Spring Boot容器另类的初始化过程

• 对于Spring Boot应用来说，它并未使用SpringServletContainerInitializer来进行容器的初始化，而是使用了TomcatStarter进行的。
• TomcatStarter存在三点因素使得它无法通过SPI机制进行初始化
  • 它没有不带参数的构造方法
  • 它的声明并非public
  • 其所在的jar包并没有META-INF/services目录
• 所以TomcatStarter并非通过SPI机制进行的查找与实例化
• 本质上，TomcatStarter是通过Spring Boot框架new出来的
• 与SpringServletContainerInitializer类似，TomcatStarter在容器的初始化过程中也是扮演一个委托或是代理的角色，真正执行的初始化动作实际上是由它所持有的ServletContextInitializer的onStartup方法。

具体代码

1. ServletContextInitializer

   package org.springframework.boot.web.servlet;
   
   import javax.servlet.ServletContext;
   import javax.servlet.ServletException;
   
   import org.springframework.web.SpringServletContainerInitializer;
   import org.springframework.web.WebApplicationInitializer;
   
   /**
   * Interface used to configure a Servlet 3.0+ {@link ServletContext context}
   * programmatically. Unlike {@link WebApplicationInitializer}, classes that implement this
   * interface (and do not implement {@link WebApplicationInitializer}) will <b>not</b> be
   * detected by {@link SpringServletContainerInitializer} and hence will not be
   * automatically bootstrapped by the Servlet container.
   * <p>
   * This interface is primarily designed to allow {@link ServletContextInitializer}s to be
   * managed by Spring and not the Servlet container.
   * <p>
   * For configuration examples see {@link WebApplicationInitializer}.
   *
   * @author Phillip Webb
   * @since 1.4.0
   * @see WebApplicationInitializer
   */
   @FunctionalInterface
   public interface ServletContextInitializer {
   
       /**
       * Configure the given {@link ServletContext} with any servlets, filters, listeners
       * context-params and attributes necessary for initialization.
       * @param servletContext the {@code ServletContext} to initialize
       * @throws ServletException if any call against the given {@code ServletContext}
       * throws a {@code ServletException}
       */
       void onStartup(ServletContext servletContext) throws ServletException;
   
   }
   

2. TomcatStarter

   /**
   * {@link ServletContainerInitializer} used to trigger {@link ServletContextInitializer
   * ServletContextInitializers} and track startup errors.
   *
   * @author Phillip Webb
   * @author Andy Wilkinson
   */
   class TomcatStarter implements ServletContainerInitializer {
   
       private static final Log logger = LogFactory.getLog(TomcatStarter.class);
   
       private final ServletContextInitializer[] initializers;
   
       private volatile Exception startUpException;
   
       TomcatStarter(ServletContextInitializer[] initializers) {
           this.initializers = initializers;
       }
   
       @Override
       public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
           try {
               for (ServletContextInitializer initializer : this.initializers) {
                   initializer.onStartup(servletContext);
               }
           }
           catch (Exception ex) {
               this.startUpException = ex;
               // Prevent Tomcat from logging and re-throwing when we know we can
               // deal with it in the main thread, but log for information here.
               if (logger.isErrorEnabled()) {
                   logger.error("Error starting Tomcat context. Exception: " + ex.getClass().getName() + ". Message: "
                           + ex.getMessage());
               }
           }
       }
   
       Exception getStartUpException() {
           return this.startUpException;
       }
   
   }
   

3. 最终会通过TomcatStarter这个委托者，获取到所有的ServletContextInitializer实例数组，然后在onStartup方法中循环调用onStartup完成初始化的配置。

和传统Spring MVC之间的对应关系

1. SpringServletContainerInitializer对应于TomcatStarter
2. WebApplicationInitializer对应于ServletContextInitializer

为什么不统一

当我们运行启动命令java -jar的时候是不会调用javax.servlet.ServletContainerInitializer这个接口的

关于传统Spring MVC的web容器初始化方式可以参考Spring MVC容器初始化方式这篇文章