SpringMVC是怎么跟着Tomcat启动的

前言

我们通过官方文档使用SpringMVC，用Tomcat来跑时，需要配置下面这段代码

package com.lnw.config;

import org.springframework.web.SpringServletContainerInitializer;
import org.springframework.web.WebApplicationInitializer;
import org.springframework.web.context.support.AnnotationConfigWebApplicationContext;
import org.springframework.web.servlet.DispatcherServlet;

import javax.servlet.ServletContainerInitializer;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRegistration;
import javax.servlet.annotation.HandlesTypes;

/**
 * 为什么实现了WebApplicationInitializer就可以被Tomcat所管理？
 * 先看一个注解和一个类
 * @see HandlesTypes
 * @see ServletContainerInitializer
 * 这两个一般成对出现的
 * 在Tomcat启动的时候，会自动去寻找所有实现该接口的类
 * 找到这个类后，里面唯一的实现方法需要一个Set参数
 * 这些参数Tomcat会帮你去找，参数的类型是@HandlesTypes里面的value值
 * Spring在org.springframework.web.SpringServletContainerInitializer中引用该注解并实现ServletContainerInitializer
 * value值就是WebApplicationInitializer
 * @see SpringServletContainerInitializer
 */

public class MyWebApplicationInitializer implements WebApplicationInitializer {

	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		System.out.println("----init  my ServletContext----");
		// Load Spring web application configuration
		AnnotationConfigWebApplicationContext ac = new AnnotationConfigWebApplicationContext();
		ac.register(AppConfig.class);
		// ac.refresh();

		// Create and register the DispatcherServlet
		DispatcherServlet servlet = new DispatcherServlet(ac);
		ServletRegistration.Dynamic registration = servletContext.addServlet("app", servlet);
		registration.setLoadOnStartup(1);
		registration.addMapping("/*");

	}
}

AppConfig.class

package com.lnw.config;

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;

@Configuration
@ComponentScan("com.lnw")
@EnableWebMvc
public class AppConfig {
}

这样，当我们整合Tomcat来跑时（一般都是Tomcat插件），就会执行onStartup方法来启动SpringMVC

源码解析

我们看下我们实现的接口WebApplicationInitializr

package org.springframework.web;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
public interface WebApplicationInitializer {

	/**
	 * Configure the given {@link ServletContext} with any servlets, filters, listeners
	 * context-params and attributes necessary for initializing this web application. See
	 * examples {@linkplain WebApplicationInitializer above}.
	 * @param servletContext the {@code ServletContext} to initialize
	 * @throws ServletException if any call against the given {@code ServletContext}
	 * throws a {@code ServletException}
	 */
	void onStartup(ServletContext servletContext) throws ServletException;

}

发现它还是Spring的东西，如果要随着Tomcat启动执行的话，那也得是Servlet或者Tomcat里面的东西吧

在Tomcat启动的时候，它会去扫描classpath下的services文件夹（包括依赖的项目），如果文件名是javax.servlet.ServletContainerInitializer，就把文件里面的值读取出来（值是个类，读取出来后加载）
SpringMVC的web项目下就有一个services文件夹，而且文件名也是那个，里面的值是SpringServletContainerInitializer


我们查看SpringMVC里面的类SpringServletContainerInitializer

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;

@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);
		}
	}

}

注解@HandlerTypes里面的value值正是我们上面的那个WebApplicationInitializer

下面我们先来说下Tomcat里面的原理

Tomcat

就只说关键部分
Tomcat在启动的时候，会配置Context，配置Context需要ContextConfig
ContextConfig的核心就是webConfig 方法，下面主要看几条主要的代码
org.apache.catalina.startup.ContextConfig#webConfig

org.apache.catalina.startup.ContextConfig#webConfig

// Step 1. 读取各个jar模块和fragments
Map<String,WebXml> fragments = processJarsForWebFragments(webXml, webXmlParser);


// Step 2. 排序fragments
Set<WebXml> orderedFragments = null;
orderedFragments =
	WebXml.orderWebFragments(webXml, fragments, sContext);

// Step 3. 寻找所有的ServletContainerInitializer实现
// SpringMVC也是基于这个过程开展的
if (ok) {
processServletContainerInitializers();
}

// Step 4. 处理/WEB-INF/classes文件夹下面的注解了@HandlesTypes
// 这一步很重要
Map<String,JavaClassCacheEntry> javaClassCache = new HashMap<>();
if (ok) {
	WebResource[] webResources =
			context.getResources().listResources("/WEB-INF/classes");


// Step 5. 处理所有的jar中有关的@HandlesTypes
if (ok) {
	processAnnotations(
			orderedFragments, webXml.isMetadataComplete(), javaClassCache);
}

// Step 6. 将所有的fragments.xml合并
if (ok) {
	ok = webXml.merge(orderedFragments);
}

// Step 7a，将所有的web.xml合并
// merge tomcat-web.xml
webXml.merge(tomcatWebXml);

。。。
 // Step 11. 将扫描到的所有实现了ServletContainerInitializer类添加到StandardContext容器中
if (ok) {
	for (Map.Entry<ServletContainerInitializer,
			Set<Class<?>>> entry :
				initializerClassMap.entrySet()) {
		if (entry.getValue().isEmpty()) {
			context.addServletContainerInitializer(
					entry.getKey(), null);
		} else {
			context.addServletContainerInitializer(
					entry.getKey(), entry.getValue());
		}
	}
}

通过注释大概可以知道每段代码的作用
其中这段代码很重要 processServletContainerInitializers();
还有就是最后的Step 11将扫描到的类添加到Context 中
也就是说，Tomcat启动的时候，会使用ClassLoader扫描classes路径下所有的类，找出实现了ServletContainerInitializer类并保存，然后在随后的启动中调用ServletContainerInitializer
的onStart方法

void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException;

集合Set是存放注解@HandlesTypes中的类，目的是在特定的ServletContainerInitializer中启动的时候做另外的操作

SpringMVC跟Tomcat

上面我们讲到Tomcat启动的时候会扫描到所有实现了ServletContainerInitializer的类，而我们的SpringServletContainerInitialzer就是SpringMVC跟Tomcat的桥接
我们再来看下SpringServletContainerInitializer的源码

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;

@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);
		}
	}

}

因为SpringServletContainerInitializer实现了ServletContainerInitializer，所以会被Tomcat扫描存储到Context，并且因为该类上面注解了@HandlesTypes，里面的value为WebApplicationInitializer.class，所以Tomcat会将实现了WebApplicationInitializer的类扫描出来封装到Set中。
Tomcat的启动中，会调用所有的ServletContainerInitializer的onStartup方法来初始化并启动StrandContext容器，所以SpringServletContainerInitiar的onStartup方法就会被调用
在org.springframework.web.SpringServletContainerInitializer#onStartup方法中，通过一个for循环启动所有的WebApplicationInitializer

for (WebApplicationInitializer initializer : initializers) {
	initializer.onStartup(servletContext);
}

就是这样，执行了SpringMVC官方文档的这段代码

public class MyWebApplicationInitializer implements WebApplicationInitializer {

	@Override
	public void onStartup(ServletContext servletContext) throws ServletException {
		System.out.println("----init  my ServletContext----");
		// Load Spring web application configuration
		AnnotationConfigWebApplicationContext ac = new AnnotationConfigWebApplicationContext();
		ac.register(AppConfig.class);
		// ac.refresh();

		// Create and register the DispatcherServlet
		DispatcherServlet servlet = new DispatcherServlet(ac);
		ServletRegistration.Dynamic registration = servletContext.addServlet("app", servlet);
		registration.setLoadOnStartup(1);
		registration.addMapping("/*");

	}
}