springboot源码解析

作者: 笔记本一号 | 来源:发表于2022-07-30 12:30 被阅读0次

1、初始化SpringApplication

启动的第一步，先调用SpringApplication的静态函数run方法，run方法会调用SpringApplication的有参构造方法来初始化SpringApplication。
而SpringApplication的初始化方法主要做以下几件事情：
1、推断当前 WEB 应用类型，一共有三种：
NONE：无内嵌的web容器启动，这种模式springboot需要运行于外部的web容器中
SERVLET：使用内嵌的web容器启动
REACTIVE：使用spring5的新特性，响应式启动
2、从META-INF/spring.factories读取对应的ApplicationContextInitializer的类装配到集合中并去重
3、从META-INF/spring.factories读取对应ApplicationListener的类装配到集合中并去重*

SpringApplication的构造函数实例化了初始化上下文的各种接口ApplicationContextInitializer以及监听器ApplicationListener，要注意的是这里的实例化，并不像平时的Spring Components一样通过注解和扫包完成，而是通过一种不依赖Spring上下文的加载方法，这样才能在Spring完成启动前做各种配置。Spring的解决方法是以接口的全限定名作为key，实现类的全限定名作为value记录在项目的META-INF/spring.factories文件中，然后通过SpringFactoriesLoader工具类提供静态方法进行类加载并缓存下来，spring.factories是Spring Boot的核心配置文件，目的是帮助Spring Boot完成自动化装配。

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

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
       .........................................
        //推断当前 WEB 应用类型
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
        //从"META-INF/spring.factories"读取对应的ApplicationContextInitializer类的装配到集合中并去重
        this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
        //从"META-INF/spring.factories"读取对应ApplicationListener类的装配到集合中并去重
        this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
        //获取Main方法所在类，并赋值给mainApplicationClass
        this.mainApplicationClass = this.deduceMainApplicationClass();
    }

spring.factories的部分内容：

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

2、执行SpringApplication.run启动容器

启动的步骤主要是以下几个步骤：
1、stopWatch.start()：记录容器启动耗费的时间
2、this.getRunListeners(args)：从"META-INF/spring.factories"获取key为SpringApplicationRunListener的运行时监听器，其实就是EventPublishingRunListener监听器
3、listeners.starting() 向运行时监听器注册监听事件
4、ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments)根据监听器准备spring容器的环境变量
5、context = this.createApplicationContext()根据WEB 应用类型创建相应的容器，一共有三种：
NONE：AnnotationConfigApplicationContext
SERVLET：AnnotationConfigServletWebServerApplicationContext
REACTIVE：AnnotationConfigReactiveWebServerApplicationContext
6、this.prepareContext(context, environment, listeners, applicationArguments, printedBanner)这里主要是给容器设置前面准备好的环境变量，并且执行之前在spring.factories中读取的ApplicationContextInitializer类的initialize方法
7、this.refreshContext(context)刷新容器，这里比较重点后面单独分析
8、this.afterRefresh(context, applicationArguments)这里是空的方法，用于子类继承做一些容器启动后的拓展
9、监听器发布ApplicationStartedEvent事件和ApplicationReadyEvent事件

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        //记录容器启动耗费的时间
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
        this.configureHeadlessProperty();
        //从"META-INF/spring.factories"获取key为SpringApplicationRunListener的监听器，其实就是EventPublishingRunListener监听器
        SpringApplicationRunListeners listeners = this.getRunListeners(args);
        //遍历并集合中的监听器，并注册需要监听事件
        listeners.starting();
        Collection exceptionReporters;
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
           //准备spring容器的环境变量            
            ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);

            this.configureIgnoreBeanInfo(environment);
            Banner printedBanner = this.printBanner(environment);
           //根据WEB 应用类型创建相应的容器
            context = this.createApplicationContext();
            exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
             //这里主要是给容器设置前面准备好的环境变量，并且执行之前在spring.factories中读取的ApplicationContextInitializer类的initialize方法
            this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
           //刷新容器
            this.refreshContext(context);
           //这里是空的方法，用于子类继承做一些容器启动后的拓展
            this.afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
            }
            //监听器发布ApplicationStartedEvent事件
            listeners.started(context);
          try {
            listeners.running(context);//发布ApplicationReadyEvent事件
           }
         ......................................
//返回容器上下文
    return context;
    }

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment, SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
        context.setEnvironment(environment);
        this.postProcessApplicationContext(context);
        this.applyInitializers(context);
        listeners.contextPrepared(context);
        ......................................
    }

3、refresh()刷新容器

1、prepareRefresh()容器的预处理：记录启动时间，设置容器的状态，预留钩子方法给子类做属性设置，保存早期监听事件
2、obtainFreshBeanFactory()获取DefaultListableBeanFactory的作为工厂对象
3、prepareBeanFactory(beanFactory)设置工厂对象的属性，包括：类加载器、表达式解析器、添加后置处理器ApplicationContextAwareProcessor,系统环境属性Environment等
4、postProcessBeanFactory(beanFactory)钩子方法，预留给子类实现，在工厂对象准备刷新容器前做一些处理
5、invokeBeanFactoryPostProcessors(beanFactory)获取BeanDefinitionRegistryPostProcessor，遍历并根据优先级(继承order接口)执行其后置方法postProcessBeanDefinitionRegistry()方法。然后在获取所有的BeanFactoryPostProcessor的，再遍历并根据优先级(继承order接口)执行后置方法postProcessBeanFactory(),由此可见后置处理器BeanDefinitionRegistryPostProcessor是在BeanFactoryPostProcessor之前执行
6、registerBeanPostProcessors(beanFactory)获取所有的BeanPostProcessor，例如：InstantiationAwareBeanPostProcessor等，根据优先级(继承order接口)将所有的BeanPostProcessor注册到工厂对象中
7、initMessageSource()进行国际化
8、initApplicationEventMulticaster() 初始化的事件派发器
9、onRefresh()钩子方法，预留给子类进行拓展处理，其中springboot的web容器AnnotationConfigServletWebServerApplicationContext会在这里启动Tomcat
10、registerListeners()注册事件监听器，监听器需要实现 ApplicationListener 接口
11、finishBeanFactoryInitialization(beanFactory)初始化所有的单实例bean,主要方法在工厂对象的preInstantiateSingletons()方法，先获取RootBeanBefinition，在判断是否是factoryBean,是则调用factoryBean的方法创建bean,不是则调用getBean()方法进行bean的创建，getBean()方法是spring创建bean的核心类，我之前在spring源码分析那里分析过，这里不再赘述，主要流程就先从缓存中获取bean,获取不到就从头开始创建bean，并经过bean的实例化、属性赋值、初始化的生命周期，中间调用Aware类的方法和上面注册的BeanPostProcessor的前后置方法，完成IOC创建
12、finishRefresh()容器调用LifecycleProcessor生命周期处理器的onRefresh()和onClose()方法，然后调用publishEvent((ApplicationEvent)(new ContextRefreshedEvent(this)))发布容器刷新完成事件

public void refresh() throws BeansException, IllegalStateException {
        synchronized(this.startupShutdownMonitor) {
            StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
            this.prepareRefresh();
            ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
            this.prepareBeanFactory(beanFactory);

            try {
                this.postProcessBeanFactory(beanFactory);
                StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
                this.invokeBeanFactoryPostProcessors(beanFactory);
                this.registerBeanPostProcessors(beanFactory);
                beanPostProcess.end();
                this.initMessageSource();
                this.initApplicationEventMulticaster();
                this.onRefresh();
                this.registerListeners();
                this.finishBeanFactoryInitialization(beanFactory);
                this.finishRefresh();
            } catch (BeansException var10) {
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var10);
                }

                this.destroyBeans();
                this.cancelRefresh(var10);
                throw var10;
            } finally {
                this.resetCommonCaches();
                contextRefresh.end();
            }

        }
    }


protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        .......................................
        beanFactory.preInstantiateSingletons();
    }

public void preInstantiateSingletons() throws BeansException {
        List<String> beanNames = new ArrayList(this.beanDefinitionNames);
        Iterator var2 = beanNames.iterator();
        while(true) {
            String beanName;
            Object bean;
            do {
                while(true) {
                    RootBeanDefinition bd;
                    do {
                        do {
                           do {
                                if (!var2.hasNext()) {
                                    var2 = beanNames.iterator();
                                    while(var2.hasNext()) {
                                        beanName = (String)var2.next();
                                        Object singletonInstance = this.getSingleton(beanName);
                                        if (singletonInstance instanceof SmartInitializingSingleton) {
                                            SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton)singletonInstance;
                                            if (System.getSecurityManager() != null) {
                                                AccessController.doPrivileged(() -> {
                                                    smartSingleton.afterSingletonsInstantiated();
                                                    return null;
                                                }, this.getAccessControlContext());
                                            } else {
                                                smartSingleton.afterSingletonsInstantiated();
                                            }
                                        }
                                    }

                                    return;
                                }

                                beanName = (String)var2.next();
                                bd = this.getMergedLocalBeanDefinition(beanName);
                            } while(bd.isAbstract());
                        } while(!bd.isSingleton());
                    } while(bd.isLazyInit());

                    if (this.isFactoryBean(beanName)) {
                        bean = this.getBean("&" + beanName);
                        break;
                    }

                    this.getBean(beanName);
                }
            } while(!(bean instanceof FactoryBean));

            FactoryBean<?> factory = (FactoryBean)bean;
            boolean isEagerInit;
            if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                SmartFactoryBean var10000 = (SmartFactoryBean)factory;
                ((SmartFactoryBean)factory).getClass();
                isEagerInit = (Boolean)AccessController.doPrivileged(var10000::isEagerInit, this.getAccessControlContext());
            } else {
                isEagerInit = factory instanceof SmartFactoryBean && ((SmartFactoryBean)factory).isEagerInit();
            }

            if (isEagerInit) {
                this.getBean(beanName);
            }
        }
    }

4、springboot的自动化装配

@SpringBootApplication上的注解@EnableAutoConfiguration，通过@Import({AutoConfigurationImportSelector.class})注入AutoConfigurationImportSelector，AutoConfigurationImportSelector通过调用getAutoConfigurationEntry方法，方法中会调用getCandidateConfigurations方法，然后通过SpringFactoriesLoader.loadFactoryNames方法，从META-INF/spring.factories读取EnableAutoConfiguration的类为key,value为需要自动装配的xxxConfiguration配置类的数组，通过SpringFactoriesLoader机制创建对应的bean，注入到容器中，完成了自动装配。

这本来需要在spring的xml配置文件中去配置bean的操作就免去了，也就是说我们可以事先在xxxConfiguration中做好各种默认配置，然后通过加入到spring.factories中让spring自动读取并装配，实现了starter默认配置达到了约定大于配置的思想，这就是也就是springboot的自动装配

简单的流程如下：@EnableAutoConfiguration->@Import({AutoConfigurationImportSelector.class})->SpringFactoriesLoader.loadFactoryNames->META-INF/spring.factories->org.springframework.boot.autoconfigure.EnableAutoConfiguration=xxxConfiguration，xxxConfiguration....

@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 {};
}

AutoConfigurationImportSelector类及对应的SpringFactoriesLoader.loadFactoryNames及SpringFactoriesLoader.loadSpringFactories方法

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
     protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(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.getConfigurationClassFilter().filter(configurations);
            this.fireAutoConfigurationImportEvents(configurations, exclusions);
            return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
        }
    }


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;
    }
}

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");
                LinkedMultiValueMap 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()) {
                        Entry<?, ?> entry = (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);
            }
        }
    }

# Auto Configure
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.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.LifecycleAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\

springboot源码解析

1、初始化SpringApplication

2、执行SpringApplication.run启动容器

3、refresh()刷新容器

4、springboot的自动化装配

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