Spring源码(十二)：Spring MVC之Spring Boot

本篇将详细讨论Spring Boot 的启动/加载、处理请求的具体流程。我们先从一个简单的Spring Boot项目日志开始分析（这里假设读者已经仔细阅读完了前面的文章，且对Spring源码有一定深度的了解，否则会看得一脸懵逼）。

本文为2024重置版，基于JDK8、Spring Boot 2.2.6.RELEASE（最初版本基于Spring MVC + XML配置，现已废弃去除）。

一、相关代码

1、pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <groupId>site.xiaokui</groupId>
        <artifactId>k-2019</artifactId>
        <version>1.0.0</version>
    </parent>

    <modelVersion>4.0.0</modelVersion>
    <artifactId>k-mini</artifactId>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
    </dependencies>
</project>

2、相关类

@SpringBootApplication
public class MiniAppRunner {

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

@Slf4j
@RequiredArgsConstructor
@RestController
public class TestController {

    @PostMapping("/test")
    @ResponseBody
    public Object test0(@RequestBody Map<String, Object> map) {
        log.info("前端传来数据:" + map);
        return map;
    }
}

POST http://127.0.0.1:7077/test?aa=1&bb=2
Content-Type: application/json

{
  "data": {
    "login_type": "08",
    "login_str": "abcd"
  }
}

3、相关配置

debug: true

server:
  port: 7077

logging:
  level.root: trace
  level.org.apache.tomcat: info
  level.sun.rmi: info

以上为相关测试代码，读者也可搭建环境自行测试（省略具体输出日志）。

二、SpringApplication

1、构造方法

// main方法入口，来自类 SpringApplication
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);
}

public SpringApplication(Class<?>... primarySources) {
    this(null, primarySources);
}

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    // 判断当前运行环境，可选 none、servlet、reactive
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    // 读取 META-INF/spring.factories，加载相关启动类
    // 加载初始化相关类，共有7个 = 5 + 2，数字刚好与下文的两份 spring.factories对上
    // 注意这里只是设置，还尚未开始调用
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    // 加载监听器，监听相关事件，共有11个 = 10 + 1
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    // 寻找main方法所在的类作为主类
    this.mainApplicationClass = deduceMainApplicationClass();
}

2、spring.factories

来自jar包 org.springframework.boot:spring-boot:2.2.6.REALEASE。

# PropertySource Loaders
org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader

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

# Error Reporters
org.springframework.boot.SpringBootExceptionReporter=\
org.springframework.boot.diagnostics.FailureAnalyzers

# Application Context Initializers，5个
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，10个
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\
org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor,\
org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor

# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanDefinitionOverrideFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoSuchMethodFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer

# FailureAnalysisReporters
org.springframework.boot.diagnostics.FailureAnalysisReporter=\
org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter

来自来自jar包 org.springframework.boot:spring-boot-autoconfigure:2.2.6.REALEASE。

# Initializers，2个
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listeners，1个
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# 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

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveRestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\
org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketRequesterAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketServerAutoConfiguration,\
org.springframework.boot.autoconfigure.rsocket.RSocketStrategiesAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.rsocket.RSocketSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyAutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

# Failure analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.flyway.FlywayMigrationScriptMissingFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\
org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer

# Template availability providers
org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProvider

3、事件周期图

下图展现SpringApplication的事件全生命周期，读者可以按照此图对照源码进行阅读。其中相关核心类有（断点神类）

• SpringApplication
• SimpleApplicationEventMulticaster

4、主干

// 主干逻辑
public ConfigurableApplicationContext run(String... args) {
    // 统计耗时
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    // WebApplicationContext
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    // 设置无图形界面服务器工作模式
    configureHeadlessProperty();
    SpringApplicationRunListeners listeners = getRunListeners(args);
    // 触发事件 ApplicationStartingEvent
    // 分别为 LoggingApplicationListener、BackgroundPreinitializer、DelegatingApplicationListener、LiquibaseServiceLocatorApplicationListener
    // 这里重点关注下 LoggingApplicationListener、BackgroundPreinitializer
    listeners.starting();
    try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        // 准备环境，包含各种系统属性、环境变量、配置值
        // 触发事件 ApplicationEnvironmentPreparedEvent
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        configureIgnoreBeanInfo(environment);
        Banner printedBanner = printBanner(environment);
        // 创建 AnnotationConfigServletWebServerApplicationContext
        context = createApplicationContext();
        exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class }, context);
        // 触发 applyInitializers
        // 触发事件 ApplicationContextInitializedEvent、ApplicationPreparedEvent
        prepareContext(context, environment, listeners, applicationArguments, printedBanner);
        // 完成刷新触发事件 ContextRefreshedEvent、ServletWebServerInitializedEvent
        refreshContext(context);
        // 空实现，留作扩展
        afterRefresh(context, applicationArguments
        // 完成统计耗时
        stopWatch.stop();
        if (this.logStartupInfo) {
            // 打印启动耗时
            // Started MiniAppRunner in 106.05 seconds (JVM running for 189.145)
            new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
        }
        // 触发事件 ApplicationStartedEvent
        listeners.started(context);
        // 调用 ApplicationRunner、CommandLineRunner
        callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, listeners);
        throw new IllegalStateException(ex);
    }

    try {
        // 发布事件 ApplicationReadyEvent，代表Spring已经完全启动了
        listeners.running(context);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, null);
        throw new IllegalStateException(ex);
    }
    return context;
}

三、系统属性初始化

1、ApplicationStartingEvent

1、LoggingApplicationListener

// 来自类 LoggingApplicationListener
@Override
public void onApplicationEvent(ApplicationEvent event) {
    // 系统开始启动事件
    if (event instanceof ApplicationStartingEvent) {
        onApplicationStartingEvent((ApplicationStartingEvent) event);
    }
    else if (event instanceof ApplicationEnvironmentPreparedEvent) {
        onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
    }
    else if (event instanceof ApplicationPreparedEvent) {
        onApplicationPreparedEvent((ApplicationPreparedEvent) event);
    }
    else if (event instanceof ContextClosedEvent
            && ((ContextClosedEvent) event).getApplicationContext().getParent() == null) {
        onContextClosedEvent();
    }
    else if (event instanceof ApplicationFailedEvent) {
        onApplicationFailedEvent();
    }
}

private void onApplicationStartingEvent(ApplicationStartingEvent event) {
    // 准备 LoggingSystem
    this.loggingSystem = LoggingSystem.get(event.getSpringApplication().getClassLoader());
    // 日志系统初始化之前的准备工作
    this.loggingSystem.beforeInitialize();
}

// 来自类 LoggingSystem
@Override
public void beforeInitialize() {
    LoggerContext loggerContext = getLoggerContext();
    if (isAlreadyInitialized(loggerContext)) {
        return;
    }
    // 会触发父类 Slf4JLoggingSystem 的初始化
    // 此步骤会尽量保证系统只有一个日志框架生效，例如jdklog、logback、log4j只会生效一个
    super.beforeInitialize();
    loggerContext.getTurboFilterList().add(FILTER);
}

2、BackgroundPreinitializer

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

// 主要作用是执行类的预热，因为组件较多，提前异步实例化，可提升系统启动速度
// 实例化好的对象，最终会保存到一个静态对象中，下次直接使用即可
private void performPreinitialization() {
    try {
        Thread thread = new Thread(new Runnable() {

            @Override
            public void run() {
                runSafely(new ConversionServiceInitializer());
                runSafely(new ValidationInitializer());
                runSafely(new MessageConverterInitializer());
                runSafely(new JacksonInitializer());
                runSafely(new CharsetInitializer());
                preinitializationComplete.countDown();
            }

            public void runSafely(Runnable runnable) {
                try {
                    runnable.run();
                }
                catch (Throwable ex) {
                    // Ignore
                }
            }

        }, "background-preinit");
        thread.start();
    }
    catch (Exception ex) {
        // This will fail on GAE where creating threads is prohibited. We can safely
        // continue but startup will be slightly slower as the initialization will now
        // happen on the main thread.
        preinitializationComplete.countDown();
    }
}

2、prepareEnvironment

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
        ApplicationArguments applicationArguments) {
    // Create and configure the environment
    // 此时具有环境变量和系统属性，但还未有配置文件属性
    ConfigurableEnvironment environment = getOrCreateEnvironment();
    configureEnvironment(environment, applicationArguments.getSourceArgs());
    ConfigurationPropertySources.attach(environment);
    // 发布环境就绪事件
    // 会触发监听类 ConfigFileApplicationListener、AnsiOutputApplicationListener、LoggingApplicationListener
	// ClasspathLoggingApplicationListener、BackgroundPreinitializer、DelegatingApplicationListener、FileEncodingApplicationListener
    listeners.environmentPrepared(environment);
    bindToSpringApplication(environment);
    if (!this.isCustomEnvironment) {
        environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,
                deduceEnvironmentClass());
    }
    ConfigurationPropertySources.attach(environment);
    return environment;
}

1、getOrCreateEnvironment

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

public class StandardServletEnvironment extends StandardEnvironment implements ConfigurableWebEnvironment {

}

// 会首先触发父类的初始化
public AbstractEnvironment() {
    // 开始读取配置
    customizePropertySources(this.propertySources);
}

// 来自类 StandardServletEnvironment
@Override
protected void customizePropertySources(MutablePropertySources propertySources) {
    propertySources.addLast(new StubPropertySource(SERVLET_CONFIG_PROPERTY_SOURCE_NAME));
    propertySources.addLast(new StubPropertySource(SERVLET_CONTEXT_PROPERTY_SOURCE_NAME));
    if (JndiLocatorDelegate.isDefaultJndiEnvironmentAvailable()) {
        propertySources.addLast(new JndiPropertySource(JNDI_PROPERTY_SOURCE_NAME));
    }
    // 回调父类 customizePropertySources
    super.customizePropertySources(propertySources);
}

// 来自类 StandardEnvironment
@Override
protected void customizePropertySources(MutablePropertySources propertySources) {
    // 底层为 (Map) System.getProperties()
    propertySources.addLast(
            new PropertiesPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties()));
    // 底层为 (Map) System.getenv()
    propertySources.addLast(
            new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
}

2、ApplicationEnvironmentPreparedEvent

限于篇幅，这里不详细展开，大概列下：

• ConfigFileApplicationListener：用于加载外部配置文件。
• AnsiOutputApplicationListener：让你的终端（可以是控制台、可以是日志文件）支持Ansi彩色输出。
• LoggingApplicationListener：初始化日志系统。
• ClasspathLoggingApplicationListener ：用于把classpath路径以log.debug()输出。
• BackgroundPreinitializer：无动作。
• DelegatingApplicationListener ：执行通过外部化配置context.listener.classes = xxx,xxx的监听器们，然后把该事件广播给他们。
• FileEncodingApplicationListener：检查文件编码file.encoding。

ConfigFileApplicationListener 部分关键代码如下

// 来自类 ConfigFileApplicationListener
@Override
public void onApplicationEvent(ApplicationEvent event) {
    if (event instanceof ApplicationEnvironmentPreparedEvent) {
        onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
    }
    if (event instanceof ApplicationPreparedEvent) {
        onApplicationPreparedEvent(event);
    }
}

private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
    // 分别为 SystemEnvironmentPropertySourceEnvironmentPostProcessor、SpringApplicationJsonEnvironmentPostProcessor
    // CloudFoundryVcapEnvironmentPostProcessor、DebugAgentEnvironmentPostProcessor
    List<EnvironmentPostProcessor> postProcessors = loadPostProcessors();
    // 注意 add this了
    // 读取外部配置，这个最重要的活是自己来干
    postProcessors.add(this);
    AnnotationAwareOrderComparator.sort(postProcessors);
    for (EnvironmentPostProcessor postProcessor : postProcessors) {
        postProcessor.postProcessEnvironment(event.getEnvironment(), event.getSpringApplication());
    }
}

// 来自类 ConfigFileApplicationListener
@Override
public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
    addPropertySources(environment, application.getResourceLoader());
}

// 鉴于代码较多，这里只列举部分关键代码
// 默认配置文件路径 DEFAULT_SEARCH_LOCATIONS = "classpath:/,classpath:/config/,file:./,file:./config/";
private void load(Profile profile, DocumentFilterFactory filterFactory, DocumentConsumer consumer) {
    getSearchLocations().forEach((location) -> {
        boolean isFolder = location.endsWith("/");
        Set<String> names = isFolder ? getSearchNames() : NO_SEARCH_NAMES;
        names.forEach((name) -> load(location, name, profile, filterFactory, consumer));
    });
}

LoggingApplicationListener 部分关键代码如下

// 来自类 LoggingApplicationListener
private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
    // 上一事件已经完成准备工作
    if (this.loggingSystem == null) {
        this.loggingSystem = LoggingSystem.get(event.getSpringApplication().getClassLoader());
    }
    // 直接跳到这里开始初始化
    initialize(event.getEnvironment(), event.getSpringApplication().getClassLoader());
}

protected void initialize(ConfigurableEnvironment environment, ClassLoader classLoader) {
    // 设置日志属性默认值
    new LoggingSystemProperties(environment).apply();
    // 日志文件名 logging.file.name、logging.file
    // 日志文件路径 logging.file.path、logging.path
    this.logFile = LogFile.get(environment);
    if (this.logFile != null) {
        this.logFile.applyToSystemProperties();
    }
    // 默认logger web & sql
    this.loggerGroups = new LoggerGroups(DEFAULT_GROUP_LOGGERS);
    // 设置early日志级别，主要是 debug/trace = true/false
    initializeEarlyLoggingLevel(environment);
    // 根据日志配置，初始化日志系统，包括 日志输出格式、日志存放路径、日志存放策略等
    initializeSystem(environment, this.loggingSystem, this.logFile);
    // 设置日志级别，含root和各个子包路径
    initializeFinalLoggingLevels(environment, this.loggingSystem);
    // 注册销毁钩子
    registerShutdownHookIfNecessary(environment, this.loggingSystem);
}

3、printBanner

打印banner，关键类为 SpringApplicationBannerPrinter，这里不做详细展开。

// 只列举部分字段
class SpringApplicationBannerPrinter {

	static final String BANNER_LOCATION_PROPERTY = "spring.banner.location";

	static final String BANNER_IMAGE_LOCATION_PROPERTY = "spring.banner.image.location";

	static final String DEFAULT_BANNER_LOCATION = "banner.txt";

	static final String[] IMAGE_EXTENSION = { "gif", "jpg", "png" };

    // 默认打印 spring boot 版本信息
	private static final Banner DEFAULT_BANNER = new SpringBootBanner();

	private final ResourceLoader resourceLoader;

	private final Banner fallbackBanner;
    // 其他略
}

四、WebApplication初始化

1、createApplicationContext

// 来自类 SpringApplication
protected ConfigurableApplicationContext createApplicationContext() {
    Class<?> contextClass = this.applicationContextClass;
    if (contextClass == null) {
        try {
            switch (this.webApplicationType) {
            case SERVLET:
                // 走此逻辑，类为 AnnotationConfigServletWebServerApplicationContext
                contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
                break;
            case REACTIVE:
                // AnnotationConfigReactiveWebServerApplicationContext
                contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
                break;
            default:
                // AnnotationConfigApplicationContext     
                contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
            }
        }
        catch (ClassNotFoundException ex) {
            throw new IllegalStateException(
                    "Unable create a default ApplicationContext, please specify an ApplicationContextClass", ex);
        }
    }
    return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

2、prepareContext

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,
        SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
    context.setEnvironment(environment);
    postProcessApplicationContext(context);
    // 触发初始化逻辑，共7个，具体参见前文
    applyInitializers(context);
    // 发布事件 ApplicationContextInitializedEvent
    listeners.contextPrepared(context);
    if (this.logStartupInfo) {
        logStartupInfo(context.getParent() == null);
        logStartupProfileInfo(context);
    }
    // Add boot specific singleton beans
    ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
    beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
    if (printedBanner != null) {
        beanFactory.registerSingleton("springBootBanner", printedBanner);
    }
    if (beanFactory instanceof DefaultListableBeanFactory) {
        ((DefaultListableBeanFactory) beanFactory)
                .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
    }
    if (this.lazyInitialization) {
        context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
    }
    // Load the sources
    Set<Object> sources = getAllSources();
    Assert.notEmpty(sources, "Sources must not be empty");
    load(context, sources.toArray(new Object[0]));
    // 发布 ApplicationPreparedEvent
    listeners.contextLoaded(context);
}

1、applyInitializers

// 触发相关类为 DelegatingApplicationContextInitializer、SharedMetadataReaderFactoryContextInitializer、ContextIdApplicationContextInitializer
// ConfigurationWarningsApplicationContextInitializer、RSocketPortInfoApplicationContextInitializer
// ServerPortInfoApplicationContextInitializer、ConditionEvaluationReportLoggingListener
protected void applyInitializers(ConfigurableApplicationContext context) {
    for (ApplicationContextInitializer initializer : getInitializers()) {
        Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(initializer.getClass(),
                ApplicationContextInitializer.class);
        Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
        initializer.initialize(context);
    }
}

2、contextPrepared

// 来自类 SpringApplicationRunListeners
// 发布WebApplication就绪事件
void contextPrepared(ConfigurableApplicationContext context) {
    for (SpringApplicationRunListener listener : this.listeners) {
        listener.contextPrepared(context);
    }
}

// 来自类 EventPublishingRunListener
@Override
public void contextPrepared(ConfigurableApplicationContext context) {
    this.initialMulticaster
            .multicastEvent(new ApplicationContextInitializedEvent(this.application, this.args, context));
}	

3、contextLoaded

// 来自类 SpringApplicationRunListeners
// 发布WebApplication加载完毕事件
void contextLoaded(ConfigurableApplicationContext context) {
    for (SpringApplicationRunListener listener : this.listeners) {
        listener.contextLoaded(context);
    }
}

// 来自类 EventPublishingRunListener
@Override
public void contextLoaded(ConfigurableApplicationContext context) {
    for (ApplicationListener<?> listener : this.application.getListeners()) {
        if (listener instanceof ApplicationContextAware) {
            ((ApplicationContextAware) listener).setApplicationContext(context);
        }
        context.addApplicationListener(listener);
    }
    this.initialMulticaster.multicastEvent(new ApplicationPreparedEvent(this.application, this.args, context));
}	

3、refreshContext

// 来自类 SpringApplication
private void refreshContext(ConfigurableApplicationContext context) {
    refresh(context);
    if (this.registerShutdownHook) {
        try {
            context.registerShutdownHook();
        }
        catch (AccessControlException ex) {
            // Not allowed in some environments.
        }
    }
}

protected void refresh(ApplicationContext applicationContext) {
    Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
    // 触发上下文的刷新动作
    // 这里不做过多展开，具体请参见前文
    ((AbstractApplicationContext) applicationContext).refresh();
}

public Collection<ApplicationListener<?>> getApplicationListeners() {
    return this.applicationListeners;
}

// 来自类 AbstractApplicationContext
@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory.
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);

        try {
            // Allows post-processing of the bean factory in context subclasses.
            postProcessBeanFactory(beanFactory);

            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);

            // Register bean processors that intercept bean creation.
            registerBeanPostProcessors(beanFactory);

            // Initialize message source for this context.
            initMessageSource();

            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            onRefresh();

            // Check for listener beans and register them.
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event.
            // 发布 ContextRefreshedEvent、ServletWebServerInitializedEvent 事件
            // 会有一次 super.finishRefresh
            finishRefresh();
        }

        catch (BeansException ex) {
            if (logger.isWarnEnabled()) {
                logger.warn("Exception encountered during context initialization - " +
                        "cancelling refresh attempt: " + ex);
            }

            // Destroy already created singletons to avoid dangling resources.
            destroyBeans();

            // Reset 'active' flag.
            cancelRefresh(ex);

            // Propagate exception to caller.
            throw ex;
        }

        finally {
            // Reset common introspection caches in Spring's core, since we
            // might not ever need metadata for singleton beans anymore...
            resetCommonCaches();
        }
    }
}

刷新完后的 AnnotationConfigServletWebServerApplicationContext 将完全具有 BeanFactory 和 ApplicationContext 的功能，剩下的就是一些收尾工作。

在完成刷新之后，会发布 ContextRefreshedEvent 、ServletWebServerInitializedEvent 事件。

4、callRunners

我们可以在Spring完全启动后，通过实现 ApplicationRunner 、CommandLineRunner 接口做一些事情。

private void callRunners(ApplicationContext context, ApplicationArguments args) {
    List<Object> runners = new ArrayList<>();
    runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
    runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
    AnnotationAwareOrderComparator.sort(runners);
    for (Object runner : new LinkedHashSet<>(runners)) {
        if (runner instanceof ApplicationRunner) {
            callRunner((ApplicationRunner) runner, args);
        }
        if (runner instanceof CommandLineRunner) {
            callRunner((CommandLineRunner) runner, args);
        }
    }
}

再最后就是发布 ApplicationReadyEvent，代表Spring已经完全启动了。

至此，Spring Boot相关源码已梳理完毕，下篇将着重分析Spring是如何处理一个请求的。