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SpringBoot源码解析-Bean的加载与自动化配置

springboot作为一个基于spring开发的框架,自然也继承了spring的容器属性。容器中的bean自然成为了springboot各种功能的基础。本节就来分析一下springboot如何将各种bean加载进容器中。


开始分析之前首先我们先概览一下springboot框架究竟加载了多少bean。在main函数中添加如下代码,运行。

public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(Application.class, args);
        Arrays.stream(context.getBeanDefinitionNames()).forEach(System.out::println);
    }
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不出意外的话,控制台会打印出上百个bean的名字。虽然我们仅仅只写了两个类而已!那么这些类的加载有何规则呢?相比于spring的xml配置文件,springboot的自动化配置又是如何实现的?这些都将在本节揭晓。

public ConfigurableApplicationContext run(String... args) {
		        ...
		        //创建ApplicationContext
			context = createApplicationContext();
			...
			//做一些初始化配置
			prepareContext(context, environment, listeners, applicationArguments,
					printedBanner);
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			...
	}
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首先我们进入SpringApplication的run方法中,在run方法中我们看到和ApplicationContext有关的代码一共有4行,第一行创建了ApplicationContext,第二行做了一些初始化配置,第三行调用了refresh方法,读过spring源码的话应该知道这个方法包含了ApplicationContext初始化最重要也最大部分的逻辑,所以这行待会会重点分析,最后一行是一个空方法,留着子类覆写。

	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;
				...
				}
			}
		}
		return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
	}
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首先进入create方法,在SpringApplication初始化的时候,我们已经知道了这是一个网络服务,所以这边创建的类是DEFAULT_SERVLET_WEB_CONTEXT_CLASS类,(org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext) 在这边直接调用了无参构造函数。先进入构造函数看一下做了那些事情。

	public AnnotationConfigServletWebServerApplicationContext() {
		this.reader = new AnnotatedBeanDefinitionReader(this);
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}
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初始化了reader和scanner组件,reader是用来注册bean的,scanner是用来扫描bean的。这两个组件初始化的逻辑都不复杂,读者可以自行理解。但是重点关注一个地方。在reader的构造函数中:

	public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {
		this(registry, getOrCreateEnvironment(registry));
	}
	
        public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
		...
		AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
	}
	
	public static void registerAnnotationConfigProcessors(BeanDefinitionRegistry registry) {
		registerAnnotationConfigProcessors(registry, null);
	}

	public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
			BeanDefinitionRegistry registry, @Nullable Object source) {

		...
		if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		...
	}
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一个ConfigurationClassPostProcessor的bean被注入到了容器中,这个地方留意一下,后面这个bean很重要。

创建完成了之后,我们看一下prepareContext方法

	private void prepareContext(ConfigurableApplicationContext context,
			ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
			ApplicationArguments applicationArguments, Banner printedBanner) {
		...
		load(context, sources.toArray(new Object[0]));
		...
	}
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prepareContext方法中,调用了一些监听器,和初始化接口,但是最重要的是load这个方法。load这个方法,将我们main方法的这个类传入了容器中。这个类上面有一个非常重要的注解SpringBootApplication。

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        ApplicationContext context = SpringApplication.run(Application.class, args);
        Arrays.stream(context.getBeanDefinitionNames()).forEach(System.out::println);
    }

}
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下面就进入到了最重要的refresh方法,如果读过《spring源码深度解析》这本书的话,这个地方的逻辑应该感到很亲切,没读过的话强烈建议读一下,不管spring怎么发展,基础还是那些的。

	@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			...
				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

			...
	}
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所以refresh方法中的逻辑我也不多介绍了,直接进入主题。invokeBeanFactoryPostProcessors方法。

	protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
		PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
		...
	}
	
public static void invokeBeanFactoryPostProcessors(
			ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

		    ...
		    List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();
			String[] postProcessorNames =
					beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
					currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					processedBeans.add(ppName);
				}
			}
			sortPostProcessors(currentRegistryProcessors, beanFactory);
			registryProcessors.addAll(currentRegistryProcessors);
			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
			currentRegistryProcessors.clear();
			...
	}
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在invokeBeanFactoryPostProcessors方法中,从容器中获取了BeanDefinitionRegistryPostProcessor类型的类,然后执行了这些类的postProcessBeanDefinitionRegistry方法。还记得上面我让你们重点关注的ConfigurationClassPostProcessor么,他就是实现了BeanDefinitionRegistryPostProcessor,所以这个地方会调用ConfigurationClassPostProcessor的postProcessBeanDefinitionRegistry方法。那么我们进入方法瞧瞧。

	public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
		...
		processConfigBeanDefinitions(registry);
	}
	
	public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
		List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
		String[] candidateNames = registry.getBeanDefinitionNames();

		for (String beanName : candidateNames) {
			BeanDefinition beanDef = registry.getBeanDefinition(beanName);
			...
			//判断@Configuration注解
			else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
				configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
			}
		}
		...
		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 {
		//解析带有@Configuration注解的类
			parser.parse(candidates);
		...
	}
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processConfigBeanDefinitions方法主要有两个逻辑,首先判断类上是否带有@Configuration注解,然后解析该类。其实在这儿,主要解析的就是@SpringBootApplication注解。因为点开@SpringBootApplication注解的源码

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

@Configuration
public @interface SpringBootConfiguration {

}
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@SpringBootApplication注解上面有@SpringBootConfiguration注解,而后者又包含了@Configuration注解,所以这个地方,解析的就是带有@SpringBootApplication注解的类。进入parse方法。

	public void parse(Set<BeanDefinitionHolder> configCandidates) {
		for (BeanDefinitionHolder holder : configCandidates) {
			BeanDefinition bd = holder.getBeanDefinition();
			try {
				if (bd instanceof AnnotatedBeanDefinition) {
					parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
				}
		...

		this.deferredImportSelectorHandler.process();
	}
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主要有两个逻辑,我们一个一个来分析。首先再次进入parse方法

	protected final void parse(AnnotationMetadata metadata, String beanName) throws IOException {
		processConfigurationClass(new ConfigurationClass(metadata, beanName));
	}

	protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
		...
		SourceClass sourceClass = asSourceClass(configClass);
		do {
		//进入这个方法
			sourceClass = doProcessConfigurationClass(configClass, sourceClass);
		}
		while (sourceClass != null);

		this.configurationClasses.put(configClass, configClass);
	}
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在doProcessConfigurationClass中,我们看到了熟悉的Component,PropertySources,ComponentScan,ImportResource,以及Import注解,上述几个注解的功能大家应该都很熟悉了,我就不多介绍了,这些注解在这儿就完成了他们的使命,经过这个方法后,我们自己写的类就会全部进入springboot容器中了。

下面开始分析this.deferredImportSelectorHandler.process();

public void process() {
			List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
			this.deferredImportSelectors = null;
			try {
				if (deferredImports != null) {
				...
		}
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进入方法后发现如果deferredImportSelectors为空的话,就什么都做不了。但是调用debug后发现这个地方是有值的,那么他是什么时候被放进来的呢。我们回头看刚刚的doProcessConfigurationClass方法。

	protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
		...
		// Process any @Import annotations
		processImports(configClass, sourceClass, getImports(sourceClass), true);
		...
	}
	
	private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
			Collection<SourceClass> importCandidates, boolean checkForCircularImports) {

		...
						if (selector instanceof DeferredImportSelector) {
							this.deferredImportSelectorHandler.handle(
									configClass, (DeferredImportSelector) selector);
						...
		}
	}

		public void handle(ConfigurationClass configClass, DeferredImportSelector importSelector) {
			...
				this.deferredImportSelectors.add(holder);
			}
		}
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在processImports发现了添加的痕迹。但是添加有个前提条件是要import导入的类selector instanceof DeferredImportSelector,这个条件是怎么实现的呢?答案就在@SpringBootApplication注解中。

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

public class AutoConfigurationImportSelector
		implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
		BeanFactoryAware, EnvironmentAware, Ordered {
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所以到这儿我们就知道了deferredImportSelectors里面有一个元素,就是这边的AutoConfigurationImportSelector。

所以到这儿,我们就可以接着分析process方法了

		public void process() {
			List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
			this.deferredImportSelectors = null;
			try {
				if (deferredImports != null) {
					DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
					deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
					//注册
					deferredImports.forEach(handler::register);
					//解析
					handler.processGroupImports();
				}
			}
			finally {
				this.deferredImportSelectors = new ArrayList<>();
			}
		}
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一个注册方法,一个解析方法,注册方法逻辑比较简单,我们直接进入解析方法。

		public void processGroupImports() {
			for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
			//这个地方看一下getImports方法
				grouping.getImports().forEach(entry -> {
					...
					//这个方法标记一下,processImport待会回来
						processImports(configurationClass, asSourceClass(configurationClass),
								asSourceClasses(entry.getImportClassName()), false);
					...
			}
		}

		public Iterable<Group.Entry> getImports() {
			for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
			//重点看process方法
				this.group.process(deferredImport.getConfigurationClass().getMetadata(),
						deferredImport.getImportSelector());
			}
			return this.group.selectImports();
		}

		public void process(AnnotationMetadata annotationMetadata,
				DeferredImportSelector deferredImportSelector) {
			...
			AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
					.getAutoConfigurationEntry(getAutoConfigurationMetadata(),
							annotationMetadata);
			...
		}

	protected AutoConfigurationEntry getAutoConfigurationEntry(
			AutoConfigurationMetadata autoConfigurationMetadata,
			AnnotationMetadata annotationMetadata) {
		...
		List<String> configurations = getCandidateConfigurations(annotationMetadata,
				attributes);
		...
	}

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

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SpringFactoriesLoader.loadFactoryNames这个方法熟悉么,一直在用,所以话不多说,先看看getSpringFactoriesLoaderFactoryClass返回了一个什么类。返回的是EnableAutoConfiguration.class; 所以进入配置文件查看。

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
...
...
...
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你应该会看到这么长长的一串配置,这里就是springboot自动化配置的中心了。我就以aop来展示一下springboot是如何简化spring的配置的。

首先经过我们刚刚的一串逻辑org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,这个类会被加载进容器中,那么这个类,和aop又有啥关系呢。

@Configuration
@ConditionalOnClass({ EnableAspectJAutoProxy.class, Aspect.class, Advice.class,
		AnnotatedElement.class })
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true)
public class AopAutoConfiguration {

	@Configuration
	@EnableAspectJAutoProxy(proxyTargetClass = false)
	@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false", matchIfMissing = false)
	public static class JdkDynamicAutoProxyConfiguration {

	}

	@Configuration
	@EnableAspectJAutoProxy(proxyTargetClass = true)
	@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true)
	public static class CglibAutoProxyConfiguration {

	}

}
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查看该类的源码,发现该类加载时有两个判断条件,容器中需要有EnableAspectJAutoProxy.class, Aspect.class, Advice.class,AnnotatedElement.class这几个注解,或者有spring.aop相关的配置。(关于Conditional条件的机制后面再详细解读,这个地方大概了解一下即可)

如果我们在启动时的类上添加了EnableAspectJAutoProxy注解的话,该注解会加载AspectJAutoProxyRegistrar类,这个类又会向容器注入AnnotationAwareAspectJAutoProxyCreator类,而后者正是aop的核心类。只要这个类进入容器,容器就带有了aop功能(aop如何实现的看我推荐的那本书,书上很详细)。

@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {
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那么如果我没有显示的添加EnableAspectJAutoProxy注解会怎样呢?如果没有显示添加的话,只要满足其他条件,AopAutoConfiguration类依然会被加载进容器,而他进入容器后,里面得到两个静态类也会被扫描进容器,而这两个类都是带有EnableAspectJAutoProxy注解的,所以aop功能依然可以实现。

所以当我们获得了自动化配置的这些支持后,就该回到刚刚标记的processImport方法了。

		public void processGroupImports() {
			for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
				grouping.getImports().forEach(entry -> {
					ConfigurationClass configurationClass = this.configurationClasses.get(
							entry.getMetadata());
					try {
					//刚刚标记的方法
						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);
					}
				});
			}
		}
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这个方法会把我们获得的自动化配置相关支持全部导入容器,这样在经过spring那一套加载逻辑之后,我们的springboot项目就可以获得各种我们配置的功能了。


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