Spring Boot源码（二）：SPI去除web.xml

SPI广泛用于dubbo，spring boot，spring cloud alibaba等

继续上篇文章

上面三句代码的意思是创建IOC容器，下面是向容器中注入DispatcherServlet。

而ContextLoaderListener这个类的作用就是启动IOC容器，所以它们作用一致。

现在来看spring web怎么写的。

官方项目就是用的SPI来创建实例，我们聚焦于SpringServletContainerInitializer

/**
*类的描述：spring应用一启动（tomcat启动的时候）就会去扫描当前应用下导入jar包的META-INF/services的javax.servlet.ServletContainerInitializer的内容，
*就是SpringServletContainerInitializer，此类中通过@HandlesTypes把WebApplicationInitializer的所有实现类作为onStartup方法的入参，
*该方法对其中不是抽象类或接口的进行实例化
*/
@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {

    /**
     * Delegate the {@code ServletContext} to any {@link WebApplicationInitializer}
     * implementations present on the application classpath.
     * <p>Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)},
     * Servlet 3.0+ containers will automatically scan the classpath for implementations
     * of Spring's {@code WebApplicationInitializer} interface and provide the set of all
     * such types to the {@code webAppInitializerClasses} parameter of this method.
     * <p>If no {@code WebApplicationInitializer} implementations are found on the classpath,
     * this method is effectively a no-op. An INFO-level log message will be issued notifying
     * the user that the {@code ServletContainerInitializer} has indeed been invoked but that
     * no {@code WebApplicationInitializer} implementations were found.
     * <p>Assuming that one or more {@code WebApplicationInitializer} types are detected,
     * they will be instantiated (and <em>sorted</em> if the @{@link
     * org.springframework.core.annotation.Order @Order} annotation is present or
     * the {@link org.springframework.core.Ordered Ordered} interface has been
     * implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)}
     * method will be invoked on each instance, delegating the {@code ServletContext} such
     * that each instance may register and configure servlets such as Spring's
     * {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener},
     * or any other Servlet API componentry such as filters.
     * @param webAppInitializerClasses all implementations of
     * {@link WebApplicationInitializer} found on the application classpath
     * @param servletContext the servlet context to be initialized
     * @see WebApplicationInitializer#onStartup(ServletContext)
     * @see AnnotationAwareOrderComparator
     */
    @Override
    public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
            throws ServletException {

        List<WebApplicationInitializer> initializers = new LinkedList<>();

        if (webAppInitializerClasses != null) {
            for (Class<?> waiClass : webAppInitializerClasses) {
                // Be defensive: Some servlet containers provide us with invalid classes,
                // no matter what @HandlesTypes says...
                if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
                        WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
                    try {
                        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);
        for (WebApplicationInitializer initializer : initializers) {
            initializer.onStartup(servletContext);
        }
    }

}

里面只有一个onStartup方法，而容器启动时会调用此方法。

我们着重看下@HandlesTypes 注解，官方解释：

/**
<p>Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)},
* Servlet 3.0+ containers will automatically scan the classpath for implementations
* of Spring's {@code WebApplicationInitializer} interface and provide the set of all
* such types to the {@code webAppInitializerClasses} parameter of this method.
*/

@HandlesTypes 注解会把WebApplicationInitializer的所有子类（可用快捷键Ctrl+Alt+B）放到Set集合中：

然后循环判断此类不为抽象类，不为接口，并且是实现了WebApplicationInitializer接口的，然后实例化对象添加到initializers中：

然后对其进行排序：

这个排序就是通过@Order来判断先后顺序的（或者实现Order接口）。

然后分别循环子类的OnStartup方法。