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前言
学习记录：tomcat的理解
提示：以下是本篇文章正文内容，下面案例可供参考
一、XML配置Servlet
ContextLoaderListener实现了ServletContextListener这个接口的contextInitialized 方法。这个是启动spring的关键。
DispatcherServlet则是启动springmvc的关键，DispatcherServlet 继承关系如下
HttpServlet<-HttpServletBean<-FrameworkServlet<-DispatcherServlet;
以上两个类是不可缺失的。
早期我们是通过xml配置来加载两个类的。如下：
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<servlet>
<servlet-name>spring-dispatcher</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:spring-dispatcher.xml</param-value>
</init-param>
<load-on-startup>0</load-on-startup>
</servlet>
那tomcat 是如何与spring进行整合的，接下来我们看看tomcat的spi机制
二、Tomcat 加载机制:SPI
下面我们自己起的一个示例，看看tomcat是如何实现SPI。
public class SpringApplication {
public static void main(String[] args) throws LifecycleException, ServletException {
run(SpringApplication.class, args);
}
public static void run(Object source, String... args) {
try {
// Tomcat容器
Tomcat tomcatServer = new Tomcat();
// 端口号
tomcatServer.setPort(9090);
// 读取项目路径 加载静态资源
StandardContext ctx = (StandardContext) tomcatServer.addWebapp("/", new File("src/main").getAbsolutePath());
// 禁止重新载入
ctx.setReloadable(false);
// class文件读取地址
File additionWebInfClasses = new File("spring-source/target/classes");
// 创建WebRoot
WebResourceRoot resources = new StandardRoot(ctx);
// tomcat内部读取Class执行
resources.addPreResources(
new DirResourceSet(resources, "/spring-demo/WEB-INF/classes", additionWebInfClasses.getAbsolutePath(), "/"));
tomcatServer.start();
// 同步等待请求
tomcatServer.getServer().await();
} catch (LifecycleException e) {
e.printStackTrace();
} catch (ServletException e) {
e.printStackTrace();
}
}
}
至此一个tomcat启动完成，接下我们需要把servlet和listener 从配置文件中去除。
- Tomcat 启动的时候会自动加载 /resources/META-INF/services/javax.servlet.ServletContainerInitializer 文件，相当于是一个SPI
//tomcat 会自动加载所有实现LoadServlet的类放在set容器中。并调用onStartup方法
@HandlesTypes(LoadServlet.class)
public class MyServletContainerInitializer implements ServletContainerInitializer {
@Override
public void onStartup(Set<Class<?>> set, ServletContext servletContext) throws ServletException {
Iterator var4;
if (set != null) {
var4 = set.iterator();
while (var4.hasNext()) {
Class<?> clazz = (Class<?>) var4.next();
if (!clazz.isInterface() && !Modifier.isAbstract(clazz.getModifiers()) && LoadServlet.class.isAssignableFrom(clazz)) {
try {
//在这里调用我们自己实现的loadOnstarp的方法
((LoadServlet) clazz.newInstance()).loadOnstarp(servletContext);
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
}
}
public interface LoadServlet {
void loadOnstarp(ServletContext servletContext);
}
public class LoadServletImpl implements LoadServlet {
@Override
public void loadOnstarp(ServletContext servletContext) {
ServletRegistration.Dynamic initServlet = servletContext.addServlet("initServlet", "com.test.servlet.InitServlet");
initServlet.setLoadOnStartup(1);
initServlet.addMapping("/init");
}
}
在LoadServletImpl 中我们自己注入Servlert.
三、Spring mvc 加载实现原理
spring-web 同样有配置文件：META-INF/services/javax.servlet.ServletContainerInitializer，那么tomcat启动的时候会加载SpringServletContainerInitializer，我们来看看这个类究竟做了什么操作。
//tomcat 会自动加载所有实现WebApplicationInitializer的类放在set容器中。并调用onStartup方法
@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);
}
}
}
简单的说：这个类其实就是加载所有实现了WebApplicationInitializer接口的类，然后调用onStartUp方法。我们先来看看AbstractContextLoaderInitializer这个类
public abstract class AbstractContextLoaderInitializer implements WebApplicationInitializer {
@Override
public void onStartup(ServletContext servletContext) throws ServletException {
registerContextLoaderListener(servletContext);
}
protected void registerContextLoaderListener(ServletContext servletContext) {
//创建spring上下文，注册了SpringContainer
WebApplicationContext rootAppContext = createRootApplicationContext();
if (rootAppContext != null) {
//创建监听器
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");
}
}
}
可以看到这个类实际上帮我们注册了监听器，接下来我们再看看AbstractDispatcherServletInitializer
public abstract class AbstractDispatcherServletInitializer extends AbstractContextLoaderInitializer {
@Override
public void onStartup(ServletContext servletContext) throws ServletException {
super.onStartup(servletContext);
//注册DispatcherServlet
registerDispatcherServlet(servletContext);
}
protected void registerDispatcherServlet(ServletContext servletContext) {
String servletName = getServletName();
Assert.hasLength(servletName, "getServletName() must not return null or empty");
//创建springmvc的上下文，注册了MvcContainer类
WebApplicationContext servletAppContext = createServletApplicationContext();
Assert.notNull(servletAppContext, "createServletApplicationContext() must not return null");
//创建DispatcherServlet
FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext);
Assert.notNull(dispatcherServlet, "createDispatcherServlet(WebApplicationContext) must not return null");
dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers());
ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet);
if (registration == null) {
throw new IllegalStateException("Failed to register servlet with name '" + servletName + "'. " +
"Check if there is another servlet registered under the same name.");
}
/*
* 如果该元素的值为负数或者没有设置，则容器会当Servlet被请求时再加载。
如果值为正整数或者0时，表示容器在应用启动时就加载并初始化这个servlet，
值越小，servlet的优先级越高，就越先被加载
* */
registration.setLoadOnStartup(1);
registration.addMapping(getServletMappings());
registration.setAsyncSupported(isAsyncSupported());
Filter[] filters = getServletFilters();
if (!ObjectUtils.isEmpty(filters)) {
for (Filter filter : filters) {
registerServletFilter(servletContext, filter);
}
}
customizeRegistration(registration);
}
}
可以看到这个类实际上帮我们注册了DispatcherServlet。
到此我们已经注册了Listener 和Servelet，接下来我们看下是再哪里加载我们spring 容器。
四、加载spring 容器
tomcat 初始化完成后会调用ContextLoaderListener 中的 contextInitialized。
调用链是：
ContextLoaderListener -> contextInitialized
-> initWebApplicationContext -> configureAndRefreshWebApplicationContext -> onrefresh
最终会调用我们spring 的refresh 方法 ，初始化spring 容器，下面是一些重要过程代码。
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
throw new IllegalStateException(
"Cannot initialize context because there is already a root application context present - " +
"check whether you have multiple ContextLoader* definitions in your web.xml!");
}
servletContext.log("Initializing Spring root WebApplicationContext");
Log logger = LogFactory.getLog(ContextLoader.class);
if (logger.isInfoEnabled()) {
logger.info("Root WebApplicationContext: initialization started");
}
long startTime = System.currentTimeMillis();
try {
// Store context in local instance variable, to guarantee that
// it is available on ServletContext shutdown.
if (this.context == null) {
this.context = createWebApplicationContext(servletContext);
}
if (this.context instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent ->
// determine parent for root web application context, if any.
ApplicationContext parent = loadParentContext(servletContext);
cwac.setParent(parent);
}
//重要，这里实际上就是初始化spring容器
configureAndRefreshWebApplicationContext(cwac, servletContext);
}
}
//把spring容器放在serveletContext中
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
ClassLoader ccl = Thread.currentThread().getContextClassLoader();
if (ccl == ContextLoader.class.getClassLoader()) {
currentContext = this.context;
}
else if (ccl != null) {
currentContextPerThread.put(ccl, this.context);
}
if (logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");
}
return this.context;
}
catch (RuntimeException | Error ex) {
logger.error("Context initialization failed", ex);
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
throw ex;
}
}
这里实际上就是调用spring refresh 方法，初始化spring容器放在ServletContext容器中，以便后面的调用。接下来我们看一下DispatchServlet。众所周知：Servlet实例化后会被tomcat 调用init 的方法，
调用链如下：
init -> initServletBean -> initWebApplicationContext -> configureAndRefreshWebApplicationContext -> refresh()
protected WebApplicationContext initWebApplicationContext() {
//这里会从servletContext中获取到父容器，就是通过监听器加载的容器
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent -> set
// the root application context (if any; may be null) as the parent
//父容器如果为空，则将 监听器加载的容器 设置为父容器。
cwac.setParent(rootContext);
}
//容器加载
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
// No context instance was injected at construction time -> see if one
// has been registered in the servlet context. If one exists, it is assumed
// that the parent context (if any) has already been set and that the
// user has performed any initialization such as setting the context id
wac = findWebApplicationContext();
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
wac = createWebApplicationContext(rootContext);
}
if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here.
synchronized (this.onRefreshMonitor) {
onRefresh(wac);
}
}
if (this.publishContext) {
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
}
return wac;
}
这里需要注意的是，ServletContextListener帮我们实例化了一个IOC容器，DispatcherServlet实际上也实例化了另一个IOC容器，listener的是spring mvc 容器主要管理我们的controller类，servlet是spring 容器主要管理service以及repo类。而他们之间则存在父子关系。spring mvc是子，spring是父，子可以调用父的bean。
总结
本文主要讲解了Spring 如何不通过xml文件来注册Listener 和Servlet，这里主要用了tomcat 的spi机制。第二则是解释了如何将spring 容器装载至ServletContext的过程。
servletContext 及spring、springmvc关系如下
servletContext 拥有spring mvc而spring mvc 拥有spring。