spring mvc源码学习笔记之九

在前面的文章中，我们简单讲了可以用 WebApplicationInitializer 接口去替换 web.xml。
本文对这一块再做个详细讲解。
在 WebApplicationInitializer 这个接口的 javadoc 中有提到可以用继承 AbstractAnnotationConfigDispatcherServletInitializer 的方式替换实现 WebApplicationInitializer 接口。
先看代码，然后再具体解释。

pom.xml 的内容如下

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><parent><groupId>com.qsm</groupId><artifactId>learn</artifactId><version>1.0.0</version></parent><groupId>com.qs</groupId><artifactId>demo-47</artifactId><properties><maven.compiler.source>8</maven.compiler.source><maven.compiler.target>8</maven.compiler.target><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding></properties><dependencies><dependency><groupId>org.springframework</groupId><artifactId>spring-webmvc</artifactId><version>5.3.28</version></dependency><dependency><groupId>javax.servlet</groupId><artifactId>javax.servlet-api</artifactId><version>4.0.1</version><scope>provided</scope></dependency></dependencies>
</project>

com.qs.demo.MyWebApplicationInitializer 的内容如下

package com.qs.demo;import com.qs.demo.config.RootApplicationContextConfig;
import com.qs.demo.config.WebApplicationContextConfig;import org.springframework.web.servlet.support.AbstractAnnotationConfigDispatcherServletInitializer;/*** @author qs* @date 2023/07/20*/
public class MyWebApplicationInitializerextends AbstractAnnotationConfigDispatcherServletInitializer {@Overrideprotected Class<?>[] getRootConfigClasses() {return new Class[] {RootApplicationContextConfig.class};}@Overrideprotected Class<?>[] getServletConfigClasses() {return new Class[] {WebApplicationContextConfig.class};}@Overrideprotected String[] getServletMappings() {return new String[] {"/"};}
}

com.qs.demo.config.RootApplicationContextConfig 的内容如下

package com.qs.demo.config;import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;/*** @author qs* @date 2023/07/20*/
@Configuration
@ComponentScan(basePackages = {"com.qs.demo.service", "com.qs.demo.dao"})
public class RootApplicationContextConfig {}

com.qs.demo.config.WebApplicationContextConfig 的内容如下

package com.qs.demo.config;import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;/*** @author qs* @date 2023/07/20*/
@Configuration
@ComponentScan(basePackages = {"com.qs.demo.controller"})
public class WebApplicationContextConfig {}

com.qs.demo.controller.PeopleController 的内容如下

package com.qs.demo.controller;import com.qs.demo.service.PeopleService;import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;/*** @author qs* @date 2023/07/20*/
@RequestMapping("/people")
@RestController
public class PeopleController {private final PeopleService peopleService;public PeopleController(PeopleService peopleService) {this.peopleService = peopleService;}@GetMapping("/01")public String a() {return peopleService.a();}
}

com.qs.demo.service.PeopleService 的内容如下

package com.qs.demo.service;import com.qs.demo.dao.PeopleDao;import org.springframework.stereotype.Service;/*** @author qs* @date 2023/07/20*/
@Service
public class PeopleService {private final PeopleDao peopleDao;public PeopleService(PeopleDao peopleDao) {this.peopleDao = peopleDao;}public String a() {return peopleDao.a();}
}

com.qs.demo.dao.PeopleDao 的内容如下

package com.qs.demo.dao;import org.springframework.stereotype.Repository;import java.util.UUID;/*** @author qs* @date 2023/07/20*/
@Repository
public class PeopleDao {public String a() {return UUID.randomUUID().toString();}}

以上就是全部代码

接下来重点分析下 AbstractAnnotationConfigDispatcherServletInitializer 这个类，源代码不多，如下

/** Copyright 2002-2018 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**      https://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.servlet.support;import org.springframework.lang.Nullable;
import org.springframework.util.ObjectUtils;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.AnnotationConfigWebApplicationContext;/*** 下面这句话意思是：这个类是一个 WebApplicationInitializer（这个抽象类实现了 WebApplicationInitializer 接口）。* 它用于注册 DispatcherServlet 。* 并且，它使用 Java-based 的 spring 配置，也就是基于注解的配置方式。** {@link org.springframework.web.WebApplicationInitializer WebApplicationInitializer}* to register a {@code DispatcherServlet} and use Java-based Spring configuration.** 这个类的实现者需要重写 getRootConfigClasses 这个抽象方法。用于指定 root 应用上下文的配置类。* 并且这个括号里边也备注了，所谓的 root 应用上下文，应该是非 web 相关的。啥意思呢，粗暴理解，就是 service | dao 这些东西放到 root 应用上下文。** 这个类的实现者需要重写 getServletConfigClasses 这个抽象方法。用于指定 DispatcherServlet 的应用上下文的配置类。* 在这个配置类中配置的是跟 spring mvc 相关的配置信息。** 对比这2个方法。我们可以看出。父子容器设定的一个出发点就是将web相关的东西和非web相关的东西隔离开。** <p>Implementations are required to implement:* <ul>* <li>{@link #getRootConfigClasses()} -- for "root" application context (non-web* infrastructure) configuration.* <li>{@link #getServletConfigClasses()} -- for {@code DispatcherServlet}* application context (Spring MVC infrastructure) configuration.* </ul>** 下面这段话意思是，如果不需要父子容器的话，可以将所有配置写到 getRootConfigClasses 指定的配置类中，* 让 getServletConfigClasses 返回 null。** <p>If an application context hierarchy is not required, applications may* return all configuration via {@link #getRootConfigClasses()} and return* {@code null} from {@link #getServletConfigClasses()}.** @author Arjen Poutsma* @author Chris Beams* @since 3.2*/
public abstract class AbstractAnnotationConfigDispatcherServletInitializerextends AbstractDispatcherServletInitializer {/*** {@inheritDoc}* <p>This implementation creates an {@link AnnotationConfigWebApplicationContext},* providing it the annotated classes returned by {@link #getRootConfigClasses()}.* Returns {@code null} if {@link #getRootConfigClasses()} returns {@code null}.*/@Override@Nullableprotected WebApplicationContext createRootApplicationContext() {System.out.println("所谓创建根web应用上下文就是 new AnnotationConfigWebApplicationContext() ");Class<?>[] configClasses = getRootConfigClasses();if (!ObjectUtils.isEmpty(configClasses)) {AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();context.register(configClasses);return context;}else {return null;}}/*** {@inheritDoc}* <p>This implementation creates an {@link AnnotationConfigWebApplicationContext},* providing it the annotated classes returned by {@link #getServletConfigClasses()}.*/@Overrideprotected WebApplicationContext createServletApplicationContext() {System.out.println("createServletApplicationContext ----> new AnnotationConfigWebApplicationContext() ");AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();Class<?>[] configClasses = getServletConfigClasses();if (!ObjectUtils.isEmpty(configClasses)) {context.register(configClasses);}return context;}/*** Specify {@code @Configuration} and/or {@code @Component} classes for the* {@linkplain #createRootApplicationContext() root application context}.* @return the configuration for the root application context, or {@code null}* if creation and registration of a root context is not desired*/@Nullableprotected abstract Class<?>[] getRootConfigClasses();/*** Specify {@code @Configuration} and/or {@code @Component} classes for the* {@linkplain #createServletApplicationContext() Servlet application context}.* @return the configuration for the Servlet application context, or* {@code null} if all configuration is specified through root config classes.*/@Nullableprotected abstract Class<?>[] getServletConfigClasses();}

最后，AbstractAnnotationConfigDispatcherServletInitializer 这个抽象类的父类 AbstractDispatcherServletInitializer以及祖父 AbstractContextLoaderInitializer 都值得看看。