SpringBoot启动流程简介

一、Application启动类中main方法启动

@SpringBootApplication(scanBasePackages = {"com.springboot.lean"})
public class Application {
 
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

1.1 @SpringBootApplication注解

1.2 SpringApplication.run方法

主要做两件事，首先根据默认设置和用户提供的参数创建一个新的SpringApplication实例

private void initialize(Object[] sources) {
    if (sources != null && sources.length > 0) {
        this.sources.addAll(Arrays.asList(sources));
    }
    this.webEnvironment = deduceWebEnvironment();
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    this.mainApplicationClass = deduceMainApplicationClass();
}

然后调用真正的run方法，启动Spring application，并创建及刷新ApplicationContext

public ConfigurableApplicationContext run(String... args) {
    //计数器，用于计算并打印容器启动时长
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    
    ConfigurableApplicationContext context = null;
    FailureAnalyzers analyzers = null;
    
    //设置java.awt.headless属性，java.awt.headless是J2SE的一种模式，
    //用于在缺失显示屏、鼠标或者键盘时的系统配置。对于后端服务来讲，很多都是需要将这个属性设置为true的
    configureHeadlessProperty();
 
    //调用getSpringFactoriesInstances从META-INF/spring.factories方法中获取并创建所有的SpringApplicationRunListener，
    //底层实际会调用SpringFactoriesLoader.loadFactoryNames(type, classLoader)
    SpringApplicationRunListeners listeners = getRunListeners(args);
    //启动监听器，例如EventPublishingRunListener
    listeners.starting();
    try {
        
        //根据用户自定义参数构建应用参数
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        
        //首先创建或配置一个ConfigurableEnvironment实例，然后将应用参数设置进环境对象中
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        
        //打印启动图形，图形文件默认在META-INF下的banner.txt文件中，关键类为SpringApplicationBannerPrinter
        Banner printedBanner = printBanner(environment);
 
        //创建application上下文，根据webEnvironment判断是加载AnnotationConfigApplicationContext类实例还是
        //AnnotationConfigEmbeddedWebApplicationContext类实例，这两个对象都会注册所有相关的注解后处理器
        //例如@Configuration，@Autowired
        context = createApplicationContext();
 
        //创建失败分析器实例
        analyzers = new FailureAnalyzers(context);
 
        prepareContext(context, environment, listeners, applicationArguments, printedBanner);
 
        //调用AbstractApplicationContext.refresh方法，进入spring启动流程
        refreshContext(context);
 
        //在容器启动刷新后做些事情，模板方法，用于自定义扩展实现
        afterRefresh(context, applicationArguments);
 
        //监听器发布容器启动完成事件
        listeners.finished(context, null);
        stopWatch.stop();
        if (this.logStartupInfo) {
            //打印容器启动时长
            new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
        }
        return context;
    }
    catch (Throwable ex) {
        //解析容器加载失败信息，并打印堆栈错误日志
        handleRunFailure(context, listeners, analyzers, ex);
        throw new IllegalStateException(ex);
    }
}