Spring Boot source (B): SPI removed web.xml SPI-Service Provider Interface

 

SPI widely used dubbo, spring boot, spring cloud alibaba etc.

About SPI, visible SPI-Service Provider Interface

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 The above three code means IOC container is created, the following are injected into the vessel DispatcherServlet.

 

 

 The role of this class is to start ContextLoaderListener IOC container, so that they are consistent with the role.

Now look at how to write spring web.

 

 

 Official project is to use the SPI to create an instance, we focus on 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。