Spring中的WebApplicationInitializer

1. WebApplicationInitializer

隨着JavaConfig配置方式逐步替代配置,WebApplicationInitializer可以看做Web.xml的替代,此接口在Web容器啓動的時候會記載這個接口的實現類,從而起到Web.xml的作用,從而我們可以通過其自定義配置Web組件.

首先我們進入這個接口,它僅有一個方法#onStartup方法,我們並不容易看出其用意何在?而此方法又是何時被調用的?

public interface WebApplicationInitializer {
    void onStartup(ServletContext var1) throws ServletException;
}

但是在這個類所屬的包下有另一個類- - -SpringServletContainerInitializer

@HandlesTypes({WebApplicationInitializer.class})
public class SpringServletContainerInitializer implements ServletContainerInitializer {
    public SpringServletContainerInitializer() {
    }

    public void onStartup(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 {
                        initializers.add((WebApplicationInitializer)waiClass.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();
                initializer.onStartup(servletContext);
            }
        }
    }
}

在SpringServletContainerInitializer#onStartup方法中,首先判斷參數WebAppInitializerClasses這個Set集合是否爲空.若不爲空並且是WebApplicationInitializer接口的實現,就將其轉存到另一個集合中,之後對集合進行排序並遍歷調用其#onStartup方法,從而我們可以知曉之前所說的WebApplicationInitializer是如何在Web流程中被調用的.

但是另外兩個問題又了:
1.SpringServletContainerInitializer#onStartup方法又是如何被調用的呢?
2.其類上註解@HandlesType又有什麼作用呢?

1.

官方聲明中指出:爲了支持無xml配置化,提供了ServletContainerInitializer,通過SPI機制,當啓動Web容器的時候,會自動到相應的jar包下找到META-INF/services下ServletContainerInitializer的全路徑名的文件,並根據其內容的全路徑進行實例化.而我們可以發現文件中的名稱就是SpringServletContainerInitializer,所以SpringServletContainerInitializer作爲ServletContainerInitializer的實現,通過SPI機制在Web容器加載的時候會被自動調用.

2.

而對於@HandlesType註解,其作用爲:讀取@HandlesType註解value的值,根據其值將掃描路徑下所有該值所表示的接口實現,並將其生成一個Set集合提供給#onStartup方法使用.

在ApplicationContext內部啓動時會通知ServletContainerInitializer#onStartup方法,而這個方法的第一個參數Set集合就是註解Value值指定的Class集合,而在這裏即爲WebApplicationInitializer實現的集合

在下列代碼中所示,在Tomcat所有容器的抽象類中ContainerMBean#addChild方法中,Tomcat在爲Host容器添加Context子容器時,會爲其分配一個ContextConfig類,其中通過#processServletContainerInitializer方法來獲取@HandlesType註解

public class ContainerMBean extends BaseModelMBean {
    public void addChild(String type, String name) throws MBeanException {
        //........
        try {
            if(contained instanceof StandardHost) {
                HostConfig config = new HostConfig();
                contained.addLifecycleListener(config);
            } else if(contained instanceof StandardContext) {
                ContextConfig config = new ContextConfig();
                contained.addLifecycleListener(config);
            }
        }
    }
}

-------------------------------------------------------------------------------------

public class ContextConfig implements LifecycleListener {
    protected void webConfig() {
            //.........
            if(this.ok) {
                this.processAnnotations(orderedFragments, webXml.isMetadataComplete(), javaClassCache);
            }
        }


    protected void processServletContainerInitializers() {
            //.........
             ServletContainerInitializer sci = (ServletContainerInitializer)var13.next();
            this.initializerClassMap.put(sci, new HashSet());

            HandlesTypes ht;
            try {
                ht = (HandlesTypes)sci.getClass().getAnnotation(HandlesTypes.class);
            } 
        }
}

2.AbStractAnnotationConfigDispatcherServletInitializer

官方文檔聲明中指出:Spring MVC中我們其實是可以創建多個DispatcherServlet的,只需要創建多個繼承自AbstractAnnotationConfigDispatcherServletInitializer的子類即可,而每個DispatcherServlet都有自己的應用上下文(servlet application context),這個應用上下文只針對當前的DispatcherServlet有用,這也就是#getServletConfigClasses的作用,用來獲取這個DispatcherServlet的應用上下文的配置類

而除了每個DispatcherServlet配置類的應用上下文,還有一個公共的跟應用上下文(root application context),這個應用上下文的作用時爲了在多個DispatcherServlet之間共享,這也就是#getRootConfigClasses的作用,用於返回根應用上下文的配置類,Spring框架的機制會保證如果當前DispatcherServlet的應用上下文中沒有找到想要的Bean,就會去根應用上下文中找.

public class SpittrWebApplicationInitializer extends AbstractAnnotationConfigDispatcherServletInitializer{
    //將DispatcherServlet映射到"/"
    @Override
    protected String[] getServletMappings(){
        return new String[]{"/"};
    }

    //配置root applicationContext應用上下文的bean
    @Override
    protected Class<?>[] getRootConfigClasses(){
        return new Class<?>[]{RootConfig.class};
    }

    //配置servlet applicationContext應用上下文的bean
    @Override
    protected Class<?>[] getServletConfigClasses(){
        return new Class<?>[]{Webconfig.class};
    }
}

--------------------------------------------------------------------------------------------------------------------------------------------------------
@Configuration
@EnableWebMvc
@ComponentScan("spitter.web")
public class WebConfig implements WebMvcConfigurer{//Spring5中此接口方法均爲默認方法,替代之前適配器類
    //配置jsp視圖解析器.否則會使用默認的BeanNameViewResolver
    @Bean
    public viewResolver viewResolver(){
        InternalResourceViewResolver resolver = new InternalResourceViewResolver();
        resolver.setPrefix("/WEB-INF.views");
        resolver.setSuffix(".jsp");
        return resolver;
    }
    //配置靜態資源處理
    @Override
    public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer){
        configurer.enable();
    }
}

--------------------------------------------------------------------------------------------------------------------------------------------------------
@Configuration
@EnableWebMvc
@ComponentScan(basePackage = {"spitter"},excludeFilters = {
    @Filter(type = FilterType.ANNOTATION, value = EnableWebMvc.class)
})
public class RootConfig{
}

3.總結

AbStractAnnotationConfigDispatcherServletInitializer就是Spring給我們通過javaConfig方式自定義DispatcherServlet組件的實現方法,如果需要了解其他相關內容:

Spring容器的相關知識可以參考:

SpringMvc自定義配置化相關知識可以參考:

Spring中關於的WebApplicationInitializer分析

Spring中的WebApplicationInitializer

1. WebApplicationInitializer

1.

2.

2.AbStractAnnotationConfigDispatcherServletInitializer

3.總結

Spring註解掃描@ComponentScan

Spring容器及其初始化

Spring中關於的WebApplicationInitializer分析

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