深入讲解spring boot中servlet的启动过程与原理

前言

本文主要介绍了关于spring boot中servlet启动过程与原理的相关内容，下面话不多说了，来一起看看详细的介绍吧

启动过程与原理：

1 spring boot 应用启动运行run方法

StopWatch stopWatch = new StopWatch();
 stopWatch.start();
 ConfigurableApplicationContext context = null;
 FailureAnalyzers analyzers = null;
 configureHeadlessProperty();
 SpringApplicationRunListeners listeners = getRunListeners(args);
 listeners.starting();
 try {
  ApplicationArguments applicationArguments = new DefaultApplicationArguments(
   args);
  ConfigurableEnvironment environment = prepareEnvironment(listeners,
   applicationArguments);
  Banner printedBanner = printBanner(environment);
   //创建一个ApplicationContext容器
  context = createApplicationContext();
  analyzers = new FailureAnalyzers(context);
  prepareContext(context, environment, listeners, applicationArguments,
   printedBanner);
   //刷新IOC容器
  refreshContext(context);
  afterRefresh(context, applicationArguments);
  listeners.finished(context, null);
  stopWatch.stop();
  if (this.logStartupInfo) {
  new StartupInfoLogger(this.mainApplicationClass)
   .logStarted(getApplicationLog(), stopWatch);
  }
  return context;
 }
 catch (Throwable ex) {
  handleRunFailure(context, listeners, analyzers, ex);
  throw new IllegalStateException(ex);
 }

2  createApplicationContext():创建IOC容器,如果是web应用则创建AnnotationConfigEmbeddedWebApplacation的IOC容器,如果不是,则创建AnnotationConfigApplication的IOC容器

public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
  + "annotation.AnnotationConfigApplicationContext";

 /**
 * The class name of application context that will be used by default for web
 * environments.
 */
 public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework."
  + "boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext";

protected ConfigurableApplicationContext createApplicationContext() {
 Class<?> contextClass = this.applicationContextClass;
 if (contextClass == null) {
  try {
　　　　　　　　　　//根据应用环境,创建不同的IOC容器
  contextClass = Class.forName(this.webEnvironment
   ? DEFAULT_WEB_CONTEXT_CLASS : DEFAULT_CONTEXT_CLASS);
  }
  catch (ClassNotFoundException ex) {
  throw new IllegalStateException(
   "Unable create a default ApplicationContext, "
    + "please specify an ApplicationContextClass",
   ex);
  }
 }
 return (ConfigurableApplicationContext) BeanUtils.instantiate(contextClass);
 }

3    refreshContext(context) spring boot刷新IOC容器(创建容器对象,并初始化容器,创建容器每一个组件)

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

4 refresh(context);刷新刚才创建的IOC容器

protected void refresh(ApplicationContext applicationContext) {
 Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
 ((AbstractApplicationContext) applicationContext).refresh();
 }

5 调用父类的refresh()的方法

public void refresh() throws BeansException, IllegalStateException {
 Object var1 = this.startupShutdownMonitor;
 synchronized(this.startupShutdownMonitor) {
  this.prepareRefresh();
  ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
  this.prepareBeanFactory(beanFactory);

  try {
  this.postProcessBeanFactory(beanFactory);
  this.invokeBeanFactoryPostProcessors(beanFactory);
  this.registerBeanPostProcessors(beanFactory);
  this.initMessageSource();
  this.initApplicationEventMulticaster();
  this.onRefresh();
  this.registerListeners();
  this.finishBeanFactoryInitialization(beanFactory);
  this.finishRefresh();
  } catch (BeansException var9) {
  if (this.logger.isWarnEnabled()) {
   this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
  }

  this.destroyBeans();
  this.cancelRefresh(var9);
  throw var9;
  } finally {
  this.resetCommonCaches();
  }

 }
 }

6  抽象父类AbstractApplicationContext类的子类EmbeddedWebApplicationContext的onRefresh方法

@Override
 protected void onRefresh() {
 super.onRefresh();
 try {
  createEmbeddedServletContainer();
 }
 catch (Throwable ex) {
  throw new ApplicationContextException("Unable to start embedded container",
   ex);
 }
 }

7  在createEmbeddedServletContainer放啊发中会获取嵌入式Servlet容器工厂,由容器工厂创建Servlet

private void createEmbeddedServletContainer() {
 EmbeddedServletContainer localContainer = this.embeddedServletContainer;
 ServletContext localServletContext = getServletContext();
 if (localContainer == null && localServletContext == null) {
　　　　　　　　　　　　　　　　//获取嵌入式Servlet容器工厂
  EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory();
　　　　　　　　　　//根据容器工厂获取对应嵌入式Servlet容器
  this.embeddedServletContainer = containerFactory
   .getEmbeddedServletContainer(getSelfInitializer());
 }
 else if (localServletContext != null) {
  try {
  getSelfInitializer().onStartup(localServletContext);
  }
  catch (ServletException ex) {
  throw new ApplicationContextException("Cannot initialize servlet context",
   ex);
  }
 }
 initPropertySources();
 }

8  从IOC容器中获取Servlet容器工厂

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

9  使用Servlet容器工厂获取嵌入式Servlet容器,具体使用哪一个容器工厂看配置环境依赖

this.embeddedServletContainer = containerFactory
  .getEmbeddedServletContainer(getSelfInitializer()); 

10  上述创建过程  首先启动IOC容器,接着启动嵌入式Servlet容器,接着将IOC容器中剩下没有创建的对象获取出来,比如自己创建的controller

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

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
 // Initialize conversion service for this context.
 if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
  beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
  beanFactory.setConversionService(
   beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
 }

 // Register a default embedded value resolver if no bean post-processor
 // (such as a PropertyPlaceholderConfigurer bean) registered any before:
 // at this point, primarily for resolution in annotation attribute values.
 if (!beanFactory.hasEmbeddedValueResolver()) {
  beanFactory.addEmbeddedValueResolver(new StringValueResolver() {
  @Override
  public String resolveStringValue(String strVal) {
   return getEnvironment().resolvePlaceholders(strVal);
  }
  });
 }

 // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
 String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
 for (String weaverAwareName : weaverAwareNames) {
  getBean(weaverAwareName);
 }

 // Stop using the temporary ClassLoader for type matching.
 beanFactory.setTempClassLoader(null);

 // Allow for caching all bean definition metadata, not expecting further changes.
 beanFactory.freezeConfiguration();

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

看看 preInstantiateSingletons方法

public void preInstantiateSingletons() throws BeansException {
  if (this.logger.isDebugEnabled()) {
   this.logger.debug("Pre-instantiating singletons in " + this);
  }

  List<String> beanNames = new ArrayList(this.beanDefinitionNames);
  Iterator var2 = beanNames.iterator();

  while(true) {
   while(true) {
    String beanName;
    RootBeanDefinition bd;
    do {
     do {
      do {
       if (!var2.hasNext()) {
        var2 = beanNames.iterator();

        while(var2.hasNext()) {
         beanName = (String)var2.next();
         Object singletonInstance = this.getSingleton(beanName);
         if (singletonInstance instanceof SmartInitializingSingleton) {
          final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton)singletonInstance;
          if (System.getSecurityManager() != null) {
           AccessController.doPrivileged(new PrivilegedAction<Object>() {
            public Object run() {
             smartSingleton.afterSingletonsInstantiated();
             return null;
            }
           }, this.getAccessControlContext());
          } else {
           smartSingleton.afterSingletonsInstantiated();
          }
         }
        }

        return;
       }

       beanName = (String)var2.next();
       bd = this.getMergedLocalBeanDefinition(beanName);
      } while(bd.isAbstract());
     } while(!bd.isSingleton());
    } while(bd.isLazyInit());

    if (this.isFactoryBean(beanName)) {
     final FactoryBean<?> factory = (FactoryBean)this.getBean("&" + beanName);
     boolean isEagerInit;
     if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
      isEagerInit = ((Boolean)AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
       public Boolean run() {
        return ((SmartFactoryBean)factory).isEagerInit();
       }
      }, this.getAccessControlContext())).booleanValue();
     } else {
      isEagerInit = factory instanceof SmartFactoryBean && ((SmartFactoryBean)factory).isEagerInit();
     }

     if (isEagerInit) {
      this.getBean(beanName);
     }
    } else {
　　　　　　　　　　　　//注册bean
     this.getBean(beanName);
    }
   }
  }
 }

是使用getBean方法来通过反射将所有未创建的实例创建出来

使用嵌入式Servlet容器:

　　　　优点:   简单,便携

　　　　 缺点:   默认不支持jsp,优化定制比较复杂

使用外置Servlet容器的步骤:

　　1  必须创建war项目,需要剑豪web项目的目录结构

　　2  嵌入式Tomcat依赖scope指定provided

　　3  编写SpringBootServletInitializer类子类,并重写configure方法

public class ServletInitializer extends SpringBootServletInitializer { 

 @Override
 protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
  return application.sources(SpringBoot04WebJspApplication.class);
 }
}

4  启动服务器

jar包和war包启动区别

jar包:执行SpringBootApplication的run方法,启动IOC容器,然后创建嵌入式Servlet容器

　war包:  先是启动Servlet服务器,服务器启动Springboot应用(springBootServletInitizer),然后启动IOC容器

Servlet 3.0+规则

　　  1  服务器启动(web应用启动),会创建当前web应用里面所有jar包里面的ServletContainerlnitializer实例

　　   2 ServletContainerInitializer的实现放在jar包的META-INF/services文件夹下

　　　3  还可以使用@HandlesTypes注解，在应用启动的时候加载指定的类。

外部Tomcat流程以及原理

　　①  启动Tomcat

　　②  根据上述描述的Servlet3.0+规则，可以在Spring的web模块里面找到有个文件名为javax.servlet.ServletContainerInitializer的文件，而文件的内容为org.springframework.web.SpringServletContainerInitializer，用于加载SpringServletContainerInitializer类

　　③看看SpringServletContainerInitializer定义

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {

 /**
  * Delegate the {@code ServletContext} to any {@link WebApplicationInitializer}
  * implementations present on the application classpath.
  * <p>Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)},
  * Servlet 3.0+ containers will automatically scan the classpath for implementations
  * of Spring's {@code WebApplicationInitializer} interface and provide the set of all
  * such types to the {@code webAppInitializerClasses} parameter of this method.
  * <p>If no {@code WebApplicationInitializer} implementations are found on the classpath,
  * this method is effectively a no-op. An INFO-level log message will be issued notifying
  * the user that the {@code ServletContainerInitializer} has indeed been invoked but that
  * no {@code WebApplicationInitializer} implementations were found.
  * <p>Assuming that one or more {@code WebApplicationInitializer} types are detected,
  * they will be instantiated (and <em>sorted</em> if the @{@link
  * org.springframework.core.annotation.Order @Order} annotation is present or
  * the {@link org.springframework.core.Ordered Ordered} interface has been
  * implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)}
  * method will be invoked on each instance, delegating the {@code ServletContext} such
  * that each instance may register and configure servlets such as Spring's
  * {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener},
  * or any other Servlet API componentry such as filters.
  * @param webAppInitializerClasses all implementations of
  * {@link WebApplicationInitializer} found on the application classpath
  * @param servletContext the servlet context to be initialized
  * @see WebApplicationInitializer#onStartup(ServletContext)
  * @see AnnotationAwareOrderComparator
  */
 @Override
 public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
   throws ServletException {

  List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();

  if (webAppInitializerClasses != null) {
   for (Class<?> waiClass : webAppInitializerClasses) {
    // Be defensive: Some servlet containers provide us with invalid classes,
    // no matter what @HandlesTypes says...
    if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
      WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
     try {
　　　　　　　　　　　　　　　　//为所有的WebApplicationInitializer类型创建实例,并加入集合中
      initializers.add((WebApplicationInitializer) waiClass.newInstance());
     }
     catch (Throwable ex) {
      throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
     }
    }
   }
  }

  if (initializers.isEmpty()) {
   servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
   return;
  }

  servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
  AnnotationAwareOrderComparator.sort(initializers);
　　　　　　//调用每一个WebApplicationInitializer实例的onstartup方法
  for (WebApplicationInitializer initializer : initializers) {
   initializer.onStartup(servletContext);
  }
 }
}

在上面一段长长的注释中可以看到，SpringServletContainerInitializer将@HandlesTypes(WebApplicationInitializer.class)标注的所有WebApplicationInitializer这个类型的类都传入到onStartup方法的Set参数中，并通过反射为这些WebApplicationInitializer类型的类创建实例；

　　④  方法最后,每一个WebApplicationInitilizer实现调用自己onstartup方法

　　⑤  而WebApplicationInitializer有个抽象实现类SpringBootServletInitializer（记住我们继承了该抽象类），则会调用每一个WebApplicationInitializer实例（包括SpringBootServletInitializer）的onStartup方法：

public abstract class SpringBootServletInitializer implements WebApplicationInitializer { 

  //other code... 

  @Override
  public void onStartup(ServletContext servletContext) throws ServletException {
    // Logger initialization is deferred in case a ordered
    // LogServletContextInitializer is being used
    this.logger = LogFactory.getLog(getClass());
    //创建IOC容器
    WebApplicationContext rootAppContext = createRootApplicationContext(
        servletContext);
    if (rootAppContext != null) {
      servletContext.addListener(new ContextLoaderListener(rootAppContext) {
        @Override
        public void contextInitialized(ServletContextEvent event) {
          // no-op because the application context is already initialized
        }
      });
    }
    else {
      this.logger.debug("No ContextLoaderListener registered, as "
          + "createRootApplicationContext() did not "
          + "return an application context");
    }
  } 

  protected WebApplicationContext createRootApplicationContext(
      ServletContext servletContext) {
    //创建Spring应用构建器，并进行相关属性设置
    SpringApplicationBuilder builder = createSpringApplicationBuilder();
    StandardServletEnvironment environment = new StandardServletEnvironment();
    environment.initPropertySources(servletContext, null);
    builder.environment(environment);
    builder.main(getClass());
    ApplicationContext parent = getExistingRootWebApplicationContext(servletContext);
    if (parent != null) {
      this.logger.info("Root context already created (using as parent).");
      servletContext.setAttribute(
          WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, null);
      builder.initializers(new ParentContextApplicationContextInitializer(parent));
    }
    builder.initializers(
        new ServletContextApplicationContextInitializer(servletContext));
    builder.contextClass(AnnotationConfigEmbeddedWebApplicationContext.class); 

    //调用configure方法，创建war类型的web项目后，由于编写SpringBootServletInitializer的子类重写configure方法，所以此处调用的是我们定义的子类重写的configure方法
    builder = configure(builder); 

    //通过构建器构建了一个Spring应用
    SpringApplication application = builder.build();
    if (application.getSources().isEmpty() && AnnotationUtils
        .findAnnotation(getClass(), Configuration.class) != null) {
      application.getSources().add(getClass());
    }
    Assert.state(!application.getSources().isEmpty(),
        "No SpringApplication sources have been defined. Either override the "
            + "configure method or add an @Configuration annotation");
    // Ensure error pages are registered
    if (this.registerErrorPageFilter) {
      application.getSources().add(ErrorPageFilterConfiguration.class);
    }
    //启动Spring应用
    return run(application);
  } 

  //Spring应用启动，创建并返回IOC容器
  protected WebApplicationContext run(SpringApplication application) {
    return (WebApplicationContext) application.run();
  }
}

SpringBootServletInitializer实例执行onStartup方法的时候会通过createRootApplicationContext方法来执行run方法，接下来的过程就同以jar包形式启动的应用的run过程一样了，在内部会创建IOC容器并返回，只是以war包形式的应用在创建IOC容器过程中，不再创建Servlet容器了。

总结

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对我们的支持。