Springboot底层启动过程是怎样的

SpringApplication构造分析

1、记录 BeanDefinition 源

spring容器刚开始是空的，要去各个源找到beanDefinition，这些源可能是配置类，可能是xml文件。在构造方法里会获取一个主源，也就是引导类，根据引导类去获取beanDefinition。

2、推断应用类型

根据jar包去判断是什么引用类型

3、记录 ApplicationContext 初始化器

对ApplicationContext做扩展

4、记录监听器

监听重要事件

5、推断主启动类

记录运行的主类。

SpringApplication run分析

1、得到 SpringApplicationRunListeners，名字取得不好，实际是事件发布器

发布 application starting 事件，在程序启动的重要节点发布事件

    public static void main(String[] args) throws Exception{
        // 添加 app 监听器
        SpringApplication app = new SpringApplication();
        app.addListeners(e -> System.out.println(e.getClass()));
        // 获取事件发送器实现类名
        List<String> names = SpringFactoriesLoader.loadFactoryNames(SpringApplicationRunListener.class, A39_2.class.getClassLoader());
        for (String name : names) {
            System.out.println(name);
            Class<?> clazz = Class.forName(name);
            Constructor<?> constructor = clazz.getConstructor(SpringApplication.class, String[].class);
            SpringApplicationRunListener publisher = (SpringApplicationRunListener) constructor.newInstance(app, args);
            // 发布事件
            DefaultBootstrapContext bootstrapContext = new DefaultBootstrapContext();
            publisher.starting(bootstrapContext); // spring boot 开始启动
            publisher.environmentPrepared(bootstrapContext, new StandardEnvironment()); // 环境信息准备完毕
            GenericApplicationContext context = new GenericApplicationContext();
            publisher.contextPrepared(context); // 在 spring 容器创建，并调用初始化器之后，发送此事件
            publisher.contextLoaded(context); // 所有 bean definition 加载完毕
            context.refresh();
            publisher.started(context); // spring 容器初始化完成(refresh 方法调用完毕)
            publisher.running(context); // spring boot 启动完毕
            publisher.failed(context, new Exception("出错了")); // spring boot 启动出错
        }

2、封装启动 args

3、准备 Environment 添加命令行参数（*）

public static void main(String[] args) throws IOException {
        ApplicationEnvironment env = new ApplicationEnvironment(); // 系统环境变量, properties, yaml
        env.getPropertySources().addLast(new ResourcePropertySource(new ClassPathResource("step3.properties")));
        env.getPropertySources().addFirst(new SimpleCommandLinePropertySource(args));
        for (PropertySource<?> ps : env.getPropertySources()) {
            System.out.println(ps);
        }
//        System.out.println(env.getProperty("JAVA_HOME"));
        System.out.println(env.getProperty("server.port"));
    }

4、ConfigurationPropertySources 处理（*）

发布 application environment 已准备事件

    public static void main(String[] args) throws IOException, NoSuchFieldException {
        ApplicationEnvironment env = new ApplicationEnvironment();
        env.getPropertySources().addLast(
                new ResourcePropertySource("step4", new ClassPathResource("step4.properties"))
        );
        ConfigurationPropertySources.attach(env);
        for (PropertySource<?> ps : env.getPropertySources()) {
            System.out.println(ps);
        }
        System.out.println(env.getProperty("user.first-name"));
        System.out.println(env.getProperty("user.middle-name"));
        System.out.println(env.getProperty("user.last-name"));
    }
}

5、通过 EnvironmentPostProcessorApplicationListener 进行 env 后处理（*）

application.properties，由 StandardConfigDataLocationResolver 解析

spring.application.json

public class Step5 {
    public static void main(String[] args) {
        SpringApplication app = new SpringApplication();
        app.addListeners(new EnvironmentPostProcessorApplicationListener());
        /*List<String> names = SpringFactoriesLoader.loadFactoryNames(EnvironmentPostProcessor.class, Step5.class.getClassLoader());
        for (String name : names) {
            System.out.println(name);
        }*/
        EventPublishingRunListener publisher = new EventPublishingRunListener(app, args);
        ApplicationEnvironment env = new ApplicationEnvironment();
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>> 增强前");
        for (PropertySource<?> ps : env.getPropertySources()) {
            System.out.println(ps);
        }
        publisher.environmentPrepared(new DefaultBootstrapContext(), env);
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>> 增强后");
        for (PropertySource<?> ps : env.getPropertySources()) {
            System.out.println(ps);
        }
    }
    private static void test1() {
        SpringApplication app = new SpringApplication();
        ApplicationEnvironment env = new ApplicationEnvironment();
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>> 增强前");
        for (PropertySource<?> ps : env.getPropertySources()) {
            System.out.println(ps);
        }
        ConfigDataEnvironmentPostProcessor postProcessor1 = new ConfigDataEnvironmentPostProcessor(new DeferredLogs(), new DefaultBootstrapContext());
        postProcessor1.postProcessEnvironment(env, app);
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>> 增强后");
        for (PropertySource<?> ps : env.getPropertySources()) {
            System.out.println(ps);
        }
        RandomValuePropertySourceEnvironmentPostProcessor postProcessor2 = new RandomValuePropertySourceEnvironmentPostProcessor(new DeferredLog());
        postProcessor2.postProcessEnvironment(env, app);
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>> 增强后");
        for (PropertySource<?> ps : env.getPropertySources()) {
            System.out.println(ps);
        }
        System.out.println(env.getProperty("server.port"));
        System.out.println(env.getProperty("random.int"));
        System.out.println(env.getProperty("random.int"));
        System.out.println(env.getProperty("random.int"));
        System.out.println(env.getProperty("random.uuid"));
        System.out.println(env.getProperty("random.uuid"));
        System.out.println(env.getProperty("random.uuid"));
    }
}

6、绑定 spring.main 到 SpringApplication 对象（*）

把配置文件中的值赋给SpringApplication的默认属性值

public class Step6 {
    // 绑定 spring.main 前缀的 key value 至 SpringApplication, 请通过 debug 查看
    public static void main(String[] args) throws IOException {
        SpringApplication application = new SpringApplication();
        ApplicationEnvironment env = new ApplicationEnvironment();
        env.getPropertySources().addLast(new ResourcePropertySource("step6", new ClassPathResource("step6.properties")));
        System.out.println(application);
        Binder.get(env).bind("spring.main", Bindable.ofInstance(application));
        System.out.println(application);
    }

7、打印 banner（*）

public class Step7 {
    public static void main(String[] args) {
        ApplicationEnvironment env = new ApplicationEnvironment();
        SpringApplicationBannerPrinter printer = new SpringApplicationBannerPrinter(
                new DefaultResourceLoader(),
                new SpringBootBanner()
        );
        // 测试文字 banner
//        env.getPropertySources().addLast(new MapPropertySource("custom", Map.of("spring.banner.location","banner1.txt")));
        // 测试图片 banner
//        env.getPropertySources().addLast(new MapPropertySource("custom", Map.of("spring.banner.image.location","banner2.png")));
        // 版本号的获取
        System.out.println(SpringBootVersion.getVersion());
        printer.print(env, Step7.class, System.out);
    }
}

8、创建容器

    private static GenericApplicationContext createApplicationContext(WebApplicationType type) {
        GenericApplicationContext context = null;
        switch (type) {
            case SERVLET -> context = new AnnotationConfigServletWebServerApplicationContext();
            case REACTIVE -> context = new AnnotationConfigReactiveWebServerApplicationContext();
            case NONE -> context = new AnnotationConfigApplicationContext();
        }
        return context;
    }

9、准备容器发布

application context 已初始化事件

10、加载 bean 定义

发布 application prepared 事件

        DefaultListableBeanFactory beanFactory = context.getDefaultListableBeanFactory();
        AnnotatedBeanDefinitionReader reader1 = new AnnotatedBeanDefinitionReader(beanFactory);
        XmlBeanDefinitionReader reader2 = new XmlBeanDefinitionReader(beanFactory);
        ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(beanFactory);
        reader1.register(Config.class);
        reader2.loadBeanDefinitions(new ClassPathResource("b03.xml"));
        scanner.scan("com.itheima.a39.sub");

11、refresh 容器

发布 application started 事件

12、执行 runner

• 发布 application ready 事件

• 这其中有异常，发布 application failed 事件

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