SpringBoot 使用外部Tomcat方法及启动原理

雨点打透心脏的1/2处 2022-05-24 05:22 194阅读 0赞

# SpringBoot 使用外部Tomcat方法及启动原理 #

> 基于 SpringBoot 2.x

## 方法 ##

*  一、必须是一个war项目，利用IDEA可以直接创建，或者是修改**pom.xml**文件

*  二、将内置Tomcat的作用范围修改成provided

dependency>
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-tomcat</artifactId>
       <scope>provided</scope>
    </dependency>

*  三、自定义一个类继承 SpringBootServletInitializer 重写其configure()方法

public class ServletInitializer extends SpringBootServletInitializer { 
    
        @Override
        protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
            // 传入SpringBoot的主程序类
            return builder.sources(SbdemoApplication.class);
        }
    }

## 启动原理 ##

**前提**

>  *  根据Servlet3.0规范，服务器启动（web应用启动）会创建当前web应用里面每一个jar包里面ServletContainerInitializer实例。
>  *  ServletContainerInitializer的实现放在jar包的META-INF/services文件夹下，有一个名为javax.servlet.ServletContainerInitializer的文件，内容就是ServletContainerInitializer的实现类的全类名。
>  *  使用@HandlesTypes，在应用启动的时候加载我们感兴趣的类。

**SpringBoot中启动流程**

*  一、SpringBoot 中的 ServletContainerInitializer 实现类位置在spring-web模块下  
    ![这里写图片描述][70]  
    文件内容：

org.springframework.web.SpringServletContainerInitializer

*  二、SpringServletContainerInitializer类

@HandlesTypes({WebApplicationInitializer.class})
    public class SpringServletContainerInitializer implements ServletContainerInitializer { 
        public SpringServletContainerInitializer() {
        }
    
        public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException {
            List<WebApplicationInitializer> initializers = new LinkedList();
            Iterator var4;
            if (webAppInitializerClasses != null) {
                var4 = webAppInitializerClasses.iterator();
    
                while(var4.hasNext()) {
                    Class<?> waiClass = (Class)var4.next();
                    if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
                        try {
                            // 将@HandlesTypes(WebApplicationInitializer.class)标注的所有这个类型的类都传入到onStartup方法的Set<Class<?>>；为这些WebApplicationInitializer类型的类创建实例。
                            initializers.add((WebApplicationInitializer)ReflectionUtils.accessibleConstructor(waiClass, new Class[0]).newInstance());
                        } catch (Throwable var7) {
                            throw new ServletException("Failed to instantiate WebApplicationInitializer class", var7);
                        }
                    }
                }
            }
    
            if (initializers.isEmpty()) {
                servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
            } else {
                servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
                AnnotationAwareOrderComparator.sort(initializers);
                var4 = initializers.iterator();
    
                while(var4.hasNext()) {
                    WebApplicationInitializer initializer = (WebApplicationInitializer)var4.next();
                    // 每一个WebApplicationInitializer都调用自己的onStartup()
                    initializer.onStartup(servletContext);
                }
    
            }
        }
    }

*  三、Tomcat 启动引导类

// ServletInitializer 继承 SpringBootServletInitializer
    public class ServletInitializer extends SpringBootServletInitializer { 
    
        @Override
        protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
            return builder.sources(SbdemoApplication.class);
        }
    }

SpringBootServletInitializer 继承 WebApplicationInitializer 类，就是***SpringServletContainerInitializer***类的***@HandlesTypes(\{WebApplicationInitializer.class\})***， 下面展示的是 onStartup() 、createRootApplicationContext() 和 configure() 方法

public abstract class SpringBootServletInitializer implements WebApplicationInitializer { 
        public void onStartup(ServletContext servletContext) throws ServletException {
            this.logger = LogFactory.getLog(this.getClass());
            // 创建web应用容器
            WebApplicationContext rootAppContext = this.createRootApplicationContext(servletContext);
            if (rootAppContext != null) {
                servletContext.addListener(new ContextLoaderListener(rootAppContext) {
                    public void contextInitialized(ServletContextEvent event) {
                    }
                });
            } else {
                this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not return an application context");
            }
    
        }
    
        protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
            // 1、创建SpringApplicationBuilder
            SpringApplicationBuilder builder = this.createSpringApplicationBuilder();
            StandardServletEnvironment environment = new StandardServletEnvironment();
            // 2、准备环境
            environment.initPropertySources(servletContext, (ServletConfig)null);
            builder.environment(environment);
            builder.main(this.getClass());
            ApplicationContext parent = this.getExistingRootWebApplicationContext(servletContext);
            if (parent != null) {
                this.logger.info("Root context already created (using as parent).");
                servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, (Object)null);
                builder.initializers(new ApplicationContextInitializer[]{
       new ParentContextApplicationContextInitializer(parent)});
            }
            // 3、初始化
            builder.initializers(new ApplicationContextInitializer[]{
       new ServletContextApplicationContextInitializer(servletContext)});
            builder.contextClass(AnnotationConfigServletWebServerApplicationContext.class);
            // 4、调用configure方法，子类重写了这个方法，将SpringBoot的主程序类传入了进来
            builder = this.configure(builder);
    
            SpringApplication application = builder.build();
            if (application.getAllSources().isEmpty() && AnnotationUtils.findAnnotation(this.getClass(), Configuration.class) != null) {
                application.addPrimarySources(Collections.singleton(this.getClass()));
            }
    
            Assert.state(!application.getAllSources().isEmpty(), "No SpringApplication sources have been defined. Either override the configure method or add an @Configuration annotation");
            if (this.registerErrorPageFilter) {
                application.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class));
            }
            // 5、启动Spring应用
            return this.run(application);
        }
    }
    
    // 此方法被启动引导类 ServletInitializer有方法重写， 传入的是应用构建器SpringApplicationBuilder， 也就是SpringBoot的主程序类
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
            return builder;
    }

> 个人随笔，如有错误，欢迎指正！

[70]: /images/20220524/838f4c583d6c4bc5bcf619eedd9d2431.png