyResolver propertyResolver) throws BeansException {

		StringValueResolver valueResolver = strVal -> {
			// resolvePlaceholders 表示如果变量无法解析则忽略
			// resolveRequiredPlaceholders 表示如果变量无法解析则抛异常
			String resolved = (this.ignoreUnresolvablePlaceholders ?
					propertyResolver.resolvePlaceholders(strVal) :
					propertyResolver.resolveRequiredPlaceholders(strVal));
			if (this.trimValues) {
				resolved = resolved.trim();
			}
			return (resolved.equals(this.nullValue) ? null : resolved);
		};

	}
}

public class ConfigFileApplicationListener
		implements EnvironmentPostProcessor, SmartApplicationListener, Ordered {
	public void load() {
		this.profiles = Collections.asLifoQueue(new LinkedList<Profile>());
		this.processedProfiles = new LinkedList<>();
		this.activatedProfiles = false;
		this.loaded = new LinkedHashMap<>();
		// 通过 profile 标记不同的环境
		initializeProfiles();
		while (!this.profiles.isEmpty()) {
			Profile profile = this.profiles.poll();
			load(profile, this::getPositiveProfileFilter,
					addToLoaded(MutablePropertySources::addLast, false));
			this.processedProfiles.add(profile);
		}
		load(null, this::getNegativeProfileFilter,
				addToLoaded(MutablePropertySources::addFirst, true));
		addLoadedPropertySources();
	}

	private void initializeProfiles() {
		Set<Profile> initialActiveProfiles = initializeActiveProfiles();
		this.profiles.addAll(getUnprocessedActiveProfiles(initialActiveProfiles));
		if (this.profiles.isEmpty()) {
			// 未配置环境，则取默认的环境
			for (String defaultProfileName : this.environment.getDefaultProfiles()) {
				Profile defaultProfile = new Profile(defaultProfileName, true);
				if (!this.profiles.contains(defaultProfile)) {
					this.profiles.add(defaultProfile);
				}
			}
		}
		// 支持不添加任何 profile 注解的 bean 的加载
		this.profiles.add(null);
	}

	private void load(Profile profile, DocumentFilterFactory filterFactory,
			DocumentConsumer consumer) {
		// Spring Boot 默认从 4 个位置查找 application.properties 文件就是从 getSearchLocations() 方法返回。
		// 1. 当前目录下的 /config 目录
		// 2. 当前目录
		// 3. 类路径下的 /config 目录
		// 4. 类路径根目录
		getSearchLocations().forEach((location) -> {
			boolean isFolder = location.endsWith("/");
			// 如果没有配置则默认从 application.properties 中加载，约定大于配置
			Set<String> names = (isFolder ? getSearchNames() : NO_SEARCH_NAMES);
			names.forEach(
					(name) -> load(location, name, profile, filterFactory, consumer));
		});
	}
}

public class SpringApplication {

	// 默认配置
	public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework.boot."
			+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";

    protected ConfigurableApplicationContext createApplicationContext() {
		Class<?> contextClass = this.applicationContextClass;
		if (contextClass == null) {
			try {
				switch (this.webApplicationType) {
				case SERVLET:
					contextClass = Class.forName(DEFAULT_WEB_CONTEXT_CLASS);
					break;
				case REACTIVE:
					contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
					break;
				default:
					contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
				}
			}
			catch (ClassNotFoundException ex) {
				throw new IllegalStateException(
						"Unable create a default ApplicationContext, "
								+ "please specify an ApplicationContextClass",
						ex);
			}
		}
		return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
	}
}

public abstract class AbstractApplicationContext extends DefaultResourceLoader
		implements ConfigurableApplicationContext {
    public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// 1. 初始化前的准备工作，例如对系统属性或者环境变量进行准备及验证。
			// 准备刷新的上下文环境
			prepareRefresh();

			// 2. 初始化 BeanFactory，并进行 XML 文件读取。
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// 3. 对 BeanFactory 进行各种功能填充。
			prepareBeanFactory(beanFactory);

			try {
				// 4. 子类覆盖方法做额外的处理。
				postProcessBeanFactory(beanFactory);

				// 5. 激活各种 BeanFactory 处理器。
				invokeBeanFactoryPostProcessors(beanFactory);

				// 6. 注册拦截 bean 创建的 bean 处理器，这里只是注册，真正的调用是在 getBean 时候。
				registerBeanPostProcessors(beanFactory);

				// 7. 为上下文初始化 Message 源，即对不同语言的消息体进行国际化处理。
				initMessageSource();

				// 8. 初始化应用消息广播器，并放入 “applicationEventMulticaster” bean 中。
				initApplicationEventMulticaster();

				// 9. 留给子类来初始化其他的 bean。
				onRefresh();

				// 10. 在所有注册的 bean 中查找 listener bean，注册到消息广播器中。
				registerListeners();

				// 11. 初始化剩下的单实例（非惰性的）。
				finishBeanFactoryInitialization(beanFactory);

				// 12. 完成刷新过程。通知生命周期处理器 lifecycleProcessor 刷新过程，同时发出 ContextRefreshEvent 通知别人。
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exc to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}
}

public class ServletWebServerApplicationContext extends GenericWebApplicationContext
		implements ConfigurableWebServerApplicationContext {
	protected void onRefresh() {
		super.onRefresh();
		try {
			createWebServer();
		}
		catch (Throwable ex) {
			throw new ApplicationContextException("Unable to start web server", ex);
		}
	}

	private void createWebServer() {
		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		if (webServer == null && servletContext == null) {
			// 核心方法，DEBUG 看下，实例为 TomcatServletWebServerFactory
			ServletWebServerFactory factory = getWebServerFactory();
			this.webServer = factory.getWebServer(getSelfInitializer());
		}
		else if (servletContext != null) {
			try {
				getSelfInitializer().onStartup(servletContext);
			}
			catch (ServletException ex) {
				throw new ApplicationContextException("Cannot initialize servlet context",
						ex);
			}
		}
		initPropertySources();
	}
}

class ServletWebServerFactoryConfiguration {
	@Configuration
	@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
	@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedTomcat {

		@Bean
		public TomcatServletWebServerFactory tomcatServletWebServerFactory() {
			return new TomcatServletWebServerFactory();
		}

	}

	/**
	 * Nested configuration if Jetty is being used.
	 */
	@Configuration
	@ConditionalOnClass({ Servlet.class, Server.class, Loader.class,
			WebAppContext.class })
	@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedJetty {

		@Bean
		public JettyServletWebServerFactory JettyServletWebServerFactory() {
			return new JettyServletWebServerFactory();
		}

	}

	/**
	 * Nested configuration if Undertow is being used.
	 */
	@Configuration
	@ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
	@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedUndertow {

		@Bean
		public UndertowServletWebServerFactory undertowServletWebServerFactory() {
			return new UndertowServletWebServerFactory();
		}

	}
}

public class TomcatServletWebServerFactory extends AbstractServletWebServerFactory
		implements ConfigurableTomcatWebServerFactory, ResourceLoaderAware {
	public WebServer getWebServer(ServletContextInitializer... initializers) {
		Tomcat tomcat = new Tomcat();
		File baseDir = (this.baseDirectory != null ? this.baseDirectory
				: createTempDir("tomcat"));
		tomcat.setBaseDir(baseDir.getAbsolutePath());
		Connector connector = new Connector(this.protocol);
		tomcat.getService().addConnector(connector);
		customizeConnector(connector);
		tomcat.setConnector(connector);
		tomcat.getHost().setAutoDeploy(false);
		configureEngine(tomcat.getEngine());
		for (Connector additionalConnector : this.additionalTomcatConnectors) {
			tomcat.getService().addConnector(additionalConnector);
		}
		prepareContext(tomcat.getHost(), initializers);
		// 异步启动 tomcat
		return getTomcatWebServer(tomcat);
	}
}