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

  war

• 二、将内置Tomcat的作用范围修改成provided

  dependency>
  org.springframework.boot
  spring-boot-starter-tomcat
  provided
  

• 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
  
  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！