SpringBoot是如何做到去xml的?

简单总结：

1. Servlet3.0 onStartup()中初始化ApplictaionContext，代替web.xml
2. @Configuration全java注解配置，代替spring配置文件applicationContext.xml
3. yaml 配置替换properties属性文件配置

1. 全注解: 去ApplicationContext.xml

使用configuration注解替换替换applicationContext.xml中的标签

@Configuration //注解配置 - 相当于xml的<beans>标签
@ComponentScan({ "com.cengel.cglab" })//启用组件扫描 - 不写将默认仅扫描此类所在包及其子包
@Import({ HibernateConfig.class, RedisConfig.class, SpringRmiConfig.class })
public class RootConfig {
    @Bean
    public Student student(){
        return new Student();
    }
}

2. 内置web容器

SpringBoot除了高度集成封装了Spring一系列框架之外，还封装了web容器.
SpringBoot支持嵌入tomcat jetty undertow三种web容器,查看EmbeddedServletContainerAutoConfiguration 类源码可知.

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
@ConditionalOnWebApplication
@Import(BeanPostProcessorsRegistrar.class)
public class EmbeddedServletContainerAutoConfiguration {
    @Configuration
    @ConditionalOnClass({ Servlet.class, Tomcat.class })
    @ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedTomcat {
        @Bean
        public TomcatEmbeddedServletContainerFactory tomcatEmbeddedServletContainerFactory() {
            return new TomcatEmbeddedServletContainerFactory();
        }
    }
    @Configuration
    @ConditionalOnClass({ Servlet.class, Server.class, Loader.class,
            WebAppContext.class })
    @ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedJetty {
        @Bean
        public JettyEmbeddedServletContainerFactory jettyEmbeddedServletContainerFactory() {
            return new JettyEmbeddedServletContainerFactory();
        }
    }
    @Configuration
    @ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
    @ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedUndertow {
        @Bean
        public UndertowEmbeddedServletContainerFactory undertowEmbeddedServletContainerFactory() {
            return new UndertowEmbeddedServletContainerFactory();
        }
    }
// .. . 其他省略
}

在spring-boot-starter-web的pom中可以看到,springweb application默认使用的是tomcat容器.

3.Servlet3.0去web.xml

在Servlet3.0环境中,Web容器会在类路径(META-INF/services)目录下查找javax.servlet.ServletContainerInitilalizer文件(文件配置其实现类)，web容器加载时，会执行该接口实现类的onStart方法

1. 为什么在web项目代码中继承AbstractAnnotationConfigDispatcherServletInitializer该类，不需要配置web.xml,spring环境就能运行？

1.实现javax.servlet.ServletContainerInitializer

@HandlesTypes({WebApplicationInitializer.class})
public class SpringServletContainerInitializer implements ServletContainerInitializer {
    public SpringServletContainerInitializer() {
    }
    public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException {
        //todo tomcat容器加载时，会执行，spring容器初始化在这里执行
        //存放spring-web项目的WebApplicationInitializer接口的所有实现类
        List<WebApplicationInitializer> initializers = new LinkedList<>();
        if (webAppInitializerClasses != null) {
            for (Class<?> waiClass : webAppInitializerClasses) {
                //  查找spring-web项目的WebApplicationInitializer接口的所有实现类
                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) {
            //逐一执行这些实现类的onStartUp方法
            //一般Spring的ApplicationContext Configure Servlet Filter等都包含在这些实现类的中,调用onStart方法时,执行这些初始化
            initializer.onStartup(servletContext);
        }
    }
}

1. 在Spring-Web的jar包META-INF下配置该接口
2. 
3. AbstractAnnotationConfigDispatcherServletInitializer 这个抽象类继承了WebApplicationInitializer
4. 也就是说,此类的onStart方法会在web容器加载时被执行
5. 而逐一向上查看此类的父类,onStart方法包含了Servlet Config Filter等的初始化

public abstract class AbstractAnnotationConfigDispatcherServletInitializer extends AbstractDispatcherServletInitializer {
    public AbstractAnnotationConfigDispatcherServletInitializer() {
    }
    protected WebApplicationContext createRootApplicationContext() {
        Class<?>[] configClasses = this.getRootConfigClasses();
        if (!ObjectUtils.isEmpty(configClasses)) {
            AnnotationConfigWebApplicationContext rootAppContext = new AnnotationConfigWebApplicationContext();
            rootAppContext.register(configClasses);
            return rootAppContext;
        } else {
            return null;
        }
    }
    protected WebApplicationContext createServletApplicationContext() {
        AnnotationConfigWebApplicationContext servletAppContext = new AnnotationConfigWebApplicationContext();
        Class<?>[] configClasses = this.getServletConfigClasses();
        if (!ObjectUtils.isEmpty(configClasses)) {
            servletAppContext.register(configClasses);
        }
        return servletAppContext;
    }
    //基于注解的bean配置类
    protected abstract Class<?>[] getRootConfigClasses();
    //基于注解的servlet配置类(webmvc viewResoler)
    protected abstract Class<?>[] getServletConfigClasses();
}

就这样,springbootApplication执行main方法时启动tomcat容器,tomcat容器初始化加载时,会执行ServletContainerInitializer的onStartup方法,Spring通过对该接口的实现,完成spring上下文,配置,servlet等的初始化装载.