springboot启动流程原理

@SpringBootApplication
public class SpringmvcApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringmvcApplication.class, args);
    }
}

//启动类调的run方法
public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
    //调的是下面的，参数是数组的run方法
    return run(new Class<?>[] { primarySource }, args);
}
//和上面的方法区别在于第一个参数是一个数组
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
    //实际上new一个SpringApplication实例，调的是一个实例方法run()
    return new SpringApplication(primarySources).run(args);
}

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    //断言primarySources不能为null，如果为null，抛出异常提示
    Assert.notNull(primarySources, "PrimarySources must not be null");
    //启动类传入的Class
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    //判断当前项目类型，有三种：NONE、SERVLET、REACTIVE
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    //设置ApplicationContextInitializer
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    //设置监听器
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    //判断主类，初始化入口类
    this.mainApplicationClass = deduceMainApplicationClass();
}
//判断主类
private Class<?> deduceMainApplicationClass() {
    try {
        StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
        for (StackTraceElement stackTraceElement : stackTrace) {
            if ("main".equals(stackTraceElement.getMethodName())) {
                return Class.forName(stackTraceElement.getClassName());
            }
        }
    }
    catch (ClassNotFoundException ex) {
        // Swallow and continue
    }
    return null;
}

//设置初始化器(Initializer),最后会调用这些初始化器
setInitializers((Collection) getSpringFactoriesInstances( ApplicationContextInitializer.class));

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
    return getSpringFactoriesInstances(type, new Class<?>[] {});
}
// 这里的入参type就是ApplicationContextInitializer.class
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
        Class<?>[] parameterTypes, Object... args) {
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    // 使用Set保存names来避免重复元素
    Set<String> names = new LinkedHashSet<>(
            SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    // 根据names来进行实例化
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
            classLoader, args, names);
    // 对实例进行排序
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
    return getSpringFactoriesInstances(type, new Class<?>[] {});
}
// 这里的入参type就是ApplicationContextInitializer.class
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
        Class<?>[] parameterTypes, Object... args) {
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    // 使用Set保存names来避免重复元素
    Set<String> names = new LinkedHashSet<>(
            SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    // 根据names来进行实例化
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
            classLoader, args, names);
    // 对实例进行排序
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

public ConfigurableApplicationContext run(String... args) {
    // 计时工具
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    // 第一步：获取并启动监听器
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting();
    try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        // 第二步：根据SpringApplicationRunListeners以及参数来准备环境
        ConfigurableEnvironment environment = prepareEnvironment(listeners,applicationArguments);
        configureIgnoreBeanInfo(environment);
        // 准备Banner打印器 - 就是启动Spring Boot的时候打印在console上的ASCII艺术字体
        Banner printedBanner = printBanner(environment);
        // 第三步：创建Spring容器
        context = createApplicationContext();
        exceptionReporters = getSpringFactoriesInstances(
                SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class }, context);
        // 第四步：Spring容器前置处理
        prepareContext(context, environment, listeners, applicationArguments,printedBanner);
        // 第五步：刷新容器
        refreshContext(context);
　　　　 // 第六步：Spring容器后置处理
        afterRefresh(context, applicationArguments);
  　　　 // 第七步：发出结束执行的事件
        listeners.started(context);
        // 第八步：执行Runners
        this.callRunners(context, applicationArguments);
        stopWatch.stop();
        // 返回容器
        return context;
    }
    catch (Throwable ex) {
        handleRunFailure(context, listeners, exceptionReporters, ex);
        throw new IllegalStateException(ex);
    }
}