深入剖析Springboot启动原理的底层源码，再也不怕面试官问了！

文章目录

image

一、入口类及其源码剖析

入口类

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

首先从注解入手，进行分析：

@SpringBootApplication 注解

Spring Boot应用标注在某个类上说明这个类是SpringBoot的主配置类，SpringBoot就应该运行这个类的main方法来启动SpringBoot应用

源码剖析

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

从源码可以看出，这个注解是@SpringBootConfiguration，@EnableAutoConfiguration以及@ComponentScan这三个注解的组合
① @SpringBootConfiguration
Spring Boot的配置类；标注在某个类上，表示一个类提供了Spring Boot应用程序
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {

@Configuration:配置类上来标注这个注解；
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {

注意：

配置类相当于配置文件；配置类也是容器中的一个组件，它使用了@Component这个注解。

② @EnableAutoConfiguration

告诉SpringBoot开启自动配置功能，这样自动配置才能生效

借助@import,扫描并实例化满足条件的自动配置的bean，然后加载到IOC容器中

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    Class[] exclude() default {};
    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    String[] excludeName() default {};
}

@AutoConfigurationPackage：自动配置包

@Import(EnableAutoConfigurationImportSelector.class)：给容器中导入组件

使用@EnableAutoConfiguration

这个注解开启自动扫描，然后使用select选择挑选满足条件的文件，并且使用SpringFactoriesLoader进行实例化。最后加载到IOC容器里面，即ApplicationContext中。

③ @ComponentScan

@ComponentScan就是自动扫描并加载符合条件的组件（比如@Component和@Repository等）或者bean定义，最终将这些bean定义加载到IOC容器中去 。

二、实例化SpringApplication对象的源码剖析

源码剖析

/**
 * Create a new {@link SpringApplication} instance. The application context will load
 * beans from the specified primary sources (see {@link SpringApplication class-level}
 * documentation for details. The instance can be customized before calling
 * {@link #run(String...)}.
 * @param resourceLoader the resource loader to use
 * @param primarySources the primary bean sources
 * @see #run(Class, String[])
 * @see #setSources(Set)
 */
@SuppressWarnings({ "unchecked", "rawtypes" })
public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {
    // 初始化资源加载器
    this.resourceLoader = resourceLoader;
    // 资源加载类不能为 null
    Assert.notNull(primarySources, "PrimarySources must not be null");
    // 初始化加载资源类集合并去重
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    // 推断应用程序是不是web应用
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    // 设置初始化器(Initializer)
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    // 设置监听器 
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    // 推断出主应用入口类
    this.mainApplicationClass = deduceMainApplicationClass();
}

其中，在推断应用程序是不是web应用的时候调用了deduceFromClasspath() 方法

源码剖析

static WebApplicationType deduceFromClasspath() {
        if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
                && !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
            // springboot2.0提出的响应式web应用 
            return WebApplicationType.REACTIVE;
        }
        for (String className : SERVLET_INDICATOR_CLASSES) {
            // 如果两个包路径都没有的话，就是普通应用
            if (!ClassUtils.isPresent(className, null)) {
                // 普通的应用
                return WebApplicationType.NONE;
            }
        }
        // 其实最后返回的就是这个servlet，因为是web应用
        return WebApplicationType.SERVLET;
    }

1. 设置初始化器(Initializer)

initializers 是 SpringApplication 中的一个实例属性

源码剖析

/**
 * Sets the {@link ApplicationContextInitializer} that will be applied to the Spring
 * {@link ApplicationContext}.
 * @param initializers the initializers to set
 */
public void setInitializers(Collection> initializers) {
    this.initializers = new ArrayList<>(initializers);
}

initailizer实现了ApplicationContextInitializer接口

源码剖析

/**
 * Callback interface for initializing a Spring {@link ConfigurableApplicationContext}
 * prior to being {@linkplain ConfigurableApplicationContext#refresh() refreshed}.
 *
 * 
Typically used within web applications that require some programmatic initialization
 * of the application context. For example, registering property sources or activating
 * profiles against the {@linkplain ConfigurableApplicationContext#getEnvironment()
 * context's environment}. See {@code ContextLoader} and {@code FrameworkServlet} support
 * for declaring a "contextInitializerClasses" context-param and init-param, respectively.
 *
 * 
{@code ApplicationContextInitializer} processors are encouraged to detect
 * whether Spring's {@link org.springframework.core.Ordered Ordered} interface has been
 * implemented or if the @{@link org.springframework.core.annotation.Order Order}
 * annotation is present and to sort instances accordingly if so prior to invocation.
 *
 * @author Chris Beams
 * @since 3.1
 * @param  the application context type
 * @see org.springframework.web.context.ContextLoader#customizeContext
 * @see org.springframework.web.context.ContextLoader#CONTEXT_INITIALIZER_CLASSES_PARAM
 * @see org.springframework.web.servlet.FrameworkServlet#setContextInitializerClasses
 * @see org.springframework.web.servlet.FrameworkServlet#applyInitializers
 */
public interface ApplicationContextInitializer {
    /**
     * Initialize the given application context.
     * @param applicationContext the application to configure
     */
     // 把初始化的ApplicationContextInitializer实现类加载到SpringApplication中
    void initialize(C applicationContext);
}

总结：

ApplicationContextInitializer接口的作用，在Spring上下文被刷新之前进行初始化的操作。典型地比如在Web应用中，注册Property Sources或者是激活Profiles。Property Sources比较好理解，就是配置文件。Profiles是Spring为了在不同环境下(如DEV，TEST，PRODUCTION等)，加载不同的配置项而抽象出来的一个实体。

调用initialize()方法，把初始化的ApplicationContextInitializer实现加载到SpringApplication中

通过getSpringFactoriesInstances(

ApplicationContextInitializer.class)方法获得实现类

源码剖析

private  Collection getSpringFactoriesInstances(Class type) {
    return getSpringFactoriesInstances(type, new Class[] {});
}
private  Collection getSpringFactoriesInstances(Class type, Class[] parameterTypes, Object... args) {
    ClassLoader classLoader = getClassLoader();
    // Use names and ensure unique to protect against duplicates
    // 使用 Set保存names
    Set names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    // 根据names进行实例化
    List instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
    // 对实例进行排序
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

2. 设置监听器

源码剖析

/**
 * Sets the {@link ApplicationListener}s that will be applied to the SpringApplication
 * and registered with the {@link ApplicationContext}.
 * @param listeners the listeners to set
 */
public void setListeners(Collection> listeners) {
    this.listeners = new ArrayList<>(listeners);
}

继承了ApplicationListener()接口

源码剖析

/**
 * Interface to be implemented by application event listeners.
 *
 * 
Based on the standard {@code java.util.EventListener} interface
 * for the Observer design pattern.
 *
 * 
As of Spring 3.0, an {@code ApplicationListener} can generically declare
 * the event type that it is interested in. When registered with a Spring
 * {@code ApplicationContext}, events will be filtered accordingly, with the
 * listener getting invoked for matching event objects only.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @param  the specific {@code ApplicationEvent} subclass to listen to
 * @see org.springframework.context.ApplicationEvent
 * @see org.springframework.context.event.ApplicationEventMulticaster
 * @see org.springframework.context.event.EventListener
 */
@FunctionalInterface
public interface ApplicationListener extends EventListener {
    /**
     * Handle an application event.
     * @param event the event to respond to
     */
    void onApplicationEvent(E event);
}

总结：

在这里使用到了观察者模式，有一个被观察者和许多观察者，当被观察者的状态发生改变时，要通知所有的观察者做一些操作。

3. 推断主应用入口类

源码剖析

private Class deduceMainApplicationClass() {
    try {
        // 构造一个异常类
        StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
        for (StackTraceElement stackTraceElement : stackTrace) {
            // 通过main的栈帧推断出入口类的名字
            if ("main".equals(stackTraceElement.getMethodName())) {
                return Class.forName(stackTraceElement.getClassName());
            }
        }
    }
    catch (ClassNotFoundException ex) {
        // Swallow and continue
    }
    return null;
}

三、run() 方法源码剖析

源码剖析

/**
 * 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) {
    // 记时器，统计应用启动的时间
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    // 初始化应用上下文和异常报告集合
    ConfigurableApplicationContext context = null;
    // SpringBootExceptionReporter 是异常处理器，启动的时候通过它把异常信息展示出来
    Collection exceptionReporters = new ArrayList<>();
     // 设置系统属性java.awt.headless的值，默认为true
    configureHeadlessProperty();
    // 监听器，SpringApplicationRunListeners实际上是一个集合
    SpringApplicationRunListeners listeners = getRunListeners(args);
    // 回调所有的获取SpringApplicationRunListener.starting()方法
    listeners.starting();
    try {
        // 初始化默认参数
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        // 准备 Spring 环境
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        // 创建环境完成后回调，配置bean
        configureIgnoreBeanInfo(environment);
        // 打印器，springboot启动的时候会打印springboot的标志以及对应的版本
        Banner printedBanner = printBanner(environment);
        // 创建Spring应用上下文，来决定创建web的ioc还是普通的ioc
        context = createApplicationContext();
        // 实例化异常报告器
        exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class }, context);
        //准备上下文环境
        // Spring上下文前置处理
        prepareContext(context, environment, listeners, applicationArguments, printedBanner);
        // prepareContext运行完成以后回调所有的SpringApplicationRunListener的contextLoaded（）；
        // Spring上下文刷新，表示刷新完成，进行后续的一些操作
        refreshContext(context);
        // Spring上下文后置处理
        afterRefresh(context, applicationArguments);
        // 停止计时器
        stopWatch.stop();
        // 输出日志记录的类名、时间信息
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
        }
        // 发布应用上下文启动完成事件
        listeners.started(context);
        // 执行所有 Runner 运行器
        callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, listeners);
        throw new IllegalStateException(ex);
    }
    try {
        // 发布应用上下文就绪事件
        listeners.running(context);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, null);
        throw new IllegalStateException(ex);
    }
    // 返回应用上下文
    return context;
}

1. 开启计时器

开启计时器，用来统计应用启动的时间

public void start() throws IllegalStateException {
        // 传入一个空字符串作为当前任务的名称
        this.start("");
    }
    public void start(String taskName) throws IllegalStateException {
        if (this.currentTaskName != null) {
            // 如果当前任务名字不为空，抛出异常
            throw new IllegalStateException("Can't start StopWatch: it's already running");
        } else {
            // 否则，记录当前任务的开始时间
            this.currentTaskName = taskName;
            this.startTimeNanos = System.nanoTime();
        }
    }

首先，传入一个空字符串作为当前任务的名称

其次，判断当前任务名是否空，如果为空，则记录当前应用启动的开始时间

2. 设置系统属性的值

系统属性的值默认是true，系统属性的值来源于System.getProperty()。

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

3. 监听器

private SpringApplicationRunListeners getRunListeners(String[] args) {
        // 类加载对应的监听器
        Class[] types = new Class[] { SpringApplication.class, String[].class };
        // 创建SpringApplicationRunListener实例
        return new SpringApplicationRunListeners(logger,
                getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
    }

4. 初始化默认参数

ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);

5.创建 Spring 环境

private ConfigurableEnvironment prepareEnvironment(
        SpringApplicationRunListeners listeners,
        ApplicationArguments applicationArguments) {
    // 获取环境。如果存在就直接返回，否则先创建一个再返回
    ConfigurableEnvironment environment = getOrCreateEnvironment();
    // 配置环境
    configureEnvironment(environment, applicationArguments.getSourceArgs());
    // 准备监听器环境
    listeners.environmentPrepared(environment);
    // 将环境绑定到SpringApplication上面
    bindToSpringApplication(environment);
    // 如果不是web应用环境，将环境转换成StandardEnvironment
    if (this.webApplicationType == WebApplicationType.NONE) {
        environment = new EnvironmentConverter(getClassLoader())
                .convertToStandardEnvironmentIfNecessary(environment);
    }
    ConfigurationPropertySources.attach(environment);
    // 返回环境
    return environment;
}

总结：

获取环境。如果存在就直接返回，否则先创建一个再返回

配置环境

准备监听器环境

将环境绑定到SpringApplication上面

如果不是web应用环境，将环境转换成StandardEnvironment

最后返回环境

6. 打印器

springboot启动的时候会打印springboot的标志以及对应的版本
private Banner printBanner(ConfigurableEnvironment environment) {
        if (this.bannerMode == Banner.Mode.OFF) {
            return null;
        }
        ResourceLoader resourceLoader = (this.resourceLoader != null) ? this.resourceLoader
                : new DefaultResourceLoader(getClassLoader());
        SpringApplicationBannerPrinter bannerPrinter = new SpringApplicationBannerPrinter(resourceLoader, this.banner);
        if (this.bannerMode == Mode.LOG) {
            return bannerPrinter.print(environment, this.mainApplicationClass, logger);
        }
        return bannerPrinter.print(environment, this.mainApplicationClass, System.out);
    }

7. 创建Spring应用上下文

protected ConfigurableApplicationContext createApplicationContext() {
    // 首先进行判断有没有指定的实现类
    Class contextClass = this.applicationContextClass;
    // 如果没有，则根据应用类型选择
    if (contextClass == null) {
        try {
            // 根据webApplicationType的类型去反射创建ConfigurableApplicationContext的具体实例
            switch (this.webApplicationType) {
            case SERVLET:
                contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
                break;
            case REACTIVE:
                contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
                break;
            default:
                contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
            }
        }
        catch (ClassNotFoundException ex) {
            throw new IllegalStateException(
                    "Unable create a default ApplicationContext, please specify an ApplicationContextClass", ex);
        }
    }
    // 通过反射，得到创建的对象
    return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

总结：

首先进行判断有没有指定的实现类； 如果没有，则根据应用类型选择；

根据webApplicationType的类型去反射创建ConfigurableApplicationContext的具体实例；

最后通过反射，得到创建的对象

对于Web应用，上下文类型是DEFAULT_WEB_CONTEXT_CLASS。

8. 实例化异常报告器

用 getSpringFactoriesInstances() 方法，获取配置的异常类名称，并实例化所有的异常类。

源码剖析

private  Collection getSpringFactoriesInstances(Class type, Class[] parameterTypes, Object... args) {
    ClassLoader classLoader = getClassLoader();
    // Use names and ensure unique to protect against duplicates
    // 使用名称并确保唯一，以防止重复
    Set names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    List instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

9. Spring上下文前置处理

源码剖析

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,
            SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
    // 给IOC容器设置一些环境属性
    context.setEnvironment(environment);
    // 给IOC容器注册一些组件
    postProcessApplicationContext(context);
    // 调用初始化方法
    applyInitializers(context);
    // 监听器，触发contextPrepared 事件
    listeners.contextPrepared(context);
    // 记录启动过程中的日志
    if (this.logStartupInfo) {
        logStartupInfo(context.getParent() == null);
        logStartupProfileInfo(context);
    }
    // Add boot specific singleton beans
    // 添加特定的单例beans
    ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
    beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
    if (printedBanner != null) {
        beanFactory.registerSingleton("springBootBanner", printedBanner);
    }
    if (beanFactory instanceof DefaultListableBeanFactory) {
        ((DefaultListableBeanFactory) beanFactory)
                .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
    }
    if (this.lazyInitialization) {
        context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
    }
    // Load the sources
    // 加载所有资源
    Set