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前言
SpringBoot是Spring的包裝，通過自動配置使得SpringBoot可以做到開箱即用，上手成本非常低，但是學習其實現原理的成本大大增加，需要先了解熟悉Spring原理。如果還不清楚Spring原理的，可以先檢視博主之前的文章，本篇主要分析SpringBoot的啟動、自動配置、Condition、事件驅動原理。
正文
啟動原理
SpringBoot啟動非常簡單，因其內建了Tomcat，所以只需要通過下面幾種方式啟動即可：
@SpringBootApplication(scanBasePackages = {"cn.dark"})
public class SpringbootDemo {
public static void main(String[] args) {
// 第一種
SpringApplication.run(SpringbootDemo .class, args);
// 第二種
new SpringApplicationBuilder(SpringbootDemo .class)).run(args);
// 第三種
SpringApplication springApplication = new SpringApplication(SpringbootDemo.class);
springApplication.run();
}
}
可以看到第一種是最簡單的，也是最常用的方式，需要注意類上面需要標註@SpringBootApplication註解，這是自動配置的核心實現，稍後分析，先來看看SpringBoot啟動做了些什麼？
在往下之前，不妨先猜測一下，run方法中需要做什麼？對比Spring原始碼，我們知道，Spring的啟動都會建立一個ApplicationContext的應用上下文物件，並呼叫其refresh方法啟動容器，SpringBoot只是Spring的一層殼，肯定也避免不了這樣的操作。另一方面，以前通過Spring搭建的專案，都需要打成War包釋出到Tomcat才行，而現在SpringBoot已經內建了Tomcat，只需要打成Jar包啟動即可，所以在run方法中肯定也會建立對應的Tomcat物件並啟動。以上只是我們的猜想，下面就來驗證，進入run方法：
public ConfigurableApplicationContext run(String... args) {
// 統計時間用的工具類
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
configureHeadlessProperty();
// 獲取實現了SpringApplicationRunListener介面的實現類，通過SPI機制載入
// META-INF/spring.factories檔案下的類
SpringApplicationRunListeners listeners = getRunListeners(args);
// 首先呼叫SpringApplicationRunListener的starting方法
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
// 處理配置資料
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
configureIgnoreBeanInfo(environment);
// 啟動時列印banner
Banner printedBanner = printBanner(environment);
// 建立上下文物件
context = createApplicationContext();
// 獲取SpringBootExceptionReporter介面的類，異常報告
exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
// 核心方法，啟動spring容器
refreshContext(context);
afterRefresh(context, applicationArguments);
// 統計結束
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
}
// 呼叫started
listeners.started(context);
// ApplicationRunner
// CommandLineRunner
// 獲取這兩個介面的實現類，並呼叫其run方法
callRunners(context, applicationArguments);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
}
try {
// 最後呼叫running方法
listeners.running(context);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
}
return context;
}
SpringBoot的啟動流程就是這個方法，先看getRunListeners方法，這個方法就是去拿到所有的SpringApplicationRunListener實現類，這些類是用於SpringBoot事件釋出的，關於事件驅動稍後分析，這裡主要看這個方法的實現原理：
private SpringApplicationRunListeners getRunListeners(String[] args) {
Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
return new SpringApplicationRunListeners(logger,
getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
}
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = getClassLoader();
// Use names and ensure unique to protect against duplicates
Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
// 載入上來後反射例項化
List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
String factoryTypeName = factoryType.getName();
return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
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()) {
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;
}
}
一步步追蹤下去可以看到最終就是通過SPI機制根據介面型別從META-INF/spring.factories檔案中載入對應的實現類並例項化，SpringBoot的自動配置也是這樣實現的。為什麼要這樣實現呢？通過註解掃描不可以麼？當然不行，這些類都在第三方jar包中，註解掃描實現是很麻煩的，當然你也可以通過@Import註解匯入，但是這種方式不適合擴充套件類特別多的情況，所以這裡採用SPI的優點就顯而易見了。
回到run方法中，可以看到呼叫了createApplicationContext方法，見名知意，這個就是去建立應用上下文物件：
public static final String DEFAULT_SERVLET_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_SERVLET_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);
}
注意這裡通過反射例項化了一個新的沒見過的上下文物件AnnotationConfigServletWebServerApplicationContext，這個是SpringBoot擴充套件的，看看其構造方法：
public AnnotationConfigServletWebServerApplicationContext() {
this.reader = new AnnotatedBeanDefinitionReader(this);
this.scanner = new ClassPathBeanDefinitionScanner(this);
}
如果你有看過Spring註解驅動的實現原理，這兩個物件肯定不會陌生，一個實支援註解解析的，另外一個是掃描包用的。
上下文建立好了，下一步自然就是呼叫refresh方法啟動容器：
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();
}
這裡首先會呼叫到其父類中ServletWebServerApplicationContext：
public final void refresh() throws BeansException, IllegalStateException {
try {
super.refresh();
}
catch (RuntimeException ex) {
stopAndReleaseWebServer();
throw ex;
}
}
可以看到是直接委託給了父類：
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
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 exception 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();
}
}
}
這個方法不會陌生吧，之前已經分析過了，這裡不再贅述，至此SpringBoot的容器就啟動了，但是Tomcat啟動是在哪裡呢？run方法中也沒有看到。實際上Tomcat的啟動也是在refresh流程中，這個方法其中一步是呼叫了onRefresh方法，在Spring中這是一個沒有實現的模板方法，而SpringBoot就通過這個方法完成了Tomcat的啟動：
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) {
ServletWebServerFactory factory = getWebServerFactory();
// 主要看這個方法
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();
}
這裡首先拿到TomcatServletWebServerFactory物件，通過該物件再去建立和啟動Tomcat：
public WebServer getWebServer(ServletContextInitializer... initializers) {
if (this.disableMBeanRegistry) {
Registry.disableRegistry();
}
Tomcat tomcat = new Tomcat();
File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath());
Connector connector = new Connector(this.protocol);
connector.setThrowOnFailure(true);
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);
return getTomcatWebServer(tomcat);
}
上面的每一步都可以對比Tomcat的配置檔案，需要注意預設只支援了http協議：
Connector connector = new Connector(this.protocol);
private String protocol = DEFAULT_PROTOCOL;
public static final String DEFAULT_PROTOCOL = "org.apache.coyote.http11.Http11NioProtocol";
如果想要擴充套件的話則可以對additionalTomcatConnectors屬性設定值，需要注意這個屬性沒有對應的setter方法，只有addAdditionalTomcatConnectors方法，也就是說我們只能通過實現BeanFactoryPostProcessor介面的postProcessBeanFactory方法，而不能通過BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法，因為前者可以通過傳入的BeanFactory物件提前獲取到TomcatServletWebServerFactory物件呼叫addAdditionalTomcatConnectors即可；而後者只能拿到BeanDefinition物件，該物件只能通過setter方法設定值。
事件驅動
Spring原本就提供了事件機制，而在SpringBoot中又對其進行擴充套件，通過釋出訂閱事件在容器的整個生命週期的不同階段進行不同的操作。我們先來看看SpringBoot啟動關閉的過程中預設會發布哪些事件，使用下面的程式碼即可：
@SpringBootApplication
public class SpringEventDemo {
public static void main(String[] args) {
new SpringApplicationBuilder(SpringEventDemo.class)
.listeners(event -> {
System.err.println("接收到事件：" + event.getClass().getSimpleName());
})
.run()
.close();
}
}
這段程式碼會在控制檯列印所有的事件名稱，按照順序如下：
- ApplicationStartingEvent：容器啟動
- ApplicationEnvironmentPreparedEvent：環境準備好
- ApplicationContextInitializedEvent：上下文初始化完成
- ApplicationPreparedEvent：上下文準備好
- ContextRefreshedEvent：上下文重新整理完
- ServletWebServerInitializedEvent：webServer初始化完成
- ApplicationStartedEvent：容器啟動完成
- ApplicationReadyEvent：容器就緒
- ContextClosedEvent：容器關閉
以上是正常啟動關閉，如果發生異常還有釋出ApplicationFailedEvent事件。事件的釋出遍佈在整個容器的啟動關閉週期中，事件釋出物件剛剛我們也看到了是通過SPI載入的SpringApplicationRunListener實現類EventPublishingRunListener，同樣事件監聽器也是在spring.factories檔案中配置的，預設實現了以下監聽器：
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener
可以看到有用於檔案編碼的（FileEncodingApplicationListener），有載入日誌框架的（LoggingApplicationListener），還有載入配置的（ConfigFileApplicationListener）等等一系列監聽器，SpringBoot也就是通過這系列監聽器將必要的配置和元件載入到容器中來，這裡不再詳細分析，感興趣的讀者可以通過其實現的onApplicationEvent方法看到每個監聽器究竟是監聽的哪一個事件，當然事件釋出和監聽我們自己也是可以擴充套件的。
自動配置原理
SpringBoot最核心的還是自動配置，為什麼它能做到開箱即用，不再需要我們手動使用@EnableXXX等註解來開啟？這一切的答案就在@SpringBootApplication註解中：
@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 {}
這裡重要的註解有三個：@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan。@ComponentScan就不用再說了，@SpringBootConfiguration等同於@Configuration，而@EnableAutoConfiguration就是開啟自動配置：
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
}
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {
}
@AutoConfigurationPackage註解的作用就是將該註解所標記類所在的包作為自動配置的包，簡單看看就行，主要看AutoConfigurationImportSelector，這個就是實現自動配置的核心類，注意這個類是實現的DeferredImportSelector介面。
在這個類中有一個selectImports方法。這個方法在我之前的文章這一次搞懂Spring事務註解的解析也有分析過，只是實現類不同，它同樣會被ConfigurationClassPostProcessor類呼叫，先來看這個方法做了些什麼：
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
// 獲取所有的自動配置類
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}
protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
}
AnnotationAttributes attributes = getAttributes(annotationMetadata);
// SPI獲取EnableAutoConfiguration為key的所有實現類
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
configurations = removeDuplicates(configurations);
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
// 把某些自動配置類過濾掉
configurations = filter(configurations, autoConfigurationMetadata);
fireAutoConfigurationImportEvents(configurations, exclusions);
// 包裝成自動配置實體類
return new AutoConfigurationEntry(configurations, exclusions);
}
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
// SPI獲取EnableAutoConfiguration為key的所有實現類
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;
}
追蹤原始碼最終可以看到也是從META-INF/spring.factories檔案中拿到所有EnableAutoConfiguration對應的值（在spring-boot-autoconfigure中）並通過反射例項化，過濾後包裝成AutoConfigurationEntry物件返回。
看到這裡你應該會覺得自動配置的實現就是通過這個selectImports方法，但實際上這個方法通常並不會被呼叫到，而是會呼叫該類的內部類AutoConfigurationGroup的process和selectImports方法，前者同樣是通過getAutoConfigurationEntry拿到所有的自動配置類，而後者這是過濾排序幷包裝後返回。
下面就來分析ConfigurationClassPostProcessor是怎麼呼叫到這裡的，直接進入processConfigBeanDefinitions方法：
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
String[] candidateNames = registry.getBeanDefinitionNames();
for (String beanName : candidateNames) {
BeanDefinition beanDef = registry.getBeanDefinition(beanName);
if (beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE) != null) {
if (logger.isDebugEnabled()) {
logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
}
}
else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
}
}
// Return immediately if no @Configuration classes were found
if (configCandidates.isEmpty()) {
return;
}
// Sort by previously determined @Order value, if applicable
configCandidates.sort((bd1, bd2) -> {
int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
return Integer.compare(i1, i2);
});
// Detect any custom bean name generation strategy supplied through the enclosing application context
SingletonBeanRegistry sbr = null;
if (registry instanceof SingletonBeanRegistry) {
sbr = (SingletonBeanRegistry) registry;
if (!this.localBeanNameGeneratorSet) {
BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(
AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR);
if (generator != null) {
this.componentScanBeanNameGenerator = generator;
this.importBeanNameGenerator = generator;
}
}
}
if (this.environment == null) {
this.environment = new StandardEnvironment();
}
// Parse each @Configuration class
ConfigurationClassParser parser = new ConfigurationClassParser(
this.metadataReaderFactory, this.problemReporter, this.environment,
this.resourceLoader, this.componentScanBeanNameGenerator, registry);
Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
do {
parser.parse(candidates);
parser.validate();
Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
configClasses.removeAll(alreadyParsed);
// Read the model and create bean definitions based on its content
if (this.reader == null) {
this.reader = new ConfigurationClassBeanDefinitionReader(
registry, this.sourceExtractor, this.resourceLoader, this.environment,
this.importBeanNameGenerator, parser.getImportRegistry());
}
this.reader.loadBeanDefinitions(configClasses);
alreadyParsed.addAll(configClasses);
// 省略。。。。
}
前面一大段主要是拿到合格的Configuration配置類，主要邏輯是在ConfigurationClassParser.parse方法中，該方法完成了對@Component、@Bean、@Import、@ComponentScans等註解的解析，這裡主要看對@Import的解析，其它的讀者可自行分析。一步步追蹤，最終會進入到processConfigurationClass方法：
protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
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);
}
這裡需要注意this.conditionEvaluator.shouldSkip方法的呼叫，這個方法就是進行Bean載入過濾的，即根據@Condition註解的匹配值判斷是否載入該Bean，具體實現稍後分析，繼續跟蹤主流程doProcessConfigurationClass：
protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
throws IOException {
省略....
// Process any @Import annotations
processImports(configClass, sourceClass, getImports(sourceClass), true);
省略....
return null;
}
這裡就是完成對一系列註解的支撐，我省略掉了，主要看processImports方法，這個方法就是處理@Import註解的：
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
Collection<SourceClass> importCandidates, boolean checkForCircularImports) {
if (importCandidates.isEmpty()) {
return;
}
if (checkForCircularImports && isChainedImportOnStack(configClass)) {
this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
}
else {
this.importStack.push(configClass);
try {
for (SourceClass candidate : importCandidates) {
if (candidate.isAssignable(ImportSelector.class)) {
// Candidate class is an ImportSelector -> delegate to it to determine imports
Class<?> candidateClass = candidate.loadClass();
ImportSelector selector = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
this.environment, this.resourceLoader, this.registry);
if (selector instanceof DeferredImportSelector) {
this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
}
else {
String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
processImports(configClass, currentSourceClass, importSourceClasses, false);
}
}
else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
Class<?> candidateClass = candidate.loadClass();
ImportBeanDefinitionRegistrar registrar =
ParserStrategyUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class,
this.environment, this.resourceLoader, this.registry);
configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
}
else {
this.importStack.registerImport(
currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
processConfigurationClass(candidate.asConfigClass(configClass));
}
}
}
}
}
剛剛我提醒過AutoConfigurationImportSelector是實現DeferredImportSelector介面的，如果不是該介面的實現類則是直接呼叫selectImports方法，反之則是呼叫DeferredImportSelectorHandler.handle方法：
private List<DeferredImportSelectorHolder> deferredImportSelectors = new ArrayList<>();
public void handle(ConfigurationClass configClass, DeferredImportSelector importSelector) {
DeferredImportSelectorHolder holder = new DeferredImportSelectorHolder(
configClass, importSelector);
if (this.deferredImportSelectors == null) {
DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
handler.register(holder);
handler.processGroupImports();
}
else {
this.deferredImportSelectors.add(holder);
}
}
首先建立了一個DeferredImportSelectorHolder物件，如果是第一次執行則是新增到deferredImportSelectors屬性中，等到ConfigurationClassParser.parse的最後呼叫process方法：
public void parse(Set<BeanDefinitionHolder> configCandidates) {
省略.....
this.deferredImportSelectorHandler.process();
}
public void process() {
List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
this.deferredImportSelectors = null;
try {
if (deferredImports != null) {
DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
deferredImports.forEach(handler::register);
handler.processGroupImports();
}
}
finally {
this.deferredImportSelectors = new ArrayList<>();
}
}
反之則是直接執行，首先通過register拿到AutoConfigurationGroup物件：
public void register(DeferredImportSelectorHolder deferredImport) {
Class<? extends Group> group = deferredImport.getImportSelector()
.getImportGroup();
DeferredImportSelectorGrouping grouping = this.groupings.computeIfAbsent(
(group != null ? group : deferredImport),
key -> new DeferredImportSelectorGrouping(createGroup(group)));
grouping.add(deferredImport);
this.configurationClasses.put(deferredImport.getConfigurationClass().getMetadata(),
deferredImport.getConfigurationClass());
}
public Class<? extends Group> getImportGroup() {
return AutoConfigurationGroup.class;
}
然後在processGroupImports方法中進行真正的處理：
public void processGroupImports() {
for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
grouping.getImports().forEach(entry -> {
ConfigurationClass configurationClass = this.configurationClasses.get(
entry.getMetadata());
try {
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);
}
});
}
}
public Iterable<Group.Entry> getImports() {
for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
this.group.process(deferredImport.getConfigurationClass().getMetadata(),
deferredImport.getImportSelector());
}
return this.group.selectImports();
}
在getImports方法中就完成了對process和selectImports方法的呼叫，拿到自動配置類後再遞迴呼叫呼叫processImports方法完成對自動配置類的載入。至此，自動配置的載入過程就分析完了，下面是時序圖：
Condition註解原理
在自動配置類中有很多Condition相關的註解，以AOP為例：
Configuration(proxyBeanMethods = false)
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true)
public class AopAutoConfiguration {
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(Advice.class)
static class AspectJAutoProxyingConfiguration {
@Configuration(proxyBeanMethods = false)
@EnableAspectJAutoProxy(proxyTargetClass = false)
@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false",
matchIfMissing = false)
static class JdkDynamicAutoProxyConfiguration {
}
@Configuration(proxyBeanMethods = false)
@EnableAspectJAutoProxy(proxyTargetClass = true)
@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
matchIfMissing = true)
static class CglibAutoProxyConfiguration {
}
}
@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingClass("org.aspectj.weaver.Advice")
@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
matchIfMissing = true)
static class ClassProxyingConfiguration {
ClassProxyingConfiguration(BeanFactory beanFactory) {
if (beanFactory instanceof BeanDefinitionRegistry) {
BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
}
}
}
}
這裡就能看到@ConditionalOnProperty、@ConditionalOnClass、@ConditionalOnMissingClass，另外還有@ConditionalOnBean、@ConditionalOnMissingBean等等很多條件匹配註解。這些註解表示條件匹配才會載入該Bean，以@ConditionalOnProperty為例，表明配置檔案中符合條件才會載入對應的Bean，prefix表示在配置檔案中的字首，name表示配置的名稱，havingValue表示配置為該值時才匹配，matchIfMissing則是表示沒有該配置是否預設載入對應的Bean。其它註解可類比理解記憶，下面主要來分析該註解的實現原理。
這裡註解點進去看會發現每個註解上都標註了@Conditional註解，並且value值都對應一個類，比如OnBeanCondition，而這些類都實現了Condition介面，看看其繼承體系：
上面只展示了幾個實現類，但實際上Condition的實現類是非常多的，我們還可以自己實現該介面來擴充套件@Condition註解。
Condition介面中有一個matches方法，這個方法返回true則表示匹配。該方法在ConfigurationClassParser中多處都有呼叫，也就是剛剛我提醒過的shouldSkip方法，具體實現是在ConditionEvaluator類中：
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<>();
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();
}
if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
return true;
}
}
return false;
}
再來看看matches的實現，但OnBeanCondition類中沒有實現該方法，而是在其父類SpringBootCondition中：
public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
String classOrMethodName = getClassOrMethodName(metadata);
try {
ConditionOutcome outcome = getMatchOutcome(context, metadata);
logOutcome(classOrMethodName, outcome);
recordEvaluation(context, classOrMethodName, outcome);
return outcome.isMatch();
}
getMatchOutcome方法也是一個模板方法，具體的匹配邏輯就在這個方法中實現，該方法返回的ConditionOutcome物件就包含了是否匹配和日誌訊息兩個欄位。進入到OnBeanCondition類中：
public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
ConditionMessage matchMessage = ConditionMessage.empty();
MergedAnnotations annotations = metadata.getAnnotations();
if (annotations.isPresent(ConditionalOnBean.class)) {
Spec<ConditionalOnBean> spec = new Spec<>(context, metadata, annotations, ConditionalOnBean.class);
MatchResult matchResult = getMatchingBeans(context, spec);
if (!matchResult.isAllMatched()) {
String reason = createOnBeanNoMatchReason(matchResult);
return ConditionOutcome.noMatch(spec.message().because(reason));
}
matchMessage = spec.message(matchMessage).found("bean", "beans").items(Style.QUOTE,
matchResult.getNamesOfAllMatches());
}
if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) {
Spec<ConditionalOnSingleCandidate> spec = new SingleCandidateSpec(context, metadata, annotations);
MatchResult matchResult = getMatchingBeans(context, spec);
if (!matchResult.isAllMatched()) {
return ConditionOutcome.noMatch(spec.message().didNotFind("any beans").atAll());
}
else if (!hasSingleAutowireCandidate(context.getBeanFactory(), matchResult.getNamesOfAllMatches(),
spec.getStrategy() == SearchStrategy.ALL)) {
return ConditionOutcome.noMatch(spec.message().didNotFind("a primary bean from beans")
.items(Style.QUOTE, matchResult.getNamesOfAllMatches()));
}
matchMessage = spec.message(matchMessage).found("a primary bean from beans").items(Style.QUOTE,
matchResult.getNamesOfAllMatches());
}
if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) {
Spec<ConditionalOnMissingBean> spec = new Spec<>(context, metadata, annotations,
ConditionalOnMissingBean.class);
MatchResult matchResult = getMatchingBeans(context, spec);
if (matchResult.isAnyMatched()) {
String reason = createOnMissingBeanNoMatchReason(matchResult);
return ConditionOutcome.noMatch(spec.message().because(reason));
}
matchMessage = spec.message(matchMessage).didNotFind("any beans").atAll();
}
return ConditionOutcome.match(matchMessage);
}
可以看到該類支援了@ConditionalOnBean、@ConditionalOnSingleCandidate、@ConditionalOnMissingBean註解，主要的匹配邏輯在getMatchingBeans方法中：
protected final MatchResult getMatchingBeans(ConditionContext context, Spec<?> spec) {
ClassLoader classLoader = context.getClassLoader();
ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
boolean considerHierarchy = spec.getStrategy() != SearchStrategy.CURRENT;
Set<Class<?>> parameterizedContainers = spec.getParameterizedContainers();
if (spec.getStrategy() == SearchStrategy.ANCESTORS) {
BeanFactory parent = beanFactory.getParentBeanFactory();
Assert.isInstanceOf(ConfigurableListableBeanFactory.class, parent,
"Unable to use SearchStrategy.ANCESTORS");
beanFactory = (ConfigurableListableBeanFactory) parent;
}
MatchResult result = new MatchResult();
Set<String> beansIgnoredByType = getNamesOfBeansIgnoredByType(classLoader, beanFactory, considerHierarchy,
spec.getIgnoredTypes(), parameterizedContainers);
for (String type : spec.getTypes()) {
Collection<String> typeMatches = getBeanNamesForType(classLoader, considerHierarchy, beanFactory, type,
parameterizedContainers);
typeMatches.removeAll(beansIgnoredByType);
if (typeMatches.isEmpty()) {
result.recordUnmatchedType(type);
}
else {
result.recordMatchedType(type, typeMatches);
}
}
for (String annotation : spec.getAnnotations()) {
Set<String> annotationMatches = getBeanNamesForAnnotation(classLoader, beanFactory, annotation,
considerHierarchy);
annotationMatches.removeAll(beansIgnoredByType);
if (annotationMatches.isEmpty()) {
result.recordUnmatchedAnnotation(annotation);
}
else {
result.recordMatchedAnnotation(annotation, annotationMatches);
}
}
for (String beanName : spec.getNames()) {
if (!beansIgnoredByType.contains(beanName) && containsBean(beanFactory, beanName, considerHierarchy)) {
result.recordMatchedName(beanName);
}
else {
result.recordUnmatchedName(beanName);
}
}
return result;
}
這裡邏輯看起來比較複雜，但實際上就做了兩件事，首先通過getNamesOfBeansIgnoredByType方法呼叫beanFactory.getBeanNamesForType拿到容器中對應的Bean例項，然後根據返回的結果判斷哪些Bean存在，哪些Bean不存在（Condition註解中是可以配置多個值的）並返回MatchResult物件，而MatchResult中只要有一個Bean沒有匹配上就返回false，也就決定了當前Bean是否需要例項化。
總結
本篇分析了SpringBoot核心原理的實現，通過本篇相信讀者也將能更加熟練地使用和擴充套件SpringBoot。另外還有一些常用的元件我沒有展開分析，如事務、MVC、監聽器的自動配置，這些我們有了Spring原始碼基礎的話下來看一下就明白了，這裡就不贅述了。最後讀者可以思考一下我們應該如何自定義starter啟動器，相信看完本篇應該難不倒你。