spring自动装配servlet

疑问：传统servlet项目是如何集成到springboot项目中的？为何无需再配置web.xml

原理：servlet3.0配置api+spring抽象实现AbstractAnnotationConfigDispatcherServletInitializer

servlet3.0配置api: 基于ServletContainerInitializer接口实现与@HandlesTypes过滤出

WebApplicationInitializer具体实现类集合，而AbstractAnnotationConfigDispatcherServletInitializer的默认实现，就配置了DisPachservlet（AbstractAnnotationConfigDispatcherServletInitializer父类中实现了），从而可以自动装备

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//
 
package org.springframework.web;
 
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
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;
 
@HandlesTypes({WebApplicationInitializer.class})
public class SpringServletContainerInitializer implements ServletContainerInitializer {
    public SpringServletContainerInitializer() {
    }
 
    public void onStartup(@Nullable Set> webAppInitializerClasses, ServletContext servletContext) throws ServletException {
        List initializers = Collections.emptyList();
        Iterator var4;
        if (webAppInitializerClasses != null) {
            initializers = new ArrayList(webAppInitializerClasses.size());
            var4 = webAppInitializerClasses.iterator();
 
            while(var4.hasNext()) {
                Class waiClass = (Class)var4.next();
                if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
                    try {
                        ((List)initializers).add((WebApplicationInitializer)ReflectionUtils.accessibleConstructor(waiClass, new Class[0]).newInstance());
                    } catch (Throwable var7) {
                        throw new ServletException("Failed to instantiate WebApplicationInitializer class", var7);
                    }
                }
            }
        }
 
        if (((List)initializers).isEmpty()) {
            servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
        } else {
            servletContext.log(((List)initializers).size() + " Spring WebApplicationInitializers detected on classpath");
            AnnotationAwareOrderComparator.sort((List)initializers);
            var4 = ((List)initializers).iterator();
 
            while(var4.hasNext()) {
                WebApplicationInitializer initializer = (WebApplicationInitializer)var4.next();
                initializer.onStartup(servletContext);
            }
 
        }
    }
}

spring抽象实现AbstractAnnotationConfigDispatcherServletInitializer：WebApplicationInitializer具体抽象，可装备dispachServlet

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//
 
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;
 
public abstract class AbstractAnnotationConfigDispatcherServletInitializer extends AbstractDispatcherServletInitializer {
    public AbstractAnnotationConfigDispatcherServletInitializer() {
    }
 
    @Nullable
    protected WebApplicationContext createRootApplicationContext() {
        Class[] configClasses = this.getRootConfigClasses();
        if (!ObjectUtils.isEmpty(configClasses)) {
            AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
            context.register(configClasses);
            return context;
        } else {
            return null;
        }
    }
 
    protected WebApplicationContext createServletApplicationContext() {
        AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
        Class[] configClasses = this.getServletConfigClasses();
        if (!ObjectUtils.isEmpty(configClasses)) {
            context.register(configClasses);
        }
 
        return context;
    }
 
    @Nullable
    protected abstract Class[] getRootConfigClasses();
 
    @Nullable
    protected abstract Class[] getServletConfigClasses();
}

springboot如何推断是web应用：

SpringApplication 在run方法之前推断，只能检查当前classLoader存在class存在，来判断是否为web应用。

判断标准：是否有Servlet和ConfigurableWebApplicationContext，有则为web servlet项目即传统springmvc

javax.servlet.Servlet", "org.springframework.web.context.ConfigurableWebApplicationContext"

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//
 
package org.springframework.boot;
 
import org.springframework.util.ClassUtils;
 
public enum WebApplicationType {
    NONE,
    SERVLET,
    REACTIVE;
 
    private static final String[] SERVLET_INDICATOR_CLASSES = new String[]{"javax.servlet.Servlet", "org.springframework.web.context.ConfigurableWebApplicationContext"};
    private static final String WEBMVC_INDICATOR_CLASS = "org.springframework.web.servlet.DispatcherServlet";
    private static final String WEBFLUX_INDICATOR_CLASS = "org.springframework.web.reactive.DispatcherHandler";
    private static final String JERSEY_INDICATOR_CLASS = "org.glassfish.jersey.servlet.ServletContainer";
 
    private WebApplicationType() {
    }
 
    static WebApplicationType deduceFromClasspath() {
        if (ClassUtils.isPresent("org.springframework.web.reactive.DispatcherHandler", (ClassLoader)null) && !ClassUtils.isPresent("org.springframework.web.servlet.DispatcherServlet", (ClassLoader)null) && !ClassUtils.isPresent("org.glassfish.jersey.servlet.ServletContainer", (ClassLoader)null)) {
            return REACTIVE;
        } else {
            String[] var0 = SERVLET_INDICATOR_CLASSES;
            int var1 = var0.length;
 
            for(int var2 = 0; var2 < var1; ++var2) {
                String className = var0[var2];
                if (!ClassUtils.isPresent(className, (ClassLoader)null)) {
                    return NONE;
                }
            }
 
            return SERVLET;
        }
    }
}

遗留问题： 

AnnotationConfigWebApplicationContext即可扫描springmvc相关的注解，从而达到自动装备

文章：摘录部分《小马哥 springBoot编程思想》