SpringBoot是如何动起来的？

程序入口

SpringApplication.run(BeautyApplication.class, args);

　　

执行此方法来加载整个SpringBoot的环境。

1. 从哪儿开始？

SpringApplication.java

    /**
     * Run the Spring application, creating and refreshing a new
     * {@link ApplicationContext}.
     * @param args the application arguments (usually passed from a Java main method)
     * @return a running {@link ApplicationContext}
     */
    public ConfigurableApplicationContext run(String... args) {
        //...
  }

调用SpringApplication.java 中的 run 方法，目的是加载Spring Application，同时返回 ApplicationContext。

2. 执行了什么？

2.1 计时

记录整个Spring Application的加载时间！

StopWatch stopWatch = new StopWatch();
stopWatch.start();
// ...
stopWatch.stop();
if (this.logStartupInfo) {
    new StartupInfoLogger(this.mainApplicationClass)
            .logStarted(getApplicationLog(), stopWatch);
}

2.2 声明

// 声明 ApplicationContext
ConfigurableApplicationContext context = null;
// 声明 一个异常报告集合
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();

2.3 指定程序运行模式

指定 java.awt.headless，默认是true 一般是在程序开始激活headless模式，告诉程序，现在你要工作在Headless mode下，就不要指望硬件帮忙了，你得自力更生，依靠系统的计算能力模拟出这些特性来。

private void configureHeadlessProperty() {
    System.setProperty(SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, System.getProperty(
            SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, Boolean.toString(this.headless)));
}

2.4 配置监听并发布应用启动事件

SpringApplicationRunListener 负责加载 ApplicationListener事件。

SpringApplicationRunListeners listeners = getRunListeners(args);
// 开始
listeners.starting();
// 处理所有 property sources 配置和 profiles 配置，准备环境，分为标准 Servlet 环境和标准环境
ConfigurableEnvironment environment = prepareEnvironment(listeners,applicationArguments);
// 准备应用上下文
prepareContext(context, environment, listeners, applicationArguments,printedBanner);
// 完成
listeners.started(context);
// 异常
handleRunFailure(context, ex, exceptionReporters, listeners);
// 执行
listeners.running(context);

getRunListeners 中根据 type = SpringApplicationRunListener.class 去拿到了所有的 Listener 并根据优先级排序。

对应的就是 META-INF/spring.factories 文件中的 

org.springframework.boot.SpringApplicationRunListener=org.springframework.boot.context.event.EventPublishingRunListener

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
            Class<?>[] parameterTypes, Object... args) {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        // Use names and ensure unique to protect against duplicates
        Set<String> names = new LinkedHashSet<>(
                SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
                classLoader, args, names);
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }

　　

在 ApplicationListener 中 , 可以针对任何一个阶段插入处理代码。

public interface SpringApplicationRunListener {

    /**
     * Called immediately when the run method has first started. Can be used for very
     * early initialization.
     */
    void starting();

    /**
     * Called once the environment has been prepared, but before the
     * {@link ApplicationContext} has been created.
     * @param environment the environment
     */
    void environmentPrepared(ConfigurableEnvironment environment);

    /**
     * Called once the {@link ApplicationContext} has been created and prepared, but
     * before sources have been loaded.
     * @param context the application context
     */
    void contextPrepared(ConfigurableApplicationContext context);

    /**
     * Called once the application context has been loaded but before it has been
     * refreshed.
     * @param context the application context
     */
    void contextLoaded(ConfigurableApplicationContext context);

    /**
     * The context has been refreshed and the application has started but
     * {@link CommandLineRunner CommandLineRunners} and {@link ApplicationRunner
     * ApplicationRunners} have not been called.
     * @param context the application context.
     * @since 2.0.0
     */
    void started(ConfigurableApplicationContext context);

    /**
     * Called immediately before the run method finishes, when the application context has
     * been refreshed and all {@link CommandLineRunner CommandLineRunners} and
     * {@link ApplicationRunner ApplicationRunners} have been called.
     * @param context the application context.
     * @since 2.0.0
     */
    void running(ConfigurableApplicationContext context);

    /**
     * Called when a failure occurs when running the application.
     * @param context the application context or {@code null} if a failure occurred before
     * the context was created
     * @param exception the failure
     * @since 2.0.0
     */
    void failed(ConfigurableApplicationContext context, Throwable exception);

}

3. 每个阶段执行的内容

3.1 listeners.starting();

在加载Spring Application之前执行，所有资源和环境未被加载。

3.2 prepareEnvironment(listeners, applicationArguments);

创建 ConfigurableEnvironment；将配置的环境绑定到Spring Application中；

  private ConfigurableEnvironment prepareEnvironment(
            SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments) {
        // Create and configure the environment
        ConfigurableEnvironment environment = getOrCreateEnvironment();
        configureEnvironment(environment, applicationArguments.getSourceArgs());
        listeners.environmentPrepared(environment);
        bindToSpringApplication(environment);
        if (this.webApplicationType == WebApplicationType.NONE) {
            environment = new EnvironmentConverter(getClassLoader())
                    .convertToStandardEnvironmentIfNecessary(environment);
        }
        ConfigurationPropertySources.attach(environment);
        return environment;
    }

3.3 prepareContext

配置忽略的Bean；

private void configureIgnoreBeanInfo(ConfigurableEnvironment environment) {
        if (System.getProperty(
                CachedIntrospectionResults.IGNORE_BEANINFO_PROPERTY_NAME) == null) {
            Boolean ignore = environment.getProperty("spring.beaninfo.ignore",
                    Boolean.class, Boolean.TRUE);
            System.setProperty(CachedIntrospectionResults.IGNORE_BEANINFO_PROPERTY_NAME,
                    ignore.toString());
        }
    }

打印日志-加载的资源

Banner printedBanner = printBanner(environment);

根据不同的WebApplicationType创建Context

context = createApplicationContext();

3.4 refreshContext

支持定制刷新

  /**
     * Register a shutdown hook with the JVM runtime, closing this context
     * on JVM shutdown unless it has already been closed at that time.
     * <p>This method can be called multiple times. Only one shutdown hook
     * (at max) will be registered for each context instance.
     * @see java.lang.Runtime#addShutdownHook
     * @see #close()
     */
    void registerShutdownHook();

3.5 afterRefresh

刷新后的实现方法暂未实现

   /**
     * Called after the context has been refreshed.
     * @param context the application context
     * @param args the application arguments
     */
    protected void afterRefresh(ConfigurableApplicationContext context,
            ApplicationArguments args) {
    }

3.6 listeners.started(context);

到此为止， Spring Application的环境和资源都加载完毕了；发布应用上下文启动完成事件；执行所有 Runner 运行器 - 执行所有 ApplicationRunner 和 CommandLineRunner 这两种运行器。

// 启动
callRunners(context, applicationArguments);

3.7 listeners.running(context);

触发所有 SpringApplicationRunListener 监听器的 running 事件方法。