Spring Boot -- 启动流程分析一

@SpringBootApplication(scanBasePackages={"com.jnu.example"})
@CoreMapperScan
@EnableAspectAutoProxy
public class App {
    public static void main(String[] args) {
        SpringApplication.run(BlogApplication.class, args);
    }
}

    /**
     * Static helper that can be used to run a {@link SpringApplication} from the
     * specified source using default settings.
     * @param primarySource the primary source to load
     * @param args the application arguments (usually passed from a Java main method)
     * @return the running {@link ApplicationContext}
     */
    public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
        return run(new Class<?>[] { primarySource }, args);
    }

    public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
        return new SpringApplication(primarySources).run(args);
    }

    /**
     * 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 primarySources the primary bean sources
     * @see #run(Class, String[])
     * @see #SpringApplication(ResourceLoader, Class...)
     * @see #setSources(Set)
     */
    public SpringApplication(Class<?>... primarySources) {
        this(null, primarySources);
    }

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    
        // resourceLoader 主要用来获取 Resource 及 ClassLoader。这里值为 null
        this.resourceLoader = resourceLoader;
        
        // 断言primarySources不能为null，否则报错
        Assert.notNull(primarySources, "PrimarySources must not be null");
        
        // primarySources是SpringApplication.run的参数，存放的是主配置类
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        
        // 进行Web应用的类型推断
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
        
        // 加载应用上下文初始化器 initializer
        setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
        
        // 加载应用事件监听器 listener
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        
        // 推断引导类，也就是找到入口类
        this.mainApplicationClass = deduceMainApplicationClass();
    }

// 进行Web应用的类型推断
this.webApplicationType = WebApplicationType.deduceFromClasspath();

/**
 * An enumeration of possible types of web application.
 *
 * @author Andy Wilkinson
 * @author Brian Clozel
 * @since 2.0.0
 */
public enum WebApplicationType {

    /**
     * The application should not run as a web application and should not start an
     * embedded web server.
     */
    NONE,

    /**
     * The application should run as a servlet-based web application and should start an
     * embedded servlet web server.
     */
    SERVLET,

    /**
     * The application should run as a reactive web application and should start an
     * embedded reactive web server.
     */
    REACTIVE;

    private static final String[] SERVLET_INDICATOR_CLASSES = { "javax.servlet.Servlet",
            "org.springframework.web.context.ConfigurableWebApplicationContext" };

    private static final String WEBMVC_INDICATOR_CLASS = "org.springframework.web.servlet.DispatcherServlet";

    private static final String WEBFLUX_INDICATOR_CLASS = "org.springframework.web.reactive.DispatcherHandler";

    private static final String JERSEY_INDICATOR_CLASS = "org.glassfish.jersey.servlet.ServletContainer";

    private static final String SERVLET_APPLICATION_CONTEXT_CLASS = "org.springframework.web.context.WebApplicationContext";

    private static final String REACTIVE_APPLICATION_CONTEXT_CLASS = "org.springframework.boot.web.reactive.context.ReactiveWebApplicationContext";

    static WebApplicationType deduceFromClasspath() {
        if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
                && !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
            return WebApplicationType.REACTIVE;
        }
        for (String className : SERVLET_INDICATOR_CLASSES) {
            if (!ClassUtils.isPresent(className, null)) {
                return WebApplicationType.NONE;
            }
        }
        return WebApplicationType.SERVLET;
    }

    static WebApplicationType deduceFromApplicationContext(Class<?> applicationContextClass) {
        if (isAssignable(SERVLET_APPLICATION_CONTEXT_CLASS, applicationContextClass)) {
            return WebApplicationType.SERVLET;
        }
        if (isAssignable(REACTIVE_APPLICATION_CONTEXT_CLASS, applicationContextClass)) {
            return WebApplicationType.REACTIVE;
        }
        return WebApplicationType.NONE;
    }

    private static boolean isAssignable(String target, Class<?> type) {
        try {
            return ClassUtils.resolveClassName(target, null).isAssignableFrom(type);
        }
        catch (Throwable ex) {
            return false;
        }
    }

}

setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class))

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
        return getSpringFactoriesInstances(type, new Class<?>[] {});
    }

    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
        ClassLoader classLoader = getClassLoader();
        // Use names and ensure unique to protect against duplicates
        Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }

# Initializers
org.springframework.context.ApplicationContextInitializer=org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

 @SuppressWarnings("unchecked")
    private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes,
            ClassLoader classLoader, Object[] args, Set<String> names) {
        List<T> instances = new ArrayList<>(names.size());
        for (String name : names) {
            try {
                Class<?> instanceClass = ClassUtils.forName(name, classLoader);
                Assert.isAssignable(type, instanceClass);
                Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);
                T instance = (T) BeanUtils.instantiateClass(constructor, args);
                instances.add(instance);
            }
            catch (Throwable ex) {
                throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);
            }
        }
        return instances;
    }

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

setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class))

# Application Listeners
org.springframework.context.ApplicationListener=org.springframework.boot.autoconfigure.BackgroundPreinitializer

    /**
     * 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<? extends ApplicationListener<?>> listeners) {
        this.listeners = new ArrayList<>(listeners);
    }

// 推断引导类，也就是找到入口类
this.mainApplicationClass = deduceMainApplicationClass();

    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;
    }

public ConfigurableApplicationContext run(String... args) {
        // 这是 Spring 的一个计时器，计算代码的执行时间(ms级别)
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        
        // 这俩变量在后面赋值处进行说明
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        
        // 用来设置java.awt.headless属性值
        configureHeadlessProperty();
        
        // 该对象属于组合模式的实现，核心是内部关联的 SpringApplicationRunListener 集合，SpringApplicationRunListener 是 Spring Boot 的运行时监听器
        SpringApplicationRunListeners listeners = getRunListeners(args);
        // 会在不同的阶段调用对应的方法，这里表示SpringApplication的run方法刚开始执行
        listeners.starting();
        
        try {
        
            // 用来获取 SpringApplication.run(args)传入的参数
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            
            // 获取 properties 配置文件
            ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
            
            // 设置 spring.beaninfo.ignore 的属性值，判断是否跳过搜索BeanInfo类
            configureIgnoreBeanInfo(environment);
            
            // 这里是项目启动时，控制台打印的 Banner
            Banner printedBanner = printBanner(environment);
            
            // 这里就是创建 Spring 应用上下文
            context = createApplicationContext();
            
            // 获取 spring.factories 中key为 SpringBootExceptionReporter 的类名集合
            exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
                    
            // 这里是准备 Spring 应用上下文
            prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            
            // 这里是启动 Spring 应用上下文，底层调用的是 ApplicationContext 的 refresh() 方法，到这里就正式进入了 Spring 的生命周期，同时，SpringBoot的自动装配特性也随之启动
            refreshContext(context);
            
            // 里面是空的，猜测应该是交由开发人员自行扩展
            afterRefresh(context, applicationArguments);
            stopWatch.stop();
            
            // 这里打印启动信息
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
            }
            
            // ApplicationContext 启动时，调用该方法
            listeners.started(context);
            
            // 项目启动后，做的一些操作，开发人员可自行扩展
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, listeners);
            throw new IllegalStateException(ex);
        }
    
        try {
        
            // ApplicationContext 启动完成时，调用该方法
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

 // 该对象属于组合模式的实现，核心是内部关联的 SpringApplicationRunListener 集合，SpringApplicationRunListener 是 Spring Boot 的运行时监听器
 SpringApplicationRunListeners listeners = getRunListeners(args);

    private SpringApplicationRunListeners getRunListeners(String[] args) {
        Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
        return new SpringApplicationRunListeners(logger,
                getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
    }

/**
 * A collection of {@link SpringApplicationRunListener}.
 *
 * @author Phillip Webb
 */
class SpringApplicationRunListeners {

    private final Log log;

    private final List<SpringApplicationRunListener> listeners;

    SpringApplicationRunListeners(Log log, Collection<? extends SpringApplicationRunListener> listeners) {
        this.log = log;
        this.listeners = new ArrayList<>(listeners);
    }

    void starting() {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.starting();
        }
    }
      ...
}

public interface SpringApplicationRunListener {

    // 在run()方法开始执行时被调用，表示应用刚刚启动，对应的 Spring Boot 事件为 ApplicationStartingEvent
    void starting();

    // ConfigurableEnvironment 构建完成时调用，对应的 Spring Boot 事件为 ApplicationEnvironmentPreparedEvent
    void environmentPrepared(ConfigurableEnvironment environment);

    // ApplicationContext 构建完成时调用，对应的 Spring Boot 事件为 ApplicationContextInitializedEvent
    void contextPrepared(ConfigurableApplicationContext context);

    // ApplicationContext 完成加载但还未启动时调用，对应的 Spring Boot 事件为 ApplicationPreparedEvent
    void contextLoaded(ConfigurableApplicationContext context);

    // ApplicationContext 已启动，但 callRunners 还未执行时调用，对应的 Spring Boot 事件为 ApplicationStartedEvent
    void started(ConfigurableApplicationContext context);

    // ApplicationContext 启动完毕被调用，对应的 Spring Boot 事件为 ApplicationReadyEvent
    void running(ConfigurableApplicationContext context);

    // 应用出错时被调用，对应的 Spring Boot 事件为 ApplicationFailedEvent
    void failed(ConfigurableApplicationContext context, Throwable exception);

}

public class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {

    private final SpringApplication application;

    private final String[] args;

    private final SimpleApplicationEventMulticaster initialMulticaster;

    public EventPublishingRunListener(SpringApplication application, String[] args) {
        this.application = application;
        this.args = args;
        this.initialMulticaster = new SimpleApplicationEventMulticaster();
        for (ApplicationListener<?> listener : application.getListeners()) {
            //添加ApplicationListener监听器
            this.initialMulticaster.addApplicationListener(listener);
        }
    }

    @Override
    public void starting() {
        //调用广播器来触发事件
        this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(this.application, this.args));
    }

    ...

}

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {

    ...
    
    @Override
    public void multicastEvent(ApplicationEvent event) {
        multicastEvent(event, resolveDefaultEventType(event));
    }

    @Override
    public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
        ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
        //获取线程池
        Executor executor = getTaskExecutor();
        //获取对当前事件感兴趣的监听器列表
        for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
            if (executor != null) {
                executor.execute(() -> invokeListener(listener, event));
            }
            else {
                invokeListener(listener, event);
            }
        }
    }
}

/**
     * Invoke the given listener with the given event.
     * @param listener the ApplicationListener to invoke
     * @param event the current event to propagate
     * @since 4.1
     */
    protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
        ErrorHandler errorHandler = getErrorHandler();
        if (errorHandler != null) {
            try {
                doInvokeListener(listener, event);
            }
            catch (Throwable err) {
                errorHandler.handleError(err);
            }
        }
        else {
            doInvokeListener(listener, event);
        }
    }

    @SuppressWarnings({"rawtypes", "unchecked"})
    private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
        try {
            listener.onApplicationEvent(event);
        }
        catch (ClassCastException ex) {
            String msg = ex.getMessage();
            if (msg == null || matchesClassCastMessage(msg, event.getClass())) {
                // Possibly a lambda-defined listener which we could not resolve the generic event type for
                // -> let‘s suppress the exception and just log a debug message.
                Log logger = LogFactory.getLog(getClass());
                if (logger.isTraceEnabled()) {
                    logger.trace("Non-matching event type for listener: " + listener, ex);
                }
            }
            else {
                throw ex;
            }
        }
    }

public class BackgroundPreinitializer implements ApplicationListener<SpringApplicationEvent> {

    ...
    
    @Override
    public void onApplicationEvent(SpringApplicationEvent event) {
        if (!Boolean.getBoolean(IGNORE_BACKGROUNDPREINITIALIZER_PROPERTY_NAME)
                && event instanceof ApplicationStartingEvent && preinitializationStarted.compareAndSet(false, true)) {
            performPreinitialization();
        }
        if ((event instanceof ApplicationReadyEvent || event instanceof ApplicationFailedEvent)
                && preinitializationStarted.get()) {
            try {
                preinitializationComplete.await();
            }
            catch (InterruptedException ex) {
                Thread.currentThread().interrupt();
            }
        }
    }
    
    ...
}

ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);

public class Test {

    @Autowired
    private ApplicationArguments applicationArguments;

    public void getArgs() {
        // 获取 args 中的所有 non option 参数
        applicationArguments.getNonOptionArgs();

        // 获取 args 中所有的 option 参数的 name
        applicationArguments.getOptionNames();

        // 获取传递给应用程序的原始未处理参数
        applicationArguments.getSourceArgs();

        // 获取 args 中指定 name 的 option 参数的值
        applicationArguments.getOptionValues("name");

        // 判断从参数中解析的 option 参数是否包含指定名称的选项
        applicationArguments.containsOption("name");
    }
}

ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);

    private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments) {
        // Create and configure the environment
        ConfigurableEnvironment environment = getOrCreateEnvironment();
        configureEnvironment(environment, applicationArguments.getSourceArgs());
        ConfigurationPropertySources.attach(environment);
        listeners.environmentPrepared(environment);
        bindToSpringApplication(environment);
        if (!this.isCustomEnvironment) {
            environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,
                    deduceEnvironmentClass());
        }
        ConfigurationPropertySources.attach(environment);
        return environment;
    }

private ConfigurableEnvironment getOrCreateEnvironment() {
        if (this.environment != null) {
            return this.environment;
        }
        switch (this.webApplicationType) {
        case SERVLET:
            return new StandardServletEnvironment();
        case REACTIVE:
            return new StandardReactiveWebEnvironment();
        default:
            return new StandardEnvironment();
        }
    }