SpringMVC全注解开发原理

基本介绍

在SpringMVC中，ContextLoaderListener 和DispatcherServlet 是mvc的两大核心组件，一般我们是在web.xml文件中配置，那如果我们想用全注解，像springboot一样，该怎么开发呢，其原理又是怎样的呢。

 

springmvc是怎么通过注解去注册ContextLoaderListener和DispatcherServlet的

注解方式的初始化其实是用到servlet3的新特性来实现的

org.springframework.web.SpringServletContainerInitializer

加载就会调用onStartup方法，最后回调用WebApplicationInitializer.onStartup(servletContext);方法，而我们的ContextLoaderListener和DispatcherServlet就是在WebApplicationInitializer的子类实现的。

package org.springframework.web;

import java.lang.reflect.Modifier;
import java.util.LinkedList;
import java.util.List;
import java.util.ServiceLoader;
import java.util.Set;
import javax.servlet.ServletContainerInitializer;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.HandlesTypes;

import org.springframework.core.annotation.AnnotationAwareOrderComparator;
import org.springframework.lang.Nullable;
import org.springframework.util.ReflectionUtils;

/**
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @author Rossen Stoyanchev
 * @since 3.1
 * @see #onStartup(Set, ServletContext)
 * @see WebApplicationInitializer
 */
@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {

	
	@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);
		}
	}

}

WebApplicationInitializer有四个子类；下图

 

AbstractContextLoaderInitializer的作用就是注册ContextLoaderListener

/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.context;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.context.ApplicationContextInitializer;
import org.springframework.lang.Nullable;
import org.springframework.web.WebApplicationInitializer;

/**
 * Convenient base class for {@link WebApplicationInitializer} implementations
 * that register a {@link ContextLoaderListener} in the servlet context.
 *
 * <p>The only method required to be implemented by subclasses is
 * {@link #createRootApplicationContext()}, which gets invoked from
 * {@link #registerContextLoaderListener(ServletContext)}.
 *
 * @author Arjen Poutsma
 * @author Chris Beams
 * @author Juergen Hoeller
 * @since 3.2
 */
public abstract class AbstractContextLoaderInitializer implements WebApplicationInitializer {

	/** Logger available to subclasses */
	protected final Log logger = LogFactory.getLog(getClass());


	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		registerContextLoaderListener(servletContext);
	}

	/**
	 * Register a {@link ContextLoaderListener} against the given servlet context. The
	 * {@code ContextLoaderListener} is initialized with the application context returned
	 * from the {@link #createRootApplicationContext()} template method.
	 * @param servletContext the servlet context to register the listener against
	 */
	protected void registerContextLoaderListener(ServletContext servletContext) {
		WebApplicationContext rootAppContext = createRootApplicationContext();
		if (rootAppContext != null) {
			ContextLoaderListener listener = new ContextLoaderListener(rootAppContext);
			listener.setContextInitializers(getRootApplicationContextInitializers());
			servletContext.addListener(listener);
		}
		else {
			logger.debug("No ContextLoaderListener registered, as " +
					"createRootApplicationContext() did not return an application context");
		}
	}

	/**
	 * Create the "<strong>root</strong>" application context to be provided to the
	 * {@code ContextLoaderListener}.
	 * <p>The returned context is delegated to
	 * {@link ContextLoaderListener#ContextLoaderListener(WebApplicationContext)} and will
	 * be established as the parent context for any {@code DispatcherServlet} application
	 * contexts. As such, it typically contains middle-tier services, data sources, etc.
	 * @return the root application context, or {@code null} if a root context is not
	 * desired
	 * @see org.springframework.web.servlet.support.AbstractDispatcherServletInitializer
	 */
	@Nullable
	protected abstract WebApplicationContext createRootApplicationContext();

	/**
	 * Specify application context initializers to be applied to the root application
	 * context that the {@code ContextLoaderListener} is being created with.
	 * @since 4.2
	 * @see #createRootApplicationContext()
	 * @see ContextLoaderListener#setContextInitializers
	 */
	@Nullable
	protected ApplicationContextInitializer<?>[] getRootApplicationContextInitializers() {
		return null;
	}

}

AbstractDispatcherServletInitializer就是注册了DispatcherServlet 

/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific langu