【SpringBoot】原理分析（四）：外置 Tomcat（war包）启动原理分析

素颜马尾好姑娘i 2022-10-23 15:26 201阅读 0赞

> 前言：
> 
>  *  关于 SpringBoot 如何打 war 包，请参考 [【SpringBoot】基本使用（一）：两种构建方式、基本配置、两种打包方式][SpringBoot]
>  *  关于内嵌 Tocmat 启动流程，请参考 [【SpringBoot】原理分析（三）：内嵌 Tomcat（jar包）启动原理分析][SpringBoot_ Tomcat_jar]

从疑问开始：我们把 SpringBoot 打成war的包时候，为什么要在启动类程序上实现 SpringBootServletInitializer 接口? 以及 configure 方法是在什么时候触发调用的?

@SpringBootApplication 
    public class DemoApplication extends SpringBootServletInitializer {
         
         public static void main(String[] args) {
           
             SpringApplication.run(DemoApplication.class, args);
         } 
        
     	@Override  
        protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
          
            return builder.sources(DemoApplication.class); 
        } 
    }

### servlet3.0 特性 ###

重点看 8.2.4 Shared libraries / runtimes pluggability 章节

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center]

图中重点段落的意思是，web应用启动会创建当前Web应用导入jar包中的 ServletContainerInitializer 类的实例 。ServletContainerInitializer 类必须放在 jar 包的 META-INF/services 目录下。文件名称为 javax.servlet.ServletContainerInitializer，文件的内容是 ServletContainerInitializer 实现类的全路径。

下面这一段也很关键

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center 1]

大致意思是，我们可以给 ServletContainerInitializer 的实现类添加 @HandlesTypes 注解，在其 onStartup 方法上便可以得到我们感兴趣的类。

> 总结：容器在启动应用的时候，会扫描当前应用每一个jar包里面 META-INF/services/javax.servlet.ServletContainerInitializer 指定的实现类，启动并运行这个实现类的方法；传入感兴趣的类型；

我们先来说一下 SpringBoot 中 war 包启动的主要流程：

1.  第一步：tomcat 启动，然后去 `org/springframework/spring-web/5.3.1/spring-web-5.3.1.jar!/META-INF/services/javax.servlet.ServletContainerInitializer` 文件中寻找 ServletContainerInitializer 实现对象
2.  第二步：将 @HandlesTypes 标注的感兴趣的类（WebApplicationInitializer）传入到 onStartup() 方法的 Set<Class<?>> 参数中，为这些感兴趣的类创建实例，然后调用实例（WebApplicationInitializer ）的 onStartup 方法

下面我们就来看一下 ServletContainerInitializer 和 WebApplicationInitializer 到底是什么情况

### ServletContainerInitializer ###

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center 2]  
可以看到 ServletContainerInitializer 的实现类是 org.springframework.web.SpringServletContainerInitializer

// 感兴趣的类为 WebApplicationInitializer
    @HandlesTypes(WebApplicationInitializer.class)
    // SpringServletContainerInitializer 实现了 ServletContainerInitializer 接口
    public class SpringServletContainerInitializer implements ServletContainerInitializer {
        
        @Override
    	public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
    			throws ServletException {
        
    		
            // 创建保存感兴趣的类的集合 
    		List<WebApplicationInitializer> initializers = Collections.emptyList();
    
    		if (webAppInitializerClasses != null) {
        
    			initializers = new ArrayList<>(webAppInitializerClasses.size());
    			for (Class<?> waiClass : webAppInitializerClasses) {
        
    				// Be defensive: Some servlet containers provide us with invalid classes,
    				// no matter what @HandlesTypes says...
                     // 如果该 lass 不是接口，不是抽象类，并且是WebApplicationInitializer类型的类
    				if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
    						WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
        
    					try {
        
                              // 通过反射创建实例并且加入到集合中 
    						initializers.add((WebApplicationInitializer)
    								ReflectionUtils.accessibleConstructor(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);
             // 循环调用集合中感兴趣类对象的onstartup的方法 
    		for (WebApplicationInitializer initializer : initializers) {
        
    			initializer.onStartup(servletContext);
    		}
    	}
    }

### WebApplicationInitializer ###

可以看到 WebApplicationInitializer 的实现类就有我们的 DemoApplication 类

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center 3]  
由于 DemoApplication 这个类没有自己的 onstart 方法，那么就调用父类 SpringBootServletInitializer 的

@Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        
       // Logger initialization is deferred in case an ordered
       // LogServletContextInitializer is being used
       this.logger = LogFactory.getLog(getClass());
       // 创建web应用的上下文对象 
       WebApplicationContext rootApplicationContext = createRootApplicationContext(servletContext);
       if (rootApplicationContext != null) {
        
           // 给容器添加监听器
          servletContext.addListener(new SpringBootContextLoaderListener(rootApplicationContext, servletContext));
       }
       else {
        
          this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not "
                + "return an application context");
       }
    }
    
    // 创建web应用的上下文对象 
    protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
        
       // 创建spring应用的构建器 
       SpringApplicationBuilder builder = createSpringApplicationBuilder();
       builder.main(getClass());
       // 设置环境 
       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.contextFactory((webApplicationType) -> new AnnotationConfigServletWebServerApplicationContext());
       // 调用我们自己启动类上的confiure方法 传入我们自己的主启动类 
       builder = configure(builder);
       builder.listeners(new WebEnvironmentPropertySourceInitializer(servletContext));
       SpringApplication application = builder.build();
       if (application.getAllSources().isEmpty()
             && MergedAnnotations.from(getClass(), SearchStrategy.TYPE_HIERARCHY).isPresent(Configuration.class)) {
        
          application.addPrimarySources(Collections.singleton(getClass()));
       }
       Assert.state(!application.getAllSources().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.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class));
       }
       application.setRegisterShutdownHook(false);
       // 调用我们类上的run方法  
       return run(application);
    }

这就回到了我们在上一篇内嵌Tomcat启动说过的 refreshContext 方法，只不过此时我们已经有了 web 应用上下文，所有后面就不会再再去创建启动内置 Tomcat。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center 4]  
总结一下，war 包启动流程图如下：

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzkzNTkyNw_size_16_color_FFFFFF_t_70_pic_center 5]

[SpringBoot]: https://blog.csdn.net/weixin_43935927/article/details/110750069
[SpringBoot_ Tomcat_jar]: https://yzhyaa.blog.csdn.net/article/details/111062009
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