承接前文springboot情操陶冶-SpringApplication(一)，本文将对run()方法作下详细的解析

SpringApplication#run()

main函数经常调用的run()方法是我们分析的关键，先上源码

    public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        // 读取java.awt.headless系统变量，默认为true.常用于linux图片的渲染
        configureHeadlessProperty();
        // 获取SpringApplicationRunListener接口集合并实例化调用公共接口starting()
        SpringApplicationRunListeners listeners = getRunListeners(args);
        listeners.starting();
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                    args);
            // environment configuration
            ConfigurableEnvironment environment = prepareEnvironment(listeners,
                    applicationArguments);
            // spring.beaninfo.ignore属性读取
            configureIgnoreBeanInfo(environment);
            // springboot的banner样图
            Banner printedBanner = printBanner(environment);
            // 创建spring应用上下文(尚未刷新)
            context = createApplicationContext();
            // SpringBootExceptionReporter接口集合读取
            exceptionReporters = getSpringFactoriesInstances(
                    SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
            // applicationContext configuration
            prepareContext(context, environment, listeners, applicationArguments,
                    printedBanner);
            // 刷新spring应用上下文
            refreshContext(context);
            afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass)
                        .logStarted(getApplicationLog(), stopWatch);
            }
            // SpringApplicationRunListener接口的started()方法调用
            listeners.started(context);
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, listeners);
            throw new IllegalStateException(ex);
        }

        try {
            // SpringApplicationRunListener接口的running()方法调用
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

以上的代码注释有点多，笔者分块来进行罗列分析

SpringApplication#getRunListeners()

获取SpringApplicationRunListener接口集合并实例化，根据前文得知，读取的是META\spring.factories文件中的对应属性，
此处笔者以org.springframework.boot.context.event.EventPublishingRunListener为例。

先观察下其构造函数

    public EventPublishingRunListener(SpringApplication application, String[] args) {
        this.application = application;
        this.args = args;
        this.initialMulticaster = new SimpleApplicationEventMulticaster();
        for (ApplicationListener<?> listener : application.getListeners()) {
            this.initialMulticaster.addApplicationListener(listener);
        }
    }

注意application.getListeners()方法，根据前文得知，其会拿到类型为ApplicationListener的集合并存入至SimpleApplicationEventMulticaster广播类中；
其余方法，比如starting()/started()/environmentPrepared()等等方法均是由其统一调用所有的ApplicationListener接口的对应事件。笔者此处以starting()为例

    @Override
    public void starting() {
        this.initialMulticaster.multicastEvent(
                new ApplicationStartingEvent(this.application, this.args));
    }

其会找寻支持响应ApplicationStartingEvent事件的Listeners，并执行相应的事件方法。响应的监听器有LoggingApplicationListener和LiquibaseServiceLocatorApplicationListener等

SpringApplication#prepareEnvironment()

Environment环境准备工作

    private ConfigurableEnvironment prepareEnvironment(
            SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments) {
        // Create and configure the environment
        ConfigurableEnvironment environment = getOrCreateEnvironment();
        // 解析args参数和spring.profiles.active配置读取
        configureEnvironment(environment, applicationArguments.getSourceArgs());
        // 触发ApplicationEnvironmentPreparedEvent事件
        listeners.environmentPrepared(environment);
        bindToSpringApplication(environment);
        if (this.webApplicationType == WebApplicationType.NONE) {
            environment = new EnvironmentConverter(getClassLoader())
                    .convertToStandardEnvironmentIfNecessary(environment);
        }
        ConfigurationPropertySources.attach(environment);
        return environment;
    }

内含代码内容过多，笔者此处针对自己的阅读作下小结

1. args参数是会被包装为SimpleCommandLinePropertySource属性源，对应key为commandLineArgs

2. spring.profiles.active系统属性读取，用于不同条件的配置

3. ApplicationEnvironmentPreparedEvent事件触发，主要的有ConfigFileApplicationListener监听类(其会默认读取application.properties/application.xml/application.yaml配置文件)

   PS:ConfigFileApplicationListener这个类比较重要，其也会去读取配置文件中的spring.profiles.active属性加载Profile；
   另外也去读取EnvironmentPostProcessor接口来统一调用。有兴趣的读者可好好分析一下

4. 将Environment含有的属性源绑定至key为configurationProperties的PropertySources类型中，方便springboot全局搜索上下文所含有的属性

SpringApplication#configureIgnoreBeanInfo()

读取spring.beaninfo.ignore属性，默认为true，主要是用于忽略bean的基本信息

SpringApplication#printBanner()

创建banner打印对象，其默认打印的样图如下(作用于console)。具体的读者感兴趣可自行阅读原理

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \  \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.0.3.RELEASE)

SpringApplication#createApplicationContext()

创建应用上下文对象，其会根据判断出来的应用类型来创建相应的上下文。

很明显，全部会应用注解方式来加载上下文。

SpringApplication#prepareContext()

对已创建的上下文对象作下预备工作

SpringApplication#applyInitializers()

启动相应的初始化类，这些初始化类均是ApplicationContextInitializer接口的实现类

    protected void applyInitializers(ConfigurableApplicationContext context) {
        for (ApplicationContextInitializer initializer : getInitializers()) {
            // 检查相应的ApplicationContextInitializer实体类所接收的泛型实体class
            Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
                    initializer.getClass(), ApplicationContextInitializer.class);
            // 只对接收的泛型为ConfigurableApplicationContext.class进行相应的初始化
            Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
            initializer.initialize(context);
        }
    }

针对上面的代码注释，初始化类基本有ConfigurationWarningsApplicationContextInitializer/ContextIdApplicationContextInitializer/DelegatingApplicationContextInitializer/ServerPortInfoApplicationContextInitializer等。
有兴趣的可自行分析相应的初始化类都执行了哪些操作

SpringApplication#load()

加载beans到相应的上下文对象ApplicationContext中

    // 此处的source一般为main函数所在的class，也可通过SpringApplication#setSource()来新增
    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);
        }
        // 加载beans
        loader.load();
    }

笔者此处最关心这个BeanDefinitionLoader会耍什么花样，继续往下

先从构造函数开始

    BeanDefinitionLoader(BeanDefinitionRegistry registry, Object... sources) {
        Assert.notNull(registry, "Registry must not be null");
        Assert.notEmpty(sources, "Sources must not be empty");
        this.sources = sources;
        // 注解解析类
        this.annotatedReader = new AnnotatedBeanDefinitionReader(registry);
        // xml解析类
        this.xmlReader = new XmlBeanDefinitionReader(registry);
        if (isGroovyPresent()) {
            this.groovyReader = new GroovyBeanDefinitionReader(registry);
        }
        // classpath扫描类，并屏蔽指定的sources类
        this.scanner = new ClassPathBeanDefinitionScanner(registry);
        this.scanner.addExcludeFilter(new ClassExcludeFilter(sources));
    }

1. AnnotatedBeanDefinitionReader注解解析类，其会注册多个postProcessor接口供后续的上下文刷新操作被调用解析@Configuration/@Autowired/@Required/。

2. AnnotatedBeanDefinitionReaderXML解析类，其会解析XML配置

3. ClassPathBeanDefinitionScanner扫描classpath环境下指定的包以及指定class类

再看下处理方法load()

    public int load() {
        int count = 0;
        for (Object source : this.sources) {
            count += load(source);
        }
        return count;
    }
    
    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());
    }
    // 笔者重点关注此处
    private int load(Class<?> source) {
        if (isGroovyPresent()
                && GroovyBeanDefinitionSource.class.isAssignableFrom(source)) {
            // Any GroovyLoaders added in beans{} DSL can contribute beans here
            GroovyBeanDefinitionSource loader = BeanUtils.instantiateClass(source,
                    GroovyBeanDefinitionSource.class);
            load(loader);
        }
        // 查看class是否被@Component修饰过。此处多用于加载main类
        if (isComponent(source)) {
            // 注册bean
            this.annotatedReaAnnotatedBeanDefinitionReaderder.register(source);
            return 1;
        }
        return 0;
    }   

此处的load()方法主要是对设置的sources类进行判断是否被@Component注解，是则注入至bean工厂。即我们常写的main()函数类一般会加上@SpringApplication注解，其最终会被AnnotatedBeanDefinitionReader处理。

SpringApplication#refresh()

刷新上下文对象，笔者此处就不展开了，可参考之前的文章Spring源码情操陶冶-AbstractApplicationContext

异常处理

springboot的异常处理机制是通过读取SpringBootExceptionReporter接口类来对不同的异常进行不同的输出，感兴趣的可自行阅读

小结

通过上述的分析，基本对springboot的工作原理有了一定的了解，最主要的其实还是其会将公共的配置放置于META\spring.factories文件中，我们以后只要多关注此文件就会明白的更多。
至于@SpringApplication注解是如何为springboot服务的，笔者后续再分析

springboot情操陶冶-SpringApplication(二)