SpringBoot 使用外部Tomcat方法及啟動原理

基於 SpringBoot 2.x

方法

• 一、必須是一個war專案，利用IDEA可以直接建立，或者是修改pom.xml檔案

<packaging>war</packaging>

• 二、將內建Tomcat的作用範圍修改成provided

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

• 三、自定義一個類繼承 SpringBootServletInitializer 重寫其configure()方法

public class ServletInitializer extends SpringBootServletInitializer {

    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        // 傳入SpringBoot的主程式類
        return builder.sources(SbdemoApplication.class);
    }
}

啟動原理

前提

• 根據Servlet3.0規範，伺服器啟動（web應用啟動）會建立當前web應用裡面每一個jar包裡面ServletContainerInitializer例項。
• ServletContainerInitializer的實現放在jar包的META-INF/services資料夾下，有一個名為javax.servlet.ServletContainerInitializer的檔案，內容就是ServletContainerInitializer的實現類的全類名。
• 使用@HandlesTypes，在應用啟動的時候載入我們感興趣的類。

SpringBoot中啟動流程

• 一、SpringBoot 中的 ServletContainerInitializer 實現類位置在spring-web模組下
  
  檔案內容：

org.springframework.web.SpringServletContainerInitializer

• 二、SpringServletContainerInitializer類

@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 {
                        // 將@HandlesTypes(WebApplicationInitializer.class)標註的所有這個型別的類都傳入到onStartup方法的Set<Class<?>>；為這些WebApplicationInitializer型別的類建立例項。
                        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();
                // 每一個WebApplicationInitializer都呼叫自己的onStartup()
                initializer.onStartup(servletContext);
            }

        }
    }
}

• 三、Tomcat 啟動引導類

// ServletInitializer 繼承 SpringBootServletInitializer
public class ServletInitializer extends SpringBootServletInitializer {

    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder.sources(SbdemoApplication.class);
    }
}

SpringBootServletInitializer 繼承 WebApplicationInitializer 類，就是SpringServletContainerInitializer類的@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());
        // 建立web應用容器
        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、建立SpringApplicationBuilder
        SpringApplicationBuilder builder = this.createSpringApplicationBuilder();
        StandardServletEnvironment environment = new StandardServletEnvironment();
        // 2、準備環境
        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、呼叫configure方法，子類重寫了這個方法，將SpringBoot的主程式類傳入了進來
        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、啟動Spring應用
        return this.run(application);
    }
}

// 此方法被啟動引導類 ServletInitializer有方法重寫， 傳入的是應用構建器SpringApplicationBuilder， 也就是SpringBoot的主程式類
protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder;
}

個人隨筆，如有錯誤，歡迎指正！