SpringBoot启动流程详细分析

1. SpringBoot原理

• SpringBoot run 会创建一个ioc容器
• ioc容器启动onRefresh会启动Tomcat
• Tomcat启动会加载所有的Servlet
• DispatcherServlet会加载九大组件的整个初始化流程

SpringBoot原理.jpg

2. 源码开始

@SpringBootApplication
public class Demo1Application {

    public static void main(String[] args) {
        SpringApplication.run(Demo1Application.class, args);
    }

}

现在我们开始两个方面的解读

• @SpringBootApplication
• SpringApplication.run(Demo1Application.class, args);

2.1 @SpringBootApplication

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
        // 指定不生效的自动化配置类
   @AliasFor(annotation = EnableAutoConfiguration.class)
   Class<?>[] exclude() default {};
    // 指定不生效的配置类的类名
   @AliasFor(annotation = EnableAutoConfiguration.class)
   String[] excludeName() default {};
    // @ComponentScan注解扫描的基包名
   @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
   String[] scanBasePackages() default {};
    // @ComponentScan扫描的基本包的类名
   @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
   Class<?>[] scanBasePackageClasses() default {};
    // 指定bean的名字生成器
   @AliasFor(annotation = ComponentScan.class, attribute = "nameGenerator")
   Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;
    // 
   @AliasFor(annotation = Configuration.class)
   boolean proxyBeanMethods() default true;

}

这边需要说明的注解是

• @SpringBootConfiguration
• @EnableAutoConfiguration

2.1.1 @SpringBootConfiguration

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
@Indexed
public @interface SpringBootConfiguration {
    @AliasFor(
        annotation = Configuration.class
    )
    boolean proxyBeanMethods() default true;
}

其实就是@Configuration

2.1.2 @EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

   /**
    * Environment property that can be used to override when auto-configuration is
    * enabled.
    */
   String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

   /**
    * Exclude specific auto-configuration classes such that they will never be applied.
    * @return the classes to exclude 
    */
  // 指定不生效的自动化配置类
   Class<?>[] exclude() default {};

   /**
    * Exclude specific auto-configuration class names such that they will never be
    * applied.
    * @return the class names to exclude
    * @since 1.3.0
    */
        // 指定不生效的配置类的类名
   String[] excludeName() default {};

}

这边实现自动配置的核心。

• @AutoConfigurationPackage
• @Import(AutoConfigurationImportSelector.class)

2.1.2.1 @AutoConfigurationPackage

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

   /**
    * Base packages that should be registered with {@link AutoConfigurationPackages}.
    * <p>
    * Use {@link #basePackageClasses} for a type-safe alternative to String-based package
    * names.
    * @return the back package names
    * @since 2.3.0
    */
   String[] basePackages() default {};

   /**
    * Type-safe alternative to {@link #basePackages} for specifying the packages to be
    * registered with {@link AutoConfigurationPackages}.
    * <p>
    * Consider creating a special no-op marker class or interface in each package that
    * serves no purpose other than being referenced by this attribute.
    * @return the base package classes
    * @since 2.3.0
    */
   Class<?>[] basePackageClasses() default {};

}

这里很重要的是@Import(AutoConfigurationPackages.Registrar.class)

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

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

   @Override
   public Set<Object> determineImports(AnnotationMetadata metadata) {
      return Collections.singleton(new PackageImports(metadata));
   }

}

实现方法

public static void register(BeanDefinitionRegistry registry, String... packageNames) {
   if (registry.containsBeanDefinition(BEAN)) {
      BasePackagesBeanDefinition beanDefinition = (BasePackagesBeanDefinition) registry.getBeanDefinition(BEAN);
      beanDefinition.addBasePackages(packageNames);
   }
   else {
      registry.registerBeanDefinition(BEAN, new BasePackagesBeanDefinition(packageNames));
   }
}

这边就是向容器中注册一个beanDefinition，定义了包名的列表。

现在我们回到@Import(AutoConfigurationImportSelector.class)

2.1.2.2 @Import(AutoConfigurationImportSelector.class)

@Import注解参数是ImportSelector接口的实现类是，容器启动时会执行接口的selectImports方法，但是实现了DeferredImportSelector接口的方法会做特殊处理，会执行接口的process方法

@Override
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);
   }
}

这里处理逻辑的方法在

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
   if (!isEnabled(annotationMetadata)) {
      return EMPTY_ENTRY;
   }
   AnnotationAttributes attributes = getAttributes(annotationMetadata);
  // 获取所有配置类名
   List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
  //过滤删除
   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);
}

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
   List<String> configurations = new ArrayList<>(
     // 这边其实就去取自动配置的spring-boot-autoconfigure jar/META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports 中的配置类导入到容器中
         SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader()));
   ImportCandidates.load(AutoConfiguration.class, getBeanClassLoader()).forEach(configurations::add);
   Assert.notEmpty(configurations,
         "No auto configuration classes found in META-INF/spring.factories nor in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports. If you "
               + "are using a custom packaging, make sure that file is correct.");
   return configurations;
}

spring-boot-autoconfigure jar/META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports 可以去看看。然后什么时候配置生效其实可以看看。都有条件注解。

到这边准备的定义类都导入到spring容器了。

2.2 SpringApplication.run

现在我们就来启动了

SpringApplication.run(Demo1Application.class, args);

public ConfigurableApplicationContext run(String... args) {
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    DefaultBootstrapContext bootstrapContext = this.createBootstrapContext();
    ConfigurableApplicationContext context = null;
    this.configureHeadlessProperty();
    SpringApplicationRunListeners listeners = this.getRunListeners(args);
    listeners.starting(bootstrapContext, this.mainApplicationClass);

    try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        ConfigurableEnvironment environment = this.prepareEnvironment(listeners, bootstrapContext, applicationArguments);
        this.configureIgnoreBeanInfo(environment);
        Banner printedBanner = this.printBanner(environment);
        context = this.createApplicationContext();// 创建IOC容器
        context.setApplicationStartup(this.applicationStartup);
        this.prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
        this.refreshContext(context);//容器刷新
        this.afterRefresh(context, applicationArguments);
        stopWatch.stop();
        if (this.logStartupInfo) {
            (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
        }

        listeners.started(context);
        this.callRunners(context, applicationArguments);
    } catch (Throwable var10) {
        this.handleRunFailure(context, var10, listeners);
        throw new IllegalStateException(var10);
    }

    try {
        listeners.running(context);
        return context;
    } catch (Throwable var9) {
        this.handleRunFailure(context, var9, (SpringApplicationRunListeners)null);
        throw new IllegalStateException(var9);
    }
}

• 最核心的方法this.refreshContext(context); 这里会去刷新容器。

protected void onRefresh() {
    super.onRefresh();

    try {
        this.createWebServer(); //创建web服务
    } catch (Throwable var2) {
        throw new ApplicationContextException("Unable to start web server", var2);
    }
}

private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = this.getServletContext();
    if (webServer == null && servletContext == null) {
        StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
        ServletWebServerFactory factory = this.getWebServerFactory();
        createWebServer.tag("factory", factory.getClass().toString());
        this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});//这里会去获取tomcat
        createWebServer.end();
        this.getBeanFactory().registerSingleton("webServerGracefulShutdown", new WebServerGracefulShutdownLifecycle(this.webServer));
        this.getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(this, this.webServer));
    } else if (servletContext != null) {
        try {
            this.getSelfInitializer().onStartup(servletContext);
        } catch (ServletException var5) {
            throw new ApplicationContextException("Cannot initialize servlet context", var5);
        }
    }

    this.initPropertySources();
}

@Override
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());
   for (LifecycleListener listener : this.serverLifecycleListeners) {
      tomcat.getServer().addLifecycleListener(listener);
   }
   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 TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
   return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown());
}

public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {
   Assert.notNull(tomcat, "Tomcat Server must not be null");
   this.tomcat = tomcat;
   this.autoStart = autoStart;
   this.gracefulShutdown = (shutdown == Shutdown.GRACEFUL) ? new GracefulShutdown(tomcat) : null;
   initialize();
}

private void initialize() throws WebServerException {
   logger.info("Tomcat initialized with port(s): " + getPortsDescription(false));
   synchronized (this.monitor) {
      try {
         addInstanceIdToEngineName();

         Context context = findContext();
         context.addLifecycleListener((event) -> {
            if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) {
               // Remove service connectors so that protocol binding doesn't
               // happen when the service is started.
               removeServiceConnectors();
            }
         });

         // Start the server to trigger initialization listeners 
         this.tomcat.start();

         // We can re-throw failure exception directly in the main thread
         rethrowDeferredStartupExceptions();

         try {
            ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
         }
         catch (NamingException ex) {
            // Naming is not enabled. Continue
         }

         // Unlike Jetty, all Tomcat threads are daemon threads. We create a
         // blocking non-daemon to stop immediate shutdown
         startDaemonAwaitThread();
      }
      catch (Exception ex) {
         stopSilently();
         destroySilently();
         throw new WebServerException("Unable to start embedded Tomcat", ex);
      }
   }
}

到这里就是启动tomcat服务了，至于tomcat怎么启动的，先挖个坑。