SpringBoot的启动流程分析(2)

我们来分析SpringApplication启动流程中的run()方法,代码如下

    public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        FailureAnalyzers analyzers = null;
        configureHeadlessProperty();
        SpringApplicationRunListeners listeners = getRunListeners(args);
        listeners.starting();
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                    args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners,
                    applicationArguments);
            Banner printedBanner = printBanner(environment);
            context = createApplicationContext();
            analyzers = new FailureAnalyzers(context);
            prepareContext(context, environment, listeners, applicationArguments,
                    printedBanner);
            refreshContext(context);
            afterRefresh(context, applicationArguments);
            listeners.finished(context, null);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass)
                        .logStarted(getApplicationLog(), stopWatch);
            }
            return context;
        }
        catch (Throwable ex) {
            handleRunFailure(context, listeners, analyzers, ex);
            throw new IllegalStateException(ex);
        }
    }

看起来方法比较长，不过我们主要看启动流程，哪些监控，失败分析的我们以后再说，所以这个方法中关键的几步如下

 context = createApplicationContext();
 prepareContext(context,environment,listeners,applicationArguments,printedBanner);
 refreshContext(context);
 afterRefresh(context, applicationArguments);

没错，看到关键的方法就是这4个，我们首先分析 createApplicationContext()方法，代码如下

　　public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework."
            + "boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext";
    public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
            + "annotation.AnnotationConfigApplicationContext";

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

很明显，在web环境下它加载并创建了 ConfigurableApplicationContext,简单点说就是创建了ApplicationContext,即创建了Bean的容器(BeanFactory)

然后看 prepareContext()方法，可以看出，这个方法是将配置，参数，监听器，以及banner信息注入applicationContext中

    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 = getSources();
        Assert.notEmpty(sources, "Sources must not be empty");
        load(context, sources.toArray(new Object[sources.size()]));
        listeners.contextLoaded(context);
    }

方法3是启动的关键函数，在这一步中springboot已经启动，因为比较复杂，我们稍后说。所以我么先看 afterRefresh(context, applicationArguments),其细节如下

    protected void afterRefresh(ConfigurableApplicationContext context,
            ApplicationArguments args) {
        callRunners(context, args);
    }

    private void callRunners(ApplicationContext context, ApplicationArguments args) {
        List<Object> runners = new ArrayList<Object>();
        runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
        runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
        AnnotationAwareOrderComparator.sort(runners);
        for (Object runner : new LinkedHashSet<Object>(runners)) {
            if (runner instanceof ApplicationRunner) {
                callRunner((ApplicationRunner) runner, args);
            }
            if (runner instanceof CommandLineRunner) {
                callRunner((CommandLineRunner) runner, args);
            }
        }
    }    

　　private void callRunner(CommandLineRunner runner, ApplicationArguments args) {
        try {
            (runner).run(args.getSourceArgs());
        }
        catch (Exception ex) {
            throw new IllegalStateException("Failed to execute CommandLineRunner", ex);
        }
    }

看到这里大家想起来什么了没有？没错，我们自己现实的 ApplicationRunner 或 CommandLineRunner 这些类就是在这里被调用执行的。

最后我们来看最重要的 refreshContext(context)，其相关的代码如下

    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();
    }

    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();
            }
        }
    }

最后落在了AbstractApplicationContext.refresh方法中。

这个方法的每一步上都有明确的注释。

prepareRefresh()准备刷新，设置应用的开启时间，设置active标志，并执行一些属性的初始化工作。

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();获取BeanFactory。

prepareBeanFactory(beanFactory)执行一些BeanFactory的初始化工作。

postProcessBeanFactory(beanFactory)设置BeanFactory的后置处理器。（可以参考Spring中Bean的生命周期）

invokeBeanFactoryPostProcessors(beanFactory)调用BeanFactory的后置处理器。

registerBeanPostProcessors(beanFactory)在容器内的所有Bean实例化之前注册Bean的前(后)置处理器。(可以参考BeanPostProcessor)

initMessageSource()设置messgeSource

initApplicationEventMulticaster()初始化Context的广播器

onRefresh()根据子类的实现方式不同做不同的事。EmbeddedWebApplicationContext会创建内置的Tomcat并启动。

registerListeners()创建监听器

finishBeanFactoryInitialization(beanFactory)实例化BeanFactory中非懒加载的单例的Bean。这个方法比较关键。

finishRefresh()初始化或启动声明周期相关的任务，发布Context已经刷新的消息。

以上是springboot启动的粗略流程，让我们大致明白启动的流程是什么样的。各个步骤解释的不是特别详细，也有可能还有错误。

以后可能会逐步的进行详细的分析。

引出的问题

BeanFactory是什么。

Tomcat中和BeanFactory中生命周期的问题。

Spring中的事件监听机制。

Tomcat和Servlet和Springboot的关联。