Spring Boot

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Spring Boot启动流程

@SpringBootApplication注解

Spring Boot的启动，首先需要一个加了@SpringBootApplication注解的启动类。

@SpringBootApplication 注解

这个注解本质上，是由@EnableAutoConfiguration、@SpringBootConfiguration、@ComponentScan三个注解连起来构成。

  • @EnableAutoConfiguration最为核心

    会导入自动配置AutoConfigurationImportSelector类，这个类会将所有符合条件的@Configuration配置都进行加载。

    @EnableAutoConfiguration 注解

  • @SpringBootConfiguration等同于@Configuration

    将当前类标记为配置类，加载到容器中。

    @SpringBootConfiguration 注解

  • @ComponentScan自动扫描并加载符合条件的Bean

SpringApplication.run方法

image-20231028185647082

在run方法开始执行后，会经历如下4个阶段：

  1. 服务构建

    构建SpringApplication本身。

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    public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        // 1. 将资源加载器、主方法类加载至内存中
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        
        // 2. 逐一判断对应的服务类是否存在，来确定Web服务的类型。默认是基于Servlet的Web服务，如Tomcat
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
        
        // 3. 加载初始化类，读取所有"META-INF/spring.factories"文件中的 
        //		注册初始化、上下文初始化、监听器这三类配置
        // Spring Boot和Spring Boot Autoconfigure这两个工程中配置了7个上下文初始化和8个监听器
        this.bootstrapRegistryInitializers = new ArrayList<>(
                getSpringFactoriesInstances(BootstrapRegistryInitializer.class)); // 没有默认的注册初始化配置
        setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        
        // 通过StackWalker判断出main方法所在的类（大概率就是启动类本身）
        this.mainApplicationClass = deduceMainApplicationClass();
    }

  2. 环境准备

    SpringApplication.run就是进入环境准备阶段。

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    public ConfigurableApplicationContext run(String... args) {
        // ...
        
        DefaultBootstrapContext bootstrapContext = createBootstrapContext();
        ConfigurableApplicationContext context = null;
        configureHeadlessProperty();
        SpringApplicationRunListeners listeners = getRunListeners(args);
        listeners.starting(bootstrapContext, this.mainApplicationClass);
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
            Banner printedBanner = printBanner(environment);
            context = createApplicationContext();
            context.setApplicationStartup(this.applicationStartup);
            prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
            refreshContext(context);
            afterRefresh(context, applicationArguments);
            Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);
            }
            listeners.started(context, timeTakenToStartup);
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            if (ex instanceof AbandonedRunException) {
                throw ex;
            }
            handleRunFailure(context, ex, listeners);
            throw new IllegalStateException(ex);
        }
        try {
            if (context.isRunning()) {
                Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);
                listeners.ready(context, timeTakenToReady);
            }
        }
        catch (Throwable ex) {
            if (ex instanceof AbandonedRunException) {
                throw ex;
            }
            handleRunFailure(context, ex, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

  3. 容器创建

  4. 填充容器