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SpringBoot自动装配及启动

SpringBoot自动装配及启动

作者: 王侦 | 来源:发表于2023-01-02 21:17 被阅读0次

    1.手写模拟SpringBoot核心流程

    手写模拟SpringBoot关键点：

    • 1）创建一个Spring容器，将启动类作为配置类传入；并且加上@ComponentScan默认将启动类所在的包路径作为扫描路径；
    • 2）创建Servlet容器，比如Tomcat，并且配置DispatcherServlet
    • 3）根据依赖启动相应容器，比如引入Tomcat依赖就启动Tomcat容器，引入Jetty依赖就启动Jetty容器。实现自动装配。
      3-1）改为面向接口编程，先获取getWebServer()，然后调用webServer.start()启动；、
      3-2）自动配置类WebServerAutoConfiguration，条件注解@Conditional加在TomcatWebServer和JettyWebServer的Bean上面，判断条件注解上的类是否存在，不存在则不加载相应的bean；
      3-3）自己工程要导入自动配置类@Import(WebServerAutoConfiguration.class)；
      3-4）如果有很多自动配置类，每一个自动配置类都要Import，不仅需要改代码，并且很麻烦。此时可以通过Java SPI（META-INF/services）或者Spring SPI（META-INF/spring.factories）机制从classpath下面工程及所有jar包的固定目录进行加载。方便第三方软件整合SpringBoot，提供对应的自动配置类。
      3-5）自动装配注解中的Import需要实现DeferredImprotSelector，这个的执行时机是在每一批次的配置类解析完之后才解析DeferredImprotSelector，此时DeferredImprotSelector才会通过SPI机制加载所有的自动配置类。是为了让程序员定义Bean优先级更好，自动配置类的Bean的条件注解会根据程序员是否定义了某个bean来决定当前bean是否注册到Spring容器。
      3-6）引入依赖时，是按照starter进行引入，starter会将某个组件需要的相关依赖一起进行引入。

    自动配置类对于没有引入依赖的那些条件注解里面的class是怎么处理的，为什么不报错？

    • 比如我们依赖的springboot-web.jar（这里面引入了springboot-tomcat.jar），这些里面的类都是class格式，都是编译好的
    • 编译不报错：我们自己工程编译时，是不会报错的，只需要编译我们自己写的代码，依赖的jar包都是已经编译好的，不用编译
    • 运行不报错（跳过类加载机制获取注解上类的字符串信息，后面再通过类加载机制进行加载，如果抛异常则catch住并返回false）：ConfigurationClassPostProcessor#postProcessBeanDefinitionRegistry()中在（ConfigurationClassParser#processConfigurationClass中的this.conditionEvaluator.shouldSkip()）处理配置类条件注解时，使用的是ASM技术进行处理。不是使用反射，使用反射会加载类，就会报错。ASM技术处理的是字节码文件，拿到字符串（类的全名），再用类加载器去加载（OnClassCondition#getMatchOutcom），如果能加载则返回true，不能加载（抛异常）则返回false。

    2.SpringBoot创建Servlet容器的地方

    2.1 创建WebServer

    AbstractApplicationContext#refresh
    调用ServletWebServerApplicationContext#onRefresh

        protected void onRefresh() {
        super.onRefresh();
        try {
            createWebServer();
        }
        catch (Throwable ex) {
            throw new ApplicationContextException("Unable to start web server", ex);
        }
    }

    ServletWebServerApplicationContext#createWebServer

        private void createWebServer() {
        WebServer webServer = this.webServer;
        ServletContext servletContext = getServletContext();
        if (webServer == null && servletContext == null) {
            StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
            ServletWebServerFactory factory = getWebServerFactory();
            createWebServer.tag("factory", factory.getClass().toString());
            this.webServer = factory.getWebServer(getSelfInitializer());
            createWebServer.end();
            getBeanFactory().registerSingleton("webServerGracefulShutdown",
                    new WebServerGracefulShutdownLifecycle(this.webServer));
            getBeanFactory().registerSingleton("webServerStartStop",
                    new WebServerStartStopLifecycle(this, this.webServer));
        }
        else if (servletContext != null) {
            try {
                getSelfInitializer().onStartup(servletContext);
            }
            catch (ServletException ex) {
                throw new ApplicationContextException("Cannot initialize servlet context", ex);
            }
        }
        initPropertySources();
    }

    来看看ServletWebServerApplicationContext#getWebServerFactory，这里表示Spring容器里面只能有一个ServletWebServerFactory类型的BD。

        protected ServletWebServerFactory getWebServerFactory() {
        // Use bean names so that we don't consider the hierarchy
        String[] beanNames = getBeanFactory().getBeanNamesForType(ServletWebServerFactory.class);
        if (beanNames.length == 0) {
            throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to missing "
                    + "ServletWebServerFactory bean.");
        }
        if (beanNames.length > 1) {
            throw new ApplicationContextException("Unable to start ServletWebServerApplicationContext due to multiple "
                    + "ServletWebServerFactory beans : " + StringUtils.arrayToCommaDelimitedString(beanNames));
        }
        return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
    }

    2.2 ServletWebServerFactoryAutoConfiguration自动配置类

    现在来着重看一下ServletWebServerFactory相关的自动配置类ServletWebServerFactoryAutoConfiguration：

    @Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
        ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
        ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
        ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {

    @Bean
    public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties,
            ObjectProvider<WebListenerRegistrar> webListenerRegistrars) {
        return new ServletWebServerFactoryCustomizer(serverProperties,
                webListenerRegistrars.orderedStream().collect(Collectors.toList()));
    }

    @Bean
    @ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")
    public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(
            ServerProperties serverProperties) {
        return new TomcatServletWebServerFactoryCustomizer(serverProperties);
    }

    @Bean
    @ConditionalOnMissingFilterBean(ForwardedHeaderFilter.class)
    @ConditionalOnProperty(value = "server.forward-headers-strategy", havingValue = "framework")
    public FilterRegistrationBean<ForwardedHeaderFilter> forwardedHeaderFilter() {
        ForwardedHeaderFilter filter = new ForwardedHeaderFilter();
        FilterRegistrationBean<ForwardedHeaderFilter> registration = new FilterRegistrationBean<>(filter);
        registration.setDispatcherTypes(DispatcherType.REQUEST, DispatcherType.ASYNC, DispatcherType.ERROR);
        registration.setOrder(Ordered.HIGHEST_PRECEDENCE);
        return registration;
    }

    /**
     * Registers a {@link WebServerFactoryCustomizerBeanPostProcessor}. Registered via
     * {@link ImportBeanDefinitionRegistrar} for early registration.
     */
    public static class BeanPostProcessorsRegistrar implements ImportBeanDefinitionRegistrar, BeanFactoryAware {

        private ConfigurableListableBeanFactory beanFactory;

        @Override
        public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
            if (beanFactory instanceof ConfigurableListableBeanFactory) {
                this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;
            }
        }

        @Override
        public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
                BeanDefinitionRegistry registry) {
            if (this.beanFactory == null) {
                return;
            }
            registerSyntheticBeanIfMissing(registry, "webServerFactoryCustomizerBeanPostProcessor",
                    WebServerFactoryCustomizerBeanPostProcessor.class,
                    WebServerFactoryCustomizerBeanPostProcessor::new);
            registerSyntheticBeanIfMissing(registry, "errorPageRegistrarBeanPostProcessor",
                    ErrorPageRegistrarBeanPostProcessor.class, ErrorPageRegistrarBeanPostProcessor::new);
        }

        private <T> void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry, String name,
                Class<T> beanClass, Supplier<T> instanceSupplier) {
            if (ObjectUtils.isEmpty(this.beanFactory.getBeanNamesForType(beanClass, true, false))) {
                RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass, instanceSupplier);
                beanDefinition.setSynthetic(true);
                registry.registerBeanDefinition(name, beanDefinition);
            }
        }

    }

}

    Import引入的EmbeddedTomcat、EmbeddedJetty和EmbeddedUndertow就是创建ServletWebServerFactory的配置类，比如EmbeddedTomcat，只有存在Tomcat依赖，并且Spring容器里面没有ServletWebServerFactory时才会创建TomcatServletWebServerFactory ：

    @Configuration(proxyBeanMethods = false)
class ServletWebServerFactoryConfiguration {

    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    static class EmbeddedTomcat {

        @Bean
        TomcatServletWebServerFactory tomcatServletWebServerFactory(
                ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
                ObjectProvider<TomcatContextCustomizer> contextCustomizers,
                ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
            TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
            factory.getTomcatConnectorCustomizers()
                    .addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getTomcatContextCustomizers()
                    .addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
            factory.getTomcatProtocolHandlerCustomizers()
                    .addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
            return factory;
        }

    }

    2.3 TomcatConnectorCustomizer对connector进行配置

    @Bean方法的入参会进行依赖查找，Spring会去容器里面查找并注入，ObjectProvider表示可以为空。TomcatConnectorCustomizer可以对Tomcat的connector进行配置。这个在哪里使用？

    详情请看TomcatServletWebServerFactory#getWebServer，其中customizeConnector就会调用之前设置到factory里面的TomcatConnectorCustomizer对connector进行配置：

        public WebServer getWebServer(ServletContextInitializer... initializers) {
        if (this.disableMBeanRegistry) {
            Registry.disableRegistry();
        }
        Tomcat tomcat = new Tomcat();
        File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
        tomcat.setBaseDir(baseDir.getAbsolutePath());
        Connector connector = new Connector(this.protocol);
        connector.setThrowOnFailure(true);
        tomcat.getService().addConnector(connector);
        customizeConnector(connector);
        tomcat.setConnector(connector);
        tomcat.getHost().setAutoDeploy(false);
        configureEngine(tomcat.getEngine());
        for (Connector additionalConnector : this.additionalTomcatConnectors) {
            tomcat.getService().addConnector(additionalConnector);
        }
        prepareContext(tomcat.getHost(), initializers);
        return getTomcatWebServer(tomcat);
    }


    protected void customizeConnector(Connector connector) {
        int port = Math.max(getPort(), 0);
        connector.setPort(port);
        if (StringUtils.hasText(getServerHeader())) {
            connector.setProperty("server", getServerHeader());
        }
        if (connector.getProtocolHandler() instanceof AbstractProtocol) {
            customizeProtocol((AbstractProtocol<?>) connector.getProtocolHandler());
        }
        invokeProtocolHandlerCustomizers(connector.getProtocolHandler());
        if (getUriEncoding() != null) {
            connector.setURIEncoding(getUriEncoding().name());
        }
        // Don't bind to the socket prematurely if ApplicationContext is slow to start
        connector.setProperty("bindOnInit", "false");
        if (getSsl() != null && getSsl().isEnabled()) {
            customizeSsl(connector);
        }
        TomcatConnectorCustomizer compression = new CompressionConnectorCustomizer(getCompression());
        compression.customize(connector);
        for (TomcatConnectorCustomizer customizer : this.tomcatConnectorCustomizers) {
            customizer.customize(connector);
        }
    }

    2.4 application.properties中的属性是怎么处理的？

    另一种配置属性的方法（比如配置server.port=8084），是在属性配置文件比如application.properties里面进行配置。那这个是怎么获取到配置文件中的属性？

    这就涉及到ServletWebServerFactoryAutoConfiguration 自动配置Import导入的另外一个类ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class。BeanPostProcessorsRegistrar是个ImportBeanDefinitionRegistrar，其registerBeanDefinitions()方法注册了WebServerFactoryCustomizerBeanPostProcessor。

    WebServerFactoryCustomizerBeanPostProcessor是个BeanPostProcessor，它主要重写了postProcessBeforeInitialization()方法，也就是在初始化Bean之前调用的。

        public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof WebServerFactory) {
            postProcessBeforeInitialization((WebServerFactory) bean);
        }
        return bean;
    }

    private void postProcessBeforeInitialization(WebServerFactory webServerFactory) {
        LambdaSafe.callbacks(WebServerFactoryCustomizer.class, getCustomizers(), webServerFactory)
                .withLogger(WebServerFactoryCustomizerBeanPostProcessor.class)
                .invoke((customizer) -> customizer.customize(webServerFactory));
    }

    该BeanPostProcessor主要处理的是WebServerFactory的初始化，比如TomcatServletWebServerFactory。

    这里主要调用的WebServerFactoryCustomizer对WebServerFactory进行定制，WebServerFactoryCustomizer在哪个地方定义了？就在ServletWebServerFactoryAutoConfiguration自动配置类里面：

        @Bean
    public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties,
            ObjectProvider<WebListenerRegistrar> webListenerRegistrars) {
        return new ServletWebServerFactoryCustomizer(serverProperties,
                webListenerRegistrars.orderedStream().collect(Collectors.toList()));
    }

    @Bean
    @ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")
    public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(
            ServerProperties serverProperties) {
        return new TomcatServletWebServerFactoryCustomizer(serverProperties);
    }

    来看一下ServletWebServerFactoryCustomizer#customize，这里面就会从配置文件获取server.port设置到factory对于的port属性里面去。

    public class ServletWebServerFactoryCustomizer
        implements WebServerFactoryCustomizer<ConfigurableServletWebServerFactory>, Ordered {

    private final ServerProperties serverProperties;


    public void customize(ConfigurableServletWebServerFactory factory) {
        PropertyMapper map = PropertyMapper.get().alwaysApplyingWhenNonNull();
        map.from(this.serverProperties::getPort).to(factory::setPort);
        map.from(this.serverProperties::getAddress).to(factory::setAddress);
        map.from(this.serverProperties.getServlet()::getContextPath).to(factory::setContextPath);
        map.from(this.serverProperties.getServlet()::getApplicationDisplayName).to(factory::setDisplayName);
        map.from(this.serverProperties.getServlet()::isRegisterDefaultServlet).to(factory::setRegisterDefaultServlet);
        map.from(this.serverProperties.getServlet()::getSession).to(factory::setSession);
        map.from(this.serverProperties::getSsl).to(factory::setSsl);
        map.from(this.serverProperties.getServlet()::getJsp).to(factory::setJsp);
        map.from(this.serverProperties::getCompression).to(factory::setCompression);
        map.from(this.serverProperties::getHttp2).to(factory::setHttp2);
        map.from(this.serverProperties::getServerHeader).to(factory::setServerHeader);
        map.from(this.serverProperties.getServlet()::getContextParameters).to(factory::setInitParameters);
        map.from(this.serverProperties.getShutdown()).to(factory::setShutdown);
        for (WebListenerRegistrar registrar : this.webListenerRegistrars) {
            registrar.register(factory);
        }
    }

    @ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)
public class ServerProperties {

    /**
     * Server HTTP port.
     */
    private Integer port;

    3.条件注解

    @Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
        ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
        ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
        ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {

    来看看该自动配置类上面的两个条件注解：

    @Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnClass {

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnWebApplicationCondition.class)
public @interface ConditionalOnWebApplication {

    重点看SpringBootCondition#matches()

        public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        String classOrMethodName = getClassOrMethodName(metadata);
        try {
            ConditionOutcome outcome = getMatchOutcome(context, metadata);
            logOutcome(classOrMethodName, outcome);
            recordEvaluation(context, classOrMethodName, outcome);
            return outcome.isMatch();
        }
        catch (NoClassDefFoundError ex) {
            throw new IllegalStateException("Could not evaluate condition on " + classOrMethodName + " due to "
                    + ex.getMessage() + " not found. Make sure your own configuration does not rely on "
                    + "that class. This can also happen if you are "
                    + "@ComponentScanning a springframework package (e.g. if you "
                    + "put a @ComponentScan in the default package by mistake)", ex);
        }
        catch (RuntimeException ex) {
            throw new IllegalStateException("Error processing condition on " + getName(metadata), ex);
        }
    }

    该方法执行步骤：

    • 1）获取条件注解写在哪个类上或者哪个方法上
    • 2）获取条件的判断结果getMatchOutcome()，结果放在ConditionOutcome
    • 3）两种日志打印，一种是即时打印，一种是容器启动完成后打印（通过ConditionEvaluationReportLoggingListener监听器监听ContextRefreshedEvent事件进行统一打印）

    3.1 ConditionalOnClass、ConditionalOnMissingClass

    现在来看子类的实现OnClassCondition#getMatchOutcome：

        public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
        ClassLoader classLoader = context.getClassLoader();
        ConditionMessage matchMessage = ConditionMessage.empty();
        List<String> onClasses = getCandidates(metadata, ConditionalOnClass.class);
        if (onClasses != null) {
            List<String> missing = filter(onClasses, ClassNameFilter.MISSING, classLoader);
            if (!missing.isEmpty()) {
                return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnClass.class)
                        .didNotFind("required class", "required classes").items(Style.QUOTE, missing));
            }
            matchMessage = matchMessage.andCondition(ConditionalOnClass.class)
                    .found("required class", "required classes")
                    .items(Style.QUOTE, filter(onClasses, ClassNameFilter.PRESENT, classLoader));
        }
        List<String> onMissingClasses = getCandidates(metadata, ConditionalOnMissingClass.class);
        if (onMissingClasses != null) {
            List<String> present = filter(onMissingClasses, ClassNameFilter.PRESENT, classLoader);
            if (!present.isEmpty()) {
                return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnMissingClass.class)
                        .found("unwanted class", "unwanted classes").items(Style.QUOTE, present));
            }
            matchMessage = matchMessage.andCondition(ConditionalOnMissingClass.class)
                    .didNotFind("unwanted class", "unwanted classes")
                    .items(Style.QUOTE, filter(onMissingClasses, ClassNameFilter.MISSING, classLoader));
        }
        return ConditionOutcome.match(matchMessage);
    }

    该方法执行步骤（ConditionalOnClass、ConditionalOnMissingClass都是走@Conditional(OnClassCondition.class)，都是在OnClassCondition中进行统一处理）：

    • 1）getCandidates()拿到ConditionOnClass注解中的value值，也就是要判断是否存在的类名
    • 2）filter()将onClasses中不存在的类找出来，ClassNameFilter.MISSING就是通过类加载器去加载这些类，看这些类是否存在，如果不存在则记录下这些类
    • 3）这里会同时处理ConditionalOnMissingClass，就是看该自动配置类上面有没有该注解，如果有则拿到该注解的value值，也就是要判断是否存在的类名
    • 4）filter()将onMissingClasses中存在的类找出来，ClassNameFilter.PRESENT就是通过类加载器去加载这些类，看这些类是否存在，如果存在则记录下这些类

    具体的filter方法是FilteringSpringBootCondition#filter：

        protected final List<String> filter(Collection<String> classNames, ClassNameFilter classNameFilter,
            ClassLoader classLoader) {
        if (CollectionUtils.isEmpty(classNames)) {
            return Collections.emptyList();
        }
        List<String> matches = new ArrayList<>(classNames.size());
        for (String candidate : classNames) {
            if (classNameFilter.matches(candidate, classLoader)) {
                matches.add(candidate);
            }
        }
        return matches;
    }

    传入的classNameFilter是ClassNameFilter.MISSING：

            MISSING {

            @Override
            public boolean matches(String className, ClassLoader classLoader) {
                return !isPresent(className, classLoader);
            }

        };


        static boolean isPresent(String className, ClassLoader classLoader) {
            if (classLoader == null) {
                classLoader = ClassUtils.getDefaultClassLoader();
            }
            try {
                resolve(className, classLoader);
                return true;
            }
            catch (Throwable ex) {
                return false;
            }
        }

    3.2 ConditionalOnBean、ConditionalOnSingleCandidate和ConditionalOnMissingBean

    这三个条件注解都走的是@Conditional(OnBeanCondition.class)，也即OnBeanCondition。核心在OnBeanCondition#getMatchOutcome方法中。

    3.3 ConditionalOnJava

    @Conditional(OnJavaCondition.class)
public @interface ConditionalOnJava {

    OnJavaCondition#getMatchOutcome去判断JDK版本时，是通过下面的特征类的特征方法进行判定的，就是看存不存在，判定时是逆序的（从高版本开始判定）。

    public enum JavaVersion {

    /**
     * Java 1.8.
     */
    EIGHT("1.8", Optional.class, "empty"),

    /**
     * Java 9.
     */
    NINE("9", Optional.class, "stream"),

    /**
     * Java 10.
     */
    TEN("10", Optional.class, "orElseThrow"),

    /**
     * Java 11.
     */
    ELEVEN("11", String.class, "strip"),

    /**
     * Java 12.
     */
    TWELVE("12", String.class, "describeConstable"),

    /**
     * Java 13.
     */
    THIRTEEN("13", String.class, "stripIndent"),

    /**
     * Java 14.
     */
    FOURTEEN("14", MethodHandles.Lookup.class, "hasFullPrivilegeAccess"),

    /**
     * Java 15.
     */
    FIFTEEN("15", CharSequence.class, "isEmpty"),

    /**
     * Java 16.
     */
    SIXTEEN("16", Stream.class, "toList");

    3.4 ConditionalOnWebApplication和ConditionalOnNotWebApplication

    这两个都是基于OnWebApplicationCondition

    3.5 ConditionalOnProperty

    @Conditional(OnPropertyCondition.class)
public @interface ConditionalOnProperty {

    核心就是看Environment对象里面有没有配置该属性。

    OnPropertyCondition#getMatchOutcome

        public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
        List<AnnotationAttributes> allAnnotationAttributes = annotationAttributesFromMultiValueMap(
                metadata.getAllAnnotationAttributes(ConditionalOnProperty.class.getName()));
        List<ConditionMessage> noMatch = new ArrayList<>();
        List<ConditionMessage> match = new ArrayList<>();
        for (AnnotationAttributes annotationAttributes : allAnnotationAttributes) {
            ConditionOutcome outcome = determineOutcome(annotationAttributes, context.getEnvironment());
            (outcome.isMatch() ? match : noMatch).add(outcome.getConditionMessage());
        }
        if (!noMatch.isEmpty()) {
            return ConditionOutcome.noMatch(ConditionMessage.of(noMatch));
        }
        return ConditionOutcome.match(ConditionMessage.of(match));
    }

    context.getEnvironment()就是获取Environment对象，Environment实现了PropertyResolver接口。

    4.@SpringBootApplication

    4.1 SpringBoot自动装配原理

    • 1.核心是@SpringBootApplication，包含三个注解：@SpringBootConfiguration、@EnableAutoConfiguration和@ComponentScan；
    • 2.@EnableAutoConfiguration表明启动类是个配置类，@ComponentScan默认情况下指定启动类所在的包路径为扫描路径；
    • 3.重点是@EnableAutoConfiguration，它导入了一个类@Import(AutoConfigurationImportSelector.class)，AutoConfigurationImportSelector是个DeferredImportSelector。
      3-1）ConfigurationClassPostProcessor#postProcessBeanDefinitionRegistry()会在每一批次的配置类解析完才处理DeferredImportSelector，这样可以让程序员自定义的Bean优先级更高；
      3-2）AutoConfigurationImportSelector.AutoConfigurationGroup#process会通过Spring SPI机制读取类路径和jar包中META-INF/spring.factories文件中的自动配置类（key为org.springframework.boot.autoconfigure.EnableAutoConfiguration），这里会根据三个Conditional注解进行初步筛选
      3-3）完整的条件注解筛选会在ConfigurationClassParser#processConfigurationClass中的this.conditionEvaluator.shouldSkip()方法里面处理。这里就会根据Starter引入依赖、程序员是否定义某个Bean等各种条件注解判定某个Bean是否需要注册到Spring容器。

    4.2 AutoConfigurationImportSelector源码剖析

    AutoConfigurationImportSelector.AutoConfigurationGroup#process

            public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
            Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
                    () -> String.format("Only %s implementations are supported, got %s",
                            AutoConfigurationImportSelector.class.getSimpleName(),
                            deferredImportSelector.getClass().getName()));
            AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
                    .getAutoConfigurationEntry(annotationMetadata);
            this.autoConfigurationEntries.add(autoConfigurationEntry);
            for (String importClassName : autoConfigurationEntry.getConfigurations()) {
                this.entries.putIfAbsent(importClassName, annotationMetadata);
            }
        }

        protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = getConfigurationClassFilter().filter(configurations);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationEntry(configurations, exclusions);
    }

    该方法执行步骤：

    • 1）getAttributes()获取@EnableAutoConfiguration的属性（exclude、excludeName）
    • 2）getCandidateConfigurations()通过Spring SPI机制获取META-INF/spring.factories中所有的AutoConfiguration自动配置类
    • 3）removeDuplicates()去重
    • 4）获取需要排除的自动配置类，@EnableAutoConfiguration的属性exclude、excludeName或者通过配置属性spring.autoconfigure.exclude进行排除
    • 5）filter()对自动配置类进行初步过滤，使用三个条件过滤器OnBeanCondition、OnClassCondition、OnWebApplciationCondition（包含的注解：ConditionOnClass、ConditionOnBean、ConditionOnSingleCandidate、ConditionalOnWebApplication），spring-boot-autoconfigure jar包里面有一个文件spring-autoconfigure-metadata.properties（编译生成的）。这里是直接读取该文件，然后根据该文件里面对各个类的条件注解（特别注意：这里的条件注解是不全的）进行过滤。
        protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class, this.beanClassLoader);
    }

    # Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

    来追踪一下filter()方法：

            List<String> filter(List<String> configurations) {
            long startTime = System.nanoTime();
            String[] candidates = StringUtils.toStringArray(configurations);
            boolean skipped = false;
            for (AutoConfigurationImportFilter filter : this.filters) {
                boolean[] match = filter.match(candidates, this.autoConfigurationMetadata);
                for (int i = 0; i < match.length; i++) {
                    if (!match[i]) {
                        candidates[i] = null;
                        skipped = true;
                    }
                }
            }
            if (!skipped) {
                return configurations;
            }
            List<String> result = new ArrayList<>(candidates.length);
            for (String candidate : candidates) {
                if (candidate != null) {
                    result.add(candidate);
                }
            }
            if (logger.isTraceEnabled()) {
                int numberFiltered = configurations.size() - result.size();
                logger.trace("Filtered " + numberFiltered + " auto configuration class in "
                        + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime) + " ms");
            }
            return result;
        }

    FilteringSpringBootCondition#match

        public boolean[] match(String[] autoConfigurationClasses, AutoConfigurationMetadata autoConfigurationMetadata) {
        ConditionEvaluationReport report = ConditionEvaluationReport.find(this.beanFactory);
        ConditionOutcome[] outcomes = getOutcomes(autoConfigurationClasses, autoConfigurationMetadata);
        boolean[] match = new boolean[outcomes.length];
        for (int i = 0; i < outcomes.length; i++) {
            match[i] = (outcomes[i] == null || outcomes[i].isMatch());
            if (!match[i] && outcomes[i] != null) {
                logOutcome(autoConfigurationClasses[i], outcomes[i]);
                if (report != null) {
                    report.recordConditionEvaluation(autoConfigurationClasses[i], this, outcomes[i]);
                }
            }
        }
        return match;
    }

    这里getOutcomes()就只在上面三个filter里面重写了，这里过滤方法就是根据条件注解进行过滤，没有本质区别，只不过这里使用的条件注解只是局限于某几个（ConditionOnClass、ConditionOnBean、ConditionOnSingleCandidate、ConditionalOnWebApplication）：


    4.3 @AutoConfigurationPackage注解

    @AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

    @Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

    会执行ImportBeanDefinitionRegistrar#registerBeanDefinitions

        static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

        @Override
        public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
            register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));
        }

            PackageImports(AnnotationMetadata metadata) {
            AnnotationAttributes attributes = AnnotationAttributes
                    .fromMap(metadata.getAnnotationAttributes(AutoConfigurationPackage.class.getName(), false));
            List<String> packageNames = new ArrayList<>(Arrays.asList(attributes.getStringArray("basePackages")));
            for (Class<?> basePackageClass : attributes.getClassArray("basePackageClasses")) {
                packageNames.add(basePackageClass.getPackage().getName());
            }
            if (packageNames.isEmpty()) {
                packageNames.add(ClassUtils.getPackageName(metadata.getClassName()));
            }
            this.packageNames = Collections.unmodifiableList(packageNames);
        }

    AutoConfigurationPackage该注解单独使用时，可以指定包路径basePackages。在这里是没有值的。

    然后就会获取启动类所在的包路径。

    最后会将该包路径封装成BasePackagesBeanDefinition，名字为AutoConfigurationPackages.class.getName()，注册到Spring容器。

    那这个路径谁用呢？Mybatis可以用，让Mybatis少一些配置，直接使用这个包路径就可以。Mybatis的扫描器就会去获取该包路径的Bean：

        public static List<String> get(BeanFactory beanFactory) {
        try {
            return beanFactory.getBean(BEAN, BasePackages.class).get();
        }
        catch (NoSuchBeanDefinitionException ex) {
            throw new IllegalStateException("Unable to retrieve @EnableAutoConfiguration base packages");
        }
    }

    4.4 @ComponentScan里面的两个exclueFilter

    @SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

    public class AutoConfigurationExcludeFilter implements TypeFilter, BeanClassLoaderAware {

    @Override
    public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory)
            throws IOException {
        return isConfiguration(metadataReader) && isAutoConfiguration(metadataReader);
    }

    private boolean isConfiguration(MetadataReader metadataReader) {
        return metadataReader.getAnnotationMetadata().isAnnotated(Configuration.class.getName());
    }

    private boolean isAutoConfiguration(MetadataReader metadataReader) {
        return getAutoConfigurations().contains(metadataReader.getClassMetadata().getClassName());
    }

    AutoConfigurationExcludeFilter会过滤掉是自动配置类的配置类，首先判断是否有@Configuration注解，然后判断自动配置类（通过SPI机制加载的key为EnableAutoConfiguration）是否包含该配置类。

    为什么要这么做？
    因为自动配置类要在每一轮配置类扫描完成后才进行处理，不需要在扫描的时候就处理。

    public class TypeExcludeFilter implements TypeFilter, BeanFactoryAware {

    @Override
    public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory)
            throws IOException {
        if (this.beanFactory instanceof ListableBeanFactory && getClass() == TypeExcludeFilter.class) {
            for (TypeExcludeFilter delegate : getDelegates()) {
                if (delegate.match(metadataReader, metadataReaderFactory)) {
                    return true;
                }
            }
        }
        return false;
    }

    private Collection<TypeExcludeFilter> getDelegates() {
        Collection<TypeExcludeFilter> delegates = this.delegates;
        if (delegates == null) {
            delegates = ((ListableBeanFactory) this.beanFactory).getBeansOfType(TypeExcludeFilter.class).values();
            this.delegates = delegates;
        }
        return delegates;
    }

    会从Spring容器中获取所有TypeExcludeFilter，然后进行匹配，匹配成功的则不进行解析。

    特别注意：

    • 1）如果这里先定义一个TypeExcludeFilter作为@Bean注册到Spring容器，并过滤某个@Component，是不会生效的
    • 2）为什么，跟顺序有关，因为@Component、@ComponentScan会先进行处理，@Bean后进行处理，@ComponentScan进行扫描时，@Bean注解TypeExcludeFilter在容器里面还不存在，所以不起作用。
    • 3）@Component注解TypeExcludeFilter也不一定生效，也有个顺序问题

    那怎么解决这个问题？

    • 关键就是要将TypeExcludeFilter在扫描之前就加入到Spring容器
    • 可以定义ApplicationContextInitializer，并配置到spring.factories文件里面
    • 然后在ApplicationContextInitializer#initialize中applicationContext.getBeanFactory().registerSingleton()向Spring容器注册TypeExcludeFilter
    • 当然也可以用BeanFactoryRegistryPostProcessor，并且是要程序手动向Spring容器添加的，这样就会在ConfigurationClassPostProcessor之前执行，但是这样做不太规范，没法实现。

    5.starter包

    Starter 其实就是一个 jar 包，在 pom 中引入一个 starter 其实就是引入了一个 jar 包。而且很多时候，这个 starter 对应的 jar 包是个空的，里面并没有任何类和接口。

    那这个空的 jar 包有何意义呢？它的意义就在于为了引入有意义的其它 jar 包，因为这些 jar 包都是基于 Maven 的，因此 jar 包的 pom 文件中包含了其它依赖。而且SpringBoot会把常用的依赖及其适合的版本号都通过依赖管理的方式包含在 pom 文件中。

    一般SpringBoot已经整合的组件，也就是SpringBoot提供的starter包里面只会有相关依赖（自动配置类已经写好了）；而对于第三方组件想要整合SpringBoot，还需要加入自动配置的jar包（比如mybatis-spring-boot-starter里面还包括mybatis-spring-boot-autoconfigure的jar包）。

    6.SpringBoot启动过程

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

    6.1 构造SpringApplication对象

        public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");

        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));

        // 1. 推测web应用类型（NONE、REACTIVE、SERVLET）
        this.webApplicationType = WebApplicationType.deduceFromClasspath();

        // 2. 从spring.factories中获取BootstrapRegistryInitializer对象
        this.bootstrapRegistryInitializers = new ArrayList<>(
                getSpringFactoriesInstances(BootstrapRegistryInitializer.class));

        // 3. 从spring.factories中获取ApplicationContextInitializer对象
        setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));

        // 4. 从spring.factories中获取ApplicationListener对象
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));

        // 5. 推测出Main类（main()方法所在的类）
        this.mainApplicationClass = deduceMainApplicationClass();
    }

    xxx\spring-boot-2.6.6-master\spring-boot-project\spring-boot\src\main\resources\META-INF\spring.factories

    # Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.rsocket.context.RSocketPortInfoApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.env.EnvironmentPostProcessorApplicationListener

    6.2 核心启动流程在run()方法里面

        public ConfigurableApplicationContext run(String... args) {
        long startTime = System.nanoTime();

        // 1、创建引导启动器，类似一个ApplicationContext，可以往里面添加一些对象
        DefaultBootstrapContext bootstrapContext = createBootstrapContext();

        ConfigurableApplicationContext context = null;
        configureHeadlessProperty();

        // 2、从spring.factories中获取SpringApplicationRunListener对象
        // 默认会拿到一个EventPublishingRunListener，它会启动过程的各个阶段发布对应的ApplicationEvent事件
        SpringApplicationRunListeners listeners = getRunListeners(args);

        // 3、发布ApplicationStartingEvent
        listeners.starting(bootstrapContext, this.mainApplicationClass);
        try {

            // 4、将run()的参数封装为DefaultApplicationArguments对象
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);

            // 5、准备Environment
            // 包括操作系统，JVM、ServletContext、properties、yaml等等配置
            // 会发布一个ApplicationEnvironmentPreparedEvent
            ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);

            // 默认spring.beaninfo.ignore=true，表示不需要jdk缓存beanInfo信息，Spring自己会缓存
            configureIgnoreBeanInfo(environment);

            Banner printedBanner = printBanner(environment);

            // 6、根据应用类型创建Spring容器
            context = createApplicationContext();
            context.setApplicationStartup(this.applicationStartup);

            // 7、利用ApplicationContextInitializer初始化Spring容器
            // 8、发布ApplicationContextInitializedEvent
            // 9、关闭DefaultBootstrapContext
            // 10、注册primarySources类，就是run方法存入进来的配置类
            // 11、发布ApplicationPreparedEvent事件
            prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);

            // 12、刷新Spring容器，会解析配置类、扫描、启动WebServer
            refreshContext(context);

            // 空方法
            afterRefresh(context, applicationArguments);

            // 启动时间
            Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);
            }

            // 13、发布ApplicationStartedEvent事件，表示Spring容器已经启动
            listeners.started(context, timeTakenToStartup);

            // 14、从Spring容器中获取ApplicationRunner和CommandLineRunner，并执行其run()
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            // 15、发布ApplicationFailedEvent事件
            handleRunFailure(context, ex, listeners);
            throw new IllegalStateException(ex);
        }

        try {
            Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);

            // 16、发布ApplicationReadyEvent事件，表示Spring容器已经准备好了
            listeners.ready(context, timeTakenToReady);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

    6.2.1 createBootstrapContext()

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

    6.2.2 SpringApplicationRunListener

    SpringApplicationRunListeners#starting

        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();
    }

    其中listeners只有一个，就是EventPublishingRunListener

    class SpringApplicationRunListeners {

    private final List<SpringApplicationRunListener> listeners;

    # Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

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

    EventPublishingRunListener#starting就是发布事件，然后调用ApplicationListener进行处理。这里可以使用SimpleApplicationEventMulticaster的taskExecutor线程池进行异步调用监听器进行处理（如果有的话）。

    6.3 prepareEnvironment()

        private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
            DefaultBootstrapContext bootstrapContext, ApplicationArguments applicationArguments) {
        // Create and configure the environment
        // 创建ApplicationServletEnvironment，里面添加了四个PropertySource
        // 1. StubPropertySource {name='servletConfigInitParams'}
        // 2. StubPropertySource {name='servletContextInitParams'}
        // 3. PropertiesPropertySource {name='systemProperties'}
        // 4. SystemEnvironmentPropertySource {name='systemEnvironment'}
        ConfigurableEnvironment environment = getOrCreateEnvironment();

        // 添加SimpleCommandLinePropertySource {name='commandLineArgs'}，放在首位
        configureEnvironment(environment, applicationArguments.getSourceArgs());

        // 把所有的PropertySources封装为一个ConfigurationPropertySourcesPropertySource
        // 然后添加到environment中，放在首位
        ConfigurationPropertySources.attach(environment);

        // 发布ApplicationEnvironmentPreparedEvent事件，表示环境已经准备好了
        // 默认EnvironmentPostProcessorApplicationListener会处理这个事件，会从spring.factories中拿出EnvironmentPostProcessor进一步处理Environment
        listeners.environmentPrepared(bootstrapContext, environment);

        // 最后，把defaultProperties移到最后
        DefaultPropertiesPropertySource.moveToEnd(environment);
        Assert.state(!environment.containsProperty("spring.main.environment-prefix"),
                "Environment prefix cannot be set via properties.");
        bindToSpringApplication(environment);
        if (!this.isCustomEnvironment) {
            environment = convertEnvironment(environment);
        }
        ConfigurationPropertySources.attach(environment);
        return environment;
    }

    6.4 createApplicationContext()

    ApplicationContextFactory中的DEFAULT里面进行创建：

        ApplicationContextFactory DEFAULT = (webApplicationType) -> {
        try {
            switch (webApplicationType) {
            case SERVLET:
                return new AnnotationConfigServletWebServerApplicationContext();
            case REACTIVE:
                return new AnnotationConfigReactiveWebServerApplicationContext();
            default:
                return new AnnotationConfigApplicationContext();
            }
        }
        catch (Exception ex) {
            throw new IllegalStateException("Unable create a default ApplicationContext instance, "
                    + "you may need a custom ApplicationContextFactory", ex);
        }
    };

    6.5 prepareContext()

    主要是做几件事：

    • 1）利用ApplicationContextInitializer初始化Spring容器
    • 2）发布ApplicationContextInitializedEvent
    • 3）关闭DefaultBootstrapContext
    • 4）注册primarySources类，就是run方法存入进来的配置类
    • 5）发布ApplicationPreparedEvent事件
        private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context,
            ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments, Banner printedBanner) {

        // 将前面生成的Environment设置到Spring容器中
        context.setEnvironment(environment);

        // 将设置在SpringApplication上的beanNameGenerator、resourceLoader设置到Spring容器中
        postProcessApplicationContext(context);

        // 利用ApplicationContextInitializer初始化Spring容器
        applyInitializers(context);

        // 发布ApplicationContextInitializedEvent事件，表示Spring容器初始化完成
        listeners.contextPrepared(context);

        // Spring容器初始化好了，就关闭DefaultBootstrapContext
        bootstrapContext.close(context);

        if (this.logStartupInfo) {
            logStartupInfo(context.getParent() == null);
            logStartupProfileInfo(context);
        }

        // 注册一些单例Bean
        // Add boot specific singleton beans
        ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
        beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
        if (printedBanner != null) {
            beanFactory.registerSingleton("springBootBanner", printedBanner);
        }

        // 设置allowCircularReferences和allowBeanDefinitionOverriding给Spring容器
        if (beanFactory instanceof AbstractAutowireCapableBeanFactory) {
            ((AbstractAutowireCapableBeanFactory) beanFactory).setAllowCircularReferences(this.allowCircularReferences);
            if (beanFactory instanceof DefaultListableBeanFactory) {
                ((DefaultListableBeanFactory) beanFactory)
                        .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
            }
        }

        if (this.lazyInitialization) {
            context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
        }

        // Load the sources
        // 拿到启动配置类（run方法传进来的类）
        Set<Object> sources = getAllSources();
        Assert.notEmpty(sources, "Sources must not be empty");

        // 将启动配置类解析为BeanDefinition注册到Spring容器中
        load(context, sources.toArray(new Object[0]));

        // 发布ApplicationPreparedEvent事件，表示Spring容器已经准备好
        listeners.contextLoaded(context);
    }

    6.6 入参

                // 4、将run()的参数封装为DefaultApplicationArguments对象
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);

            // 14、从Spring容器中获取ApplicationRunner和CommandLineRunner，并执行其run()
            callRunners(context, applicationArguments);

    callRunners()会去Spring容器查找ApplicationRunner和CommandLineRunner执行，执行时会传入入参applicationArguments。

    7.重点分析一下各种配置的解析prepareEnvironment()

        private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
            DefaultBootstrapContext bootstrapContext, ApplicationArguments applicationArguments) {
        // Create and configure the environment
        // 创建ApplicationServletEnvironment，里面添加了四个PropertySource
        // 1. StubPropertySource {name='servletConfigInitParams'}
        // 2. StubPropertySource {name='servletContextInitParams'}
        // 3. PropertiesPropertySource {name='systemProperties'}
        // 4. SystemEnvironmentPropertySource {name='systemEnvironment'}
        ConfigurableEnvironment environment = getOrCreateEnvironment();

        // 添加SimpleCommandLinePropertySource {name='commandLineArgs'}，放在首位
        configureEnvironment(environment, applicationArguments.getSourceArgs());

        // 把所有的PropertySources封装为一个ConfigurationPropertySourcesPropertySource
        // 然后添加到environment中，放在首位
        ConfigurationPropertySources.attach(environment);

        // 发布ApplicationEnvironmentPreparedEvent事件，表示环境已经准备好了
        // 默认EnvironmentPostProcessorApplicationListener会处理这个事件，会从spring.factories中拿出EnvironmentPostProcessor进一步处理Environment
        listeners.environmentPrepared(bootstrapContext, environment);

        // 最后，把defaultProperties移到最后
        DefaultPropertiesPropertySource.moveToEnd(environment);
        Assert.state(!environment.containsProperty("spring.main.environment-prefix"),
                "Environment prefix cannot be set via properties.");
        bindToSpringApplication(environment);
        if (!this.isCustomEnvironment) {
            environment = convertEnvironment(environment);
        }
        ConfigurationPropertySources.attach(environment);
        return environment;
    }

    public class MutablePropertySources implements PropertySources {

    private final List<PropertySource<?>> propertySourceList = new CopyOnWriteArrayList<>();

    一个PropertySource就代表一个配置文件（解析完后的内容）。

        private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
        ConfigurableEnvironment environment = event.getEnvironment();
        SpringApplication application = event.getSpringApplication();

        // 从spring.factories中拿出EnvironmentPostProcessor进一步处理Environment
        // RandomValuePropertySourceEnvironmentPostProcessor
        // SystemEnvironmentPropertySourceEnvironmentPostProcessor
        // SpringApplicationJsonEnvironmentPostProcessor
        // CloudFoundryVcapEnvironmentPostProcessor
        // ConfigDataEnvironmentPostProcessor
        // IntegrationPropertiesEnvironmentPostProcessor
        // DebugAgentEnvironmentPostProcessor
        for (EnvironmentPostProcessor postProcessor : getEnvironmentPostProcessors(application.getResourceLoader(),
                event.getBootstrapContext())) {
            postProcessor.postProcessEnvironment(environment, application);
        }
    }

    来看看这些EnvironmentPostProcessor的作用

    • RandomValuePropertySourceEnvironmentPostProcessor添加了一个random
    • SystemEnvironmentPropertySourceEnvironmentPostProcessor有前缀的情况下会替换systemEnvironment（操作系统的环境变量）。Environment variables中进行配置。这个前缀可用于区分环境，操作系统级别的环境。比如如果配置了前缀zhouyu，必须配置成zhouyu.k5=v5才能获取到k5的值。
    • SpringApplicationJsonEnvironmentPostProcessor会去获取spring.application.json里面配置的json字符串，然后解析出来后放到PropertySource。只能配置在VM options里面或者Program arguments里面，也就是只能配置在已经解析完成的属性文件里面。此时properties还没解析，配置无用。
    • CloudFoundryVcapEnvironmentPostProcessor
    • ConfigDataEnvironmentPostProcessor在三个参数指定的路径下面去找properties或者yaml文件

    整体流程：






    SpringBoot完整的配置优先级
    https://docs.spring.io/spring-boot/docs/current/reference/html/features.html#features.external-config

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