1、首先问自己一个问题，为啥要读SpringBoot源码呢？

我们在干一件事情前必须要有自己的目的，不要盲目的跟风。

就比如现在，我们为啥要读SpringBoot源码呢？

对于我来说，最主要的原因是目前技术更新太快，框架层出不穷，越学越迷茫，我到底学了个啥？

像SpringBoot，你难道不好奇他的自动化配置、IOC和AOP的实现原理吗？

带着上面的问题，我们来Debug看看！

2、环境准备

Ⅰ、我的maven依赖关系(不同的SpringBoot版本、不同的依赖关系，运行结果会不一样，因为SpringBoot会自动配置)

<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-redis</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
            <version>2.1.4</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-aop</artifactId>
        </dependency>
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-boot-starter</artifactId>
            <version>3.0.0</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>cn.hutool</groupId>
            <artifactId>hutool-all</artifactId>
            <version>5.7.2</version>
        </dependency>
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>3.4.0</version>
        </dependency>
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-generator</artifactId>
            <version>3.4.0</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

Ⅱ、Debug前的准备

• clean
• compile
• install

3、开始调试

就想我们学习C语言时的HelloWorld一样，SpringBoot也有一个开场白：

package com.study.redis;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

/**
 * @author lemon
 */
@SpringBootApplication
public class RedisApplication {

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

然后我们Ctrl+鼠标左键点进SpringApplication的run方法可以看到下图：

首先是调用形参为单个class的run方法，然后调用形参为class数组形式的run方法。

另外需要注意的是上面的那个Decompiled .class file,bytecode version: 52.0(Java 8)    Download Sources Choose Sources...

为什么要注意这个呢，因为现在是对.class文件Debug，而不是Java文件，稍后Debug就会出现错误：

Source code does not match the byte code

我们选择Download Sources，如若有必要可以再Choose Sources，选择刚刚下载的Sources，然后就会看到带注释的方法了：

/**
     * 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);
    }

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

这里最终实例化了一个SpringApplication对象，并调用它的run方法，我们先看看初始化函数：

/**
     * 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);
    }

    /**
     * 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) {
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
        this.bootstrapRegistryInitializers = getBootstrapRegistryInitializersFromSpringFactories();
        setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        this.mainApplicationClass = deduceMainApplicationClass();
    }

初始化函数就先看到这儿，等要用的时候我们再回来看，接下来我们来看run方法：

/**
     * 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) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        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);
            configureIgnoreBeanInfo(environment);
            Banner printedBanner = printBanner(environment);
            context = createApplicationContext();
            context.setApplicationStartup(this.applicationStartup);
            prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
            refreshContext(context);
            afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
            }
            listeners.started(context);
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, listeners);
            throw new IllegalStateException(ex);
        }

        try {
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

首先实例化了一个StopWatch对象：

/**
     * Construct a new {@code StopWatch}.
     * <p>Does not start any task.
     */
    public StopWatch() {
        this("");
    }

    /**
     * Construct a new {@code StopWatch} with the given ID.
     * <p>The ID is handy when we have output from multiple stop watches and need
     * to distinguish between them.
     * <p>Does not start any task.
     * @param id identifier for this stop watch
     */
    public StopWatch(String id) {
        this.id = id;
    }

然后调用了start方法，我们看看它的start方法干了啥：

/**
     * Start an unnamed task.
     * <p>The results are undefined if {@link #stop()} or timing methods are
     * called without invoking this method first.
     * @see #start(String)
     * @see #stop()
     */
    public void start() throws IllegalStateException {
        start("");
    }

    /**
     * Start a named task.
     * <p>The results are undefined if {@link #stop()} or timing methods are
     * called without invoking this method first.
     * @param taskName the name of the task to start
     * @see #start()
     * @see #stop()
     */
    public void start(String taskName) throws IllegalStateException {
        if (this.currentTaskName != null) {
            throw new IllegalStateException("Can't start StopWatch: it's already running");
        }
        this.currentTaskName = taskName;
        this.startTimeNanos = System.nanoTime();
    }

感觉啥也没干哈哈(菜鸟级理解)

接下来就是createBootstrapContext方法：

private DefaultBootstrapContext createBootstrapContext() {
        DefaultBootstrapContext bootstrapContext = new DefaultBootstrapContext();
        this.bootstrapRegistryInitializers.forEach((initializer) -> initializer.initialize(bootstrapContext));
        return bootstrapContext;
    }

首先实例化了一个DefaultBootstrapContext，调用的是默认的构造函数，无代码展示。

然后将bootstrapRegistryInitializer里面的每一个实例都初始化，那我们回来之前SpringApplication的构造函数：

this.bootstrapRegistryInitializers = getBootstrapRegistryInitializersFromSpringFactories();

@SuppressWarnings("deprecation")
    private List<BootstrapRegistryInitializer> getBootstrapRegistryInitializersFromSpringFactories() {
        ArrayList<BootstrapRegistryInitializer> initializers = new ArrayList<>();
        getSpringFactoriesInstances(Bootstrapper.class).stream()
                .map((bootstrapper) -> ((BootstrapRegistryInitializer) bootstrapper::initialize))
                .forEach(initializers::add);
        initializers.addAll(getSpringFactoriesInstances(BootstrapRegistryInitializer.class));
        return initializers;
    }

看到这个@SuppressWarnings("deprecation")注解，作者估计是不想用这个了吧。

可以看到initializers里面的元素都是从getSpringFactoriesInstances方法中来的：

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;
    }

首先它实例化了一个类加载器，其实也就是下面这个：

Thread.currentThread().getContextClassLoader()

然后通过SpringFactoriesLoade的LoadFactoryName方法实例实例化了一个Set<String>集合：

/**
     * Load the fully qualified class names of factory implementations of the
     * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
     * class loader.
     * <p>As of Spring Framework 5.3, if a particular implementation class name
     * is discovered more than once for the given factory type, duplicates will
     * be ignored.
     * @param factoryType the interface or abstract class representing the factory
     * @param classLoader the ClassLoader to use for loading resources; can be
     * {@code null} to use the default
     * @throws IllegalArgumentException if an error occurs while loading factory names
     * @see #loadFactories
     */
    public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        ClassLoader classLoaderToUse = classLoader;
        if (classLoaderToUse == null) {
            classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
        }
        String factoryTypeName = factoryType.getName();
        return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
    }

最终还是调用的loadSpringFactories方法：

private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
        Map<String, List<String>> result = cache.get(classLoader);
        if (result != null) {
            return result;
        }

        result = new HashMap<>();
        try {
            Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                UrlResource resource = new UrlResource(url);
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                for (Map.Entry<?, ?> entry : properties.entrySet()) {
                    String factoryTypeName = ((String) entry.getKey()).trim();
                    String[] factoryImplementationNames =
                            StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
                    for (String factoryImplementationName : factoryImplementationNames) {
                        result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
                                .add(factoryImplementationName.trim());
                    }
                }
            }

            // Replace all lists with unmodifiable lists containing unique elements
            result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
                    .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
            cache.put(classLoader, result);
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
        return result;
    }

我们先来看看cache是啥：

static final Map<ClassLoader, Map<String, List<String>>> cache = new ConcurrentReferenceHashMap<>();

很明细，里面是空的，接下来我们看看下面这句：

Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

通过Debug，我们得到如下结果：

算了，这个urls里面东西太多了，反正就是一些包相关信息，我们直接看result吧，反正信息都是从类加载器取出来的：

现在我们回到LoadFactoryNames方法，由于返回的result中没有对应Bootstrapper.class类型的键，所以返回一个空列表。

那我们再看看是否有对应的BootstrapRegistryInitializer.class类型的键，同样也没有。

所以初始化构造的bootstrapRegistryInitializer是一个空列表。

但是initializer和listeners有值：

接下来我们回到createBootstrapContext方法，由于bootstrapRegistryInitializer是一个空列表，

所以该方法相当于直接返回一个默认的DefaultBootstrapContext，没有做其他操作。

现在再回到run方法中的configureHeadlessProperty方法：

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

这一步就简单的设置一下系统所属性，接下来我们看看getRunListeners方法：

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

我们先看看构造函数：

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

简单的赋值，没啥好说的，我们回到getSpringFactoriesInstances方法：

可以看到返回了一个listeners，我们再回到run方法：

listeners.starting(bootstrapContext, this.mainApplicationClass);

void starting(ConfigurableBootstrapContext bootstrapContext, Class<?> mainApplicationClass) {
        doWithListeners("spring.boot.application.starting", (listener) -> listener.starting(bootstrapContext),
                (step) -> {
                    if (mainApplicationClass != null) {
                        step.tag("mainApplicationClass", mainApplicationClass.getName());
                    }
                });
    }

private void doWithListeners(String stepName, Consumer<SpringApplicationRunListener> listenerAction,
            Consumer<StartupStep> stepAction) {
        StartupStep step = this.applicationStartup.start(stepName);
        this.listeners.forEach(listenerAction);
        if (stepAction != null) {
            stepAction.accept(step);
        }
        step.end();
    }

通过调用listeners的start方法，最终调用该实例的的listeners的starting方法，我们再看看step干了啥：

private static final DefaultStartupStep DEFAULT_STARTUP_STEP = new DefaultStartupStep();

    @Override
    public DefaultStartupStep start(String name) {
        return DEFAULT_STARTUP_STEP;
    }

虽然方法名叫start，但是只是实例化一个DefaultStartupStep对象，接下来再看listener的starting方法：

@Override
    public void starting(ConfigurableBootstrapContext bootstrapContext) {
        this.initialMulticaster
                .multicastEvent(new ApplicationStartingEvent(bootstrapContext, this.application, this.args));
    }

该方法属于EventPublishingRunListener类：

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

可以看到initialMulticaster是SimpleApplicationEventMulticaster的实例，再看看ApplicationStartingEvent构造函数：

/**
     * Create a new {@link ApplicationStartingEvent} instance.
     * @param bootstrapContext the bootstrap context
     * @param application the current application
     * @param args the arguments the application is running with
     */
    public ApplicationStartingEvent(ConfigurableBootstrapContext bootstrapContext, SpringApplication application,
            String[] args) {
        super(application, args);
        this.bootstrapContext = bootstrapContext;
    }

反正就是构造一个实例就完了，那我们再回到initialMulticaster的multicasEvent方法：

@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));
        Executor executor = getTaskExecutor();
        for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
            if (executor != null) {
                executor.execute(() -> invokeListener(listener, event));
            }
            else {
                invokeListener(listener, event);
            }
        }
    }

event就是ApplicationStartingEvent的一个实例，由于SimpleApplicationEventMulticaster的构造函数如下：

/**
     * Create a new SimpleApplicationEventMulticaster.
     */
    public SimpleApplicationEventMulticaster() {
    }

所以Executor是空的，我们看看getApplicationListeners方法：

    /**
     * Return a Collection of ApplicationListeners matching the given
     * event type. Non-matching listeners get excluded early.
     * @param event the event to be propagated. Allows for excluding
     * non-matching listeners early, based on cached matching information.
     * @param eventType the event type
     * @return a Collection of ApplicationListeners
     * @see org.springframework.context.ApplicationListener
     */
    protected Collection<ApplicationListener<?>> getApplicationListeners(
            ApplicationEvent event, ResolvableType eventType) {

        Object source = event.getSource();
        Class<?> sourceType = (source != null ? source.getClass() : null);
        ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);

        // Potential new retriever to populate
        CachedListenerRetriever newRetriever = null;

        // Quick check for existing entry on ConcurrentHashMap
        CachedListenerRetriever existingRetriever = this.retrieverCache.get(cacheKey);
        if (existingRetriever == null) {
            // Caching a new ListenerRetriever if possible
            if (this.beanClassLoader == null ||
                    (ClassUtils.isCacheSafe(event.getClass(), this.beanClassLoader) &&
                            (sourceType == null || ClassUtils.isCacheSafe(sourceType, this.beanClassLoader)))) {
                newRetriever = new CachedListenerRetriever();
                existingRetriever = this.retrieverCache.putIfAbsent(cacheKey, newRetriever);
                if (existingRetriever != null) {
                    newRetriever = null;  // no need to populate it in retrieveApplicationListeners
                }
            }
        }

        if (existingRetriever != null) {
            Collection<ApplicationListener<?>> result = existingRetriever.getApplicationListeners();
            if (result != null) {
                return result;
            }
            // If result is null, the existing retriever is not fully populated yet by another thread.
            // Proceed like caching wasn't possible for this current local attempt.
        }

        return retrieveApplicationListeners(eventType, sourceType, newRetriever);
    }

我们来看看对应的event、eventType是什么：

source就是之前构造的SpringApplication实例：

接下来根据eventType和sourceType创建了一个cacheKey:

public ListenerCacheKey(ResolvableType eventType, @Nullable Class<?> sourceType) {
            Assert.notNull(eventType, "Event type must not be null");
            this.eventType = eventType;
            this.sourceType = sourceType;
        }

就是简单的赋值，没啥好说的，然后根据该key获取一个值：

CachedListenerRetriever existingRetriever = this.retrieverCache.get(cacheKey);

final Map<ListenerCacheKey, CachedListenerRetriever> retrieverCache = new ConcurrentHashMap<>(64);

由于retrieverCache是一个空的ConcurrentHashMap()，为啥要用并发Map，我们先不管。

由于existingRetriever是空的，并且beanClassLoader也是空的，所以我们直接看retrieveApplicationListeners方法：

/**
     * Actually retrieve the application listeners for the given event and source type.
     * @param eventType the event type
     * @param sourceType the event source type
     * @param retriever the ListenerRetriever, if supposed to populate one (for caching purposes)
     * @return the pre-filtered list of application listeners for the given event and source type
     */
    private Collection<ApplicationListener<?>> retrieveApplicationListeners(
            ResolvableType eventType, @Nullable Class<?> sourceType, @Nullable CachedListenerRetriever retriever) {

        List<ApplicationListener<?>> allListeners = new ArrayList<>();
        Set<ApplicationListener<?>> filteredListeners = (retriever != null ? new LinkedHashSet<>() : null);
        Set<String> filteredListenerBeans = (retriever != null ? new LinkedHashSet<>() : null);

        Set<ApplicationListener<?>> listeners;
        Set<String> listenerBeans;
        synchronized (this.defaultRetriever) {
            listeners = new LinkedHashSet<>(this.defaultRetriever.applicationListeners);
            listenerBeans = new LinkedHashSet<>(this.defaultRetriever.applicationListenerBeans);
        }

        // Add programmatically registered listeners, including ones coming
        // from ApplicationListenerDetector (singleton beans and inner beans).
        for (ApplicationListener<?> listener : listeners) {
            if (supportsEvent(listener, eventType, sourceType)) {
                if (retriever != null) {
                    filteredListeners.add(listener);
                }
                allListeners.add(listener);
            }
        }

        // Add listeners by bean name, potentially overlapping with programmatically
        // registered listeners above - but here potentially with additional metadata.
        if (!listenerBeans.isEmpty()) {
            ConfigurableBeanFactory beanFactory = getBeanFactory();
            for (String listenerBeanName : listenerBeans) {
                try {
                    if (supportsEvent(beanFactory, listenerBeanName, eventType)) {
                        ApplicationListener<?> listener =
                                beanFactory.getBean(listenerBeanName, ApplicationListener.class);
                        if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {
                            if (retriever != null) {
                                if (beanFactory.isSingleton(listenerBeanName)) {
                                    filteredListeners.add(listener);
                                }
                                else {
                                    filteredListenerBeans.add(listenerBeanName);
                                }
                            }
                            allListeners.add(listener);
                        }
                    }
                    else {
                        // Remove non-matching listeners that originally came from
                        // ApplicationListenerDetector, possibly ruled out by additional
                        // BeanDefinition metadata (e.g. factory method generics) above.
                        Object listener = beanFactory.getSingleton(listenerBeanName);
                        if (retriever != null) {
                            filteredListeners.remove(listener);
                        }
                        allListeners.remove(listener);
                    }
                }
                catch (NoSuchBeanDefinitionException ex) {
                    // Singleton listener instance (without backing bean definition) disappeared -
                    // probably in the middle of the destruction phase
                }
            }
        }

        AnnotationAwareOrderComparator.sort(allListeners);
        if (retriever != null) {
            if (filteredListenerBeans.isEmpty()) {
                retriever.applicationListeners = new LinkedHashSet<>(allListeners);
                retriever.applicationListenerBeans = filteredListenerBeans;
            }
            else {
                retriever.applicationListeners = filteredListeners;
                retriever.applicationListenerBeans = filteredListenerBeans;
            }
        }
        return allListeners;
    }

我们直接来到下面这段代码：

synchronized (this.defaultRetriever) {
            listeners = new LinkedHashSet<>(this.defaultRetriever.applicationListeners);
            listenerBeans = new LinkedHashSet<>(this.defaultRetriever.applicationListenerBeans);
        }

发现这个defaultRetriever.applicationListeners直接有值了

它是从哪儿来的呢？

private final DefaultListenerRetriever defaultRetriever = new DefaultListenerRetriever();

/**
     * Helper class that encapsulates a general set of target listeners.
     */
    private class DefaultListenerRetriever {

        public final Set<ApplicationListener<?>> applicationListeners = new LinkedHashSet<>();

        public final Set<String> applicationListenerBeans = new LinkedHashSet<>();

        public Collection<ApplicationListener<?>> getApplicationListeners() {
            List<ApplicationListener<?>> allListeners = new ArrayList<>(
                    this.applicationListeners.size() + this.applicationListenerBeans.size());
            allListeners.addAll(this.applicationListeners);
            if (!this.applicationListenerBeans.isEmpty()) {
                BeanFactory beanFactory = getBeanFactory();
                for (String listenerBeanName : this.applicationListenerBeans) {
                    try {
                        ApplicationListener<?> listener =
                                beanFactory.getBean(listenerBeanName, ApplicationListener.class);
                        if (!allListeners.contains(listener)) {
                            allListeners.add(listener);
                        }
                    }
                    catch (NoSuchBeanDefinitionException ex) {
                        // Singleton listener instance (without backing bean definition) disappeared -
                        // probably in the middle of the destruction phase
                    }
                }
            }
            AnnotationAwareOrderComparator.sort(allListeners);
            return allListeners;
        }
    }

这里我们并未找到该值的来源，那我们找一下哪些地方用到了这个defaultRetriever：

@Override
    public void addApplicationListener(ApplicationListener<?> listener) {
        synchronized (this.defaultRetriever) {
            // Explicitly remove target for a proxy, if registered already,
            // in order to avoid double invocations of the same listener.
            Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);
            if (singletonTarget instanceof ApplicationListener) {
                this.defaultRetriever.applicationListeners.remove(singletonTarget);
            }
            this.defaultRetriever.applicationListeners.add(listener);
            this.retrieverCache.clear();
        }
    }

这里向里面添加listener的，我们再看看有那个地方调用了这个方法：

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

发现原来是在最开始初始化EventPublishRunListener的时候添加的。

接下来就是判断是否支持事件，添加到对应的listeners中去，另外由于listenersBean是空的，所以没啥好说的。

我们直接看返回结果：

分析这么多，我们终于可以回到以下代码了：

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()) ||
                    (event instanceof PayloadApplicationEvent &&
                            matchesClassCastMessage(msg, ((PayloadApplicationEvent) event).getPayload().getClass()))) {
                // Possibly a lambda-defined listener which we could not resolve the generic event type for
                // -> let's suppress the exception.
                Log loggerToUse = this.lazyLogger;
                if (loggerToUse == null) {
                    loggerToUse = LogFactory.getLog(getClass());
                    this.lazyLogger = loggerToUse;
                }
                if (loggerToUse.isTraceEnabled()) {
                    loggerToUse.trace("Non-matching event type for listener: " + listener, ex);
                }
            }
            else {
                throw ex;
            }
        }
    }

最终调用的是onApplicationEvent方法，不同的listener该方法不一样：

以上就是为了让listeners跑起来，感觉啥也没干哈哈，可能我想要的东西在下面，接下来我们看看run方法中try语句块的代码：

try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
            configureIgnoreBeanInfo(environment);
            Banner printedBanner = printBanner(environment);
            context = createApplicationContext();
            context.setApplicationStartup(this.applicationStartup);
            prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
            refreshContext(context);
            afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
            }
            listeners.started(context);
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, listeners);
            throw new IllegalStateException(ex);
        }

        try {
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, null);
            throw new IllegalStateException(ex);
        }
        return context;

自动化配置应该就是通过使用类加载器机制实现的吧...

草，写不下去了，好多，下一篇吧！！！