SpringBoot内置tomcat启动原理
前言
我们知道SpringBoot工程是可以被打成jar包，直接运行jar包启动的，那么为什么不用部署到web服务器也能够像web一样访问呢？其本质原因就是springBoot工程内嵌了一个tomcat，jar包方式以main方法作为入口执行代码的时候，底层启动了一个tomcat。
内置tomcat依赖
如果我们用springBoot开发web工程，通常都会导入如下依赖：
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
点击进去就能看到SpringBoot tomcat的启动依赖：
开发阶段对我们来说使用内置的tomcat是非常够用了，当然也可以使用jetty。
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}
这里是main函数入口，两句代码最耀眼，分别是SpringBootApplication注解和SpringApplication.run()方法。
war包方式发布
发布的时候，目前大多数的做法还是排除内置的tomcat，打瓦包（war）然后部署在生产的tomcat中，好吧，那打包的时候应该怎么处理？
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<!-- 移除嵌入式tomcat依赖 -->
<exclusions>
<exclusion>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
</exclusion>
</exclusions>
</dependency>
<!--添加servlet-api依赖--->
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<scope>provided</scope>
</dependency>
更新main函数，主要是继承SpringBootServletInitializer，并重写configure()方法。
@SpringBootApplication
public class Application extends SpringBootServletInitializer {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
return builder.sources(this.getClass());
}
}
关于为什么war包方式发布需要继承SpringBootServletInitializer，并重写configure方法：
- 1.启动入口不一样
首先jar包方式启动的入口是main方法，底层会启动一个内嵌的tomcat
而war包方式发布入口就是由外部的tomcat管理了，其会加载web.xml - 2.至与为什么要继承SpringBootServletInitializer，根本原因是servlet3.0 的新特性，不再需要web.xml配置文件了，这里就不具体说明了，推荐一篇博文：SpringBoot为什么没有web.xml了
从main函数说起
public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
return run(new Class<?>[] {
primarySource }, args);
}
//这里run方法返回的是ConfigurableApplicationContext
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
//这里对比war包方式继承SpringBootServletInitializer重写configure方法
//中的builder.sources(this.getClass())
//其实目的是一样的，让SpringApplication知道配置信息
return new SpringApplication(primarySources).run(args);
}
主要看run方法：
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
configureHeadlessProperty();
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
//准备环境，这里会用监听器方式加载各种配置文件，例如application.yml,properties...
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
configureIgnoreBeanInfo(environment);
//打印banner，这里你可以自己涂鸦一下，换成自己项目的logo
Banner printedBanner = printBanner(environment);
//创建应用上下文
context = createApplicationContext();
exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
new Class[] {
ConfigurableApplicationContext.class }, context);
//预处理上下文
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
//刷新上下文
refreshContext(context);
//再刷新上下文
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
}
listeners.started(context);
callRunners(context, applicationArguments);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
}
try {
listeners.running(context);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
}
return context;
}
既然我们想知道tomcat在SpringBoot中是怎么启动的，那么run方法中，重点关注创建应用上下文（createApplicationContext）和刷新上下文（refreshContext）。
创建上下文
//创建上下文
protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
switch (this.webApplicationType) {
case SERVLET:
//内嵌tomcat走这
//创建AnnotationConfigServletWebServerApplicationContext
contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
break;
case REACTIVE:
//Spring Boot的特性： Spring WebFlux相关的不用管
contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
break;
default:
//不需要内嵌web服务器走这
contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
}
}
catch (ClassNotFoundException ex) {
throw new IllegalStateException(
"Unable create a default ApplicationContext, " + "please specify an ApplicationContextClass",
ex);
}
}
return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}
这里会创建AnnotationConfigServletWebServerApplicationContext类。
而AnnotationConfigServletWebServerApplicationContext类继承了ServletWebServerApplicationContext，而这个类是最终集成了AbstractApplicationContext。
刷新上下文
//SpringApplication.java
//刷新上下文
private void refreshContext(ConfigurableApplicationContext context) {
refresh(context);
if (this.registerShutdownHook) {
try {
context.registerShutdownHook();
}
catch (AccessControlException ex) {
// Not allowed in some environments.
}
}
}
//这里直接调用最终父类AbstractApplicationContext.refresh()方法
protected void refresh(ApplicationContext applicationContext) {
Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
((AbstractApplicationContext) applicationContext).refresh();
}
//AbstractApplicationContext.java
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
prepareRefresh();
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
prepareBeanFactory(beanFactory);
try {
postProcessBeanFactory(beanFactory);
invokeBeanFactoryPostProcessors(beanFactory);
registerBeanPostProcessors(beanFactory);
initMessageSource();
initApplicationEventMulticaster();
//调用各个子类的onRefresh()方法，也就说这里要回到子类：ServletWebServerApplicationContext，调用该类的onRefresh()方法
onRefresh();
registerListeners();
finishBeanFactoryInitialization(beanFactory);
finishRefresh();
}catch (BeansException ex) {
...
throw ex;
}finally {
resetCommonCaches();
}
}
}
//ServletWebServerApplicationContext.java
//在这个方法里看到了熟悉的面孔，this.createWebServer，神秘的面纱就要揭开了。
protected void onRefresh() {
super.onRefresh();
try {
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
//ServletWebServerApplicationContext.java
//这里是创建webServer，但是还没有启动tomcat，这里是通过ServletWebServerFactory创建，那么接着看下ServletWebServerFactory
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
getBeanFactory().registerSingleton("webServerGracefulShutdown",
new WebServerGracefulShutdownLifecycle(this.webServer));
getBeanFactory().registerSingleton("webServerStartStop",
new WebServerStartStopLifecycle(this, this.webServer));
}
else if (servletContext != null) {
try {
getSelfInitializer().onStartup(servletContext);
}
catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context", ex);
}
}
initPropertySources();
}
//接口
public interface ServletWebServerFactory {
WebServer getWebServer(ServletContextInitializer... initializers);
}
//实现
AbstractServletWebServerFactory
JettyServletWebServerFactory
TomcatServletWebServerFactory
UndertowServletWebServerFactory
而其中我们常用的有两个：TomcatServletWebServerFactory和JettyServletWebServerFactory。
//TomcatServletWebServerFactory.java
//这里我们使用的tomcat，所以我们查看TomcatServletWebServerFactory。到这里总算是看到了tomcat的踪迹。
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
if (this.disableMBeanRegistry) {
Registry.disableRegistry();
}
Tomcat tomcat = new Tomcat();
File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath());
// 创建Connector对象
Connector connector = new Connector(this.protocol);
connector.setThrowOnFailure(true);
tomcat.getService().addConnector(connector);
customizeConnector(connector);
tomcat.setConnector(connector);
tomcat.getHost().setAutoDeploy(false);
configureEngine(tomcat.getEngine());
for (Connector additionalConnector : this.additionalTomcatConnectors) {
tomcat.getService().addConnector(additionalConnector);
}
prepareContext(tomcat.getHost(), initializers);
return getTomcatWebServer(tomcat);
}
protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
return new TomcatWebServer(tomcat, getPort() >= 0, getShutdown());
}
//Tomcat.java
//返回Engine容器，看到这里，如果熟悉tomcat源码的话，对engine不会感到陌生。
public Engine getEngine() {
Service service = getServer().findServices()[0];
if (service.getContainer() != null) {
return service.getContainer();
}
Engine engine = new StandardEngine();
engine.setName( "Tomcat" );
engine.setDefaultHost(hostname);
engine.setRealm(createDefaultRealm());
service.setContainer(engine);
return engine;
}
//Engine是最高级别容器，Host是Engine的子容器，Context是Host的子容器，Wrapper是Context的子容器
getWebServer这个方法创建了Tomcat对象，并且做了两件重要的事情：把Connector对象添加到tomcat中，configureEngine(tomcat.getEngine());
getWebServer方法返回的是TomcatWebServer。
//TomcatWebServer.java
//这里调用构造函数实例化TomcatWebServer
public TomcatWebServer(Tomcat tomcat, boolean autoStart) {
Assert.notNull(tomcat, "Tomcat Server must not be null");
this.tomcat = tomcat;
this.autoStart = autoStart;
initialize();
}
private void initialize() throws WebServerException {
//在控制台会看到这句日志
logger.info("Tomcat initialized with port(s): " + getPortsDescription(false));
synchronized (this.monitor) {
try {
addInstanceIdToEngineName();
Context context = findContext();
context.addLifecycleListener((event) -> {
if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) {
removeServiceConnectors();
}
});
//===启动tomcat服务===
this.tomcat.start();
rethrowDeferredStartupExceptions();
try {
ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
}
catch (NamingException ex) {
}
//开启阻塞非守护进程
startDaemonAwaitThread();
}
catch (Exception ex) {
stopSilently();
destroySilently();
throw new WebServerException("Unable to start embedded Tomcat", ex);
}
}
}
//Tomcat.java
public void start() throws LifecycleException {
getServer();
server.start();
}
public void stop() throws LifecycleException {
getServer();
server.stop();
}
// 一个Tomcat会有一个Server
public Server getServer() {
if (server != null) {
return server;
}
System.setProperty("catalina.useNaming", "false");
server = new StandardServer();
initBaseDir();
// Set configuration source
ConfigFileLoader.setSource(new CatalinaBaseConfigurationSource(new File(basedir), null));
server.setPort( -1 );
Service service = new StandardService();
service.setName("Tomcat");
server.addService(service);
return server;
}
//TomcatWebServer.java
//启动tomcat服务
@Override
public void start() throws WebServerException {
synchronized (this.monitor) {
if (this.started) {
return;
}
try {
addPreviouslyRemovedConnectors();
Connector connector = this.tomcat.getConnector();
if (connector != null && this.autoStart) {
performDeferredLoadOnStartup();
}
checkThatConnectorsHaveStarted();
this.started = true;
//在控制台打印这句日志，如果在yml设置了上下文，这里会打印
logger.info("Tomcat started on port(s): " + getPortsDescription(true) + " with context path '"
+ getContextPath() + "'");
}
catch (ConnectorStartFailedException ex) {
stopSilently();
throw ex;
}
catch (Exception ex) {
throw new WebServerException("Unable to start embedded Tomcat server", ex);
}
finally {
Context context = findContext();
ContextBindings.unbindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
}
}
}
//关闭tomcat服务
@Override
public void stop() throws WebServerException {
synchronized (this.monitor) {
boolean wasStarted = this.started;
try {
this.started = false;
try {
stopTomcat();
this.tomcat.destroy();
}
catch (LifecycleException ex) {
}
}
catch (Exception ex) {
throw new WebServerException("Unable to stop embedded Tomcat", ex);
}
finally {
if (wasStarted) {
containerCounter.decrementAndGet();
}
}
}
}
TomcatStarter 初始化servlet容器
内嵌tomcat的创建和启动已经看到了，那么它是怎么初始化servlet容器的呢？ springboot 内嵌的 tomcat 并没有完全遵守 servlet3.0 规范，主要区别就是对ServletContainerInitializer这个接口的处理
- 如果是外部tomcat，那么ServletContainerInitializer的实现类是通过SPI服务发现机制进行加载的
- 而内置的tomcat，是专门new了一个
TomcatStarter作为ServletContainerInitializer的实现类进行容器初始化的
ServletContainerInitializer 是 Servlet 3.0 新增的一个接口，WEB容器在启动时使用 JAR 服务 API(JAR Service API) 来发现 ServletContainerInitializer 的实现类，并且容器将 WEB-INF/lib 目录下 JAR 包中的类都交给该类的 onStartup() 方法处理，我们通常需要在该实现类上使用 @HandlesTypes 注解来指定希望被处理的类，过滤掉不希望给 onStartup() 处理的类。
比如spring容器以前是通过web.xml配置servlet监听器实现的，现在按照Servlet 3.0规范，只要专门实现ServletContainerInitializer接口，在其onStartup方法中通过代码将spring的servlet监听器加到ServletContext上下文中即可，当然Spring Web确实实现了ServletContainerInitializer这个接口，但并不是把之前的监听器换种方式加入到Servlet容器这么简单，而是委托给一个新的接口WebApplicationinitializer：
关于servlet3.0 规范，推荐一篇博文：SpringBoot为什么没有web.xml了
继续看，先找到TomcatStarter在哪加载的，入口就是之前获取TomcatWebServer的地方：
//TomcatServletWebServerFactory.java
public WebServer getWebServer(ServletContextInitializer... initializers) {
Tomcat tomcat = new Tomcat();
File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath());
Connector connector = new Connector(this.protocol);
tomcat.getService().addConnector(connector);
customizeConnector(connector);
tomcat.setConnector(connector);
tomcat.getHost().setAutoDeploy(false);
configureEngine(tomcat.getEngine());
for (Connector additionalConnector : this.additionalTomcatConnectors) {
tomcat.getService().addConnector(additionalConnector);
}
//为tomcat准备上下文，可以理解为用代码写web.xml，tomcat启动的时候会去加载这里设置的信息
prepareContext(tomcat.getHost(), initializers);
return getTomcatWebServer(tomcat);
}
//TomcatServletWebServerFactory.java
protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
File documentRoot = getValidDocumentRoot();
//tomcat内嵌上下文
TomcatEmbeddedContext context = new TomcatEmbeddedContext();
if (documentRoot != null) {
context.setResources(new LoaderHidingResourceRoot(context));
}
context.setName(getContextPath());
context.setDisplayName(getDisplayName());
context.setPath(getContextPath());
File docBase = (documentRoot != null) ? documentRoot : createTempDir("tomcat-docbase");
context.setDocBase(docBase.getAbsolutePath());
context.addLifecycleListener(new FixContextListener());
context.setParentClassLoader((this.resourceLoader != null) ? this.resourceLoader.getClassLoader()
: ClassUtils.getDefaultClassLoader());
resetDefaultLocaleMapping(context);
addLocaleMappings(context);
context.setUseRelativeRedirects(false);
try {
context.setCreateUploadTargets(true);
}
catch (NoSuchMethodError ex) {
// Tomcat is < 8.5.39. Continue.
}
configureTldSkipPatterns(context);
WebappLoader loader = new WebappLoader(context.getParentClassLoader());
loader.setLoaderClass(TomcatEmbeddedWebappClassLoader.class.getName());
loader.setDelegate(true);
context.setLoader(loader);
if (isRegisterDefaultServlet()) {
addDefaultServlet(context);
}
if (shouldRegisterJspServlet()) {
addJspServlet(context);
addJasperInitializer(context);
}
context.addLifecycleListener(new StaticResourceConfigurer(context));
ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);
host.addChild(context);
//设置上下文
configureContext(context, initializersToUse);
postProcessContext(context);
}
//TomcatServletWebServerFactory.java
protected void configureContext(Context context, ServletContextInitializer[] initializers) {
//这个TomcatStarter 实现的就是 servlet3.0 规范的ServletContainerInitializer接口
TomcatStarter starter = new TomcatStarter(initializers);
if (context instanceof TomcatEmbeddedContext) {
//如果context是内嵌的tomcat上下文
TomcatEmbeddedContext embeddedContext = (TomcatEmbeddedContext) context;
embeddedContext.setStarter(starter);
embeddedContext.setFailCtxIfServletStartFails(true);
}
//给context添加ServletContainerInitializer!
context.addServletContainerInitializer(starter, NO_CLASSES);
for (LifecycleListener lifecycleListener : this.contextLifecycleListeners) {
context.addLifecycleListener(lifecycleListener);
}
for (Valve valve : this.contextValves) {
context.getPipeline().addValve(valve);
}
for (ErrorPage errorPage : getErrorPages()) {
new TomcatErrorPage(errorPage).addToContext(context);
}
for (MimeMappings.Mapping mapping : getMimeMappings()) {
context.addMimeMapping(mapping.getExtension(), mapping.getMimeType());
}
configureSession(context);
new DisableReferenceClearingContextCustomizer().customize(context);
for (TomcatContextCustomizer customizer : this.tomcatContextCustomizers) {
customizer.customize(context);
}
}
找到TomcatStarter了，就是在这里加载的，但是其onStartup方法是等内嵌的tomcat启动后触发的，需要注意的是内嵌的tomcat只会加载TomcatStarter，不会加载其他的ServletContainerInitializer 的实现类：
class TomcatStarter implements ServletContainerInitializer {
private static final Log logger = LogFactory.getLog(TomcatStarter.class);
private final ServletContextInitializer[] initializers;
private volatile Exception startUpException;
TomcatStarter(ServletContextInitializer[] initializers) {
this.initializers = initializers;
}
@Override
public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
try {
for (ServletContextInitializer initializer : this.initializers) {
//ServletContextInitializer 才是初始化的关键
initializer.onStartup(servletContext);
}
}
catch (Exception ex) {
this.startUpException = ex;
// Prevent Tomcat from logging and re-throwing when we know we can
// deal with it in the main thread, but log for information here.
if (logger.isErrorEnabled()) {
logger.error("Error starting Tomcat context. Exception: " + ex.getClass().getName() + ". Message: "
+ ex.getMessage());
}
}
}
Exception getStartUpException() {
return this.startUpException;
}
}
可以看到TomcatStarter 中的 org.springframework.boot.context.embedded.ServletContextInitializer 是 springboot 初始化 servlet，filter，listener 的关键。
主要关注ServletWebServerApplicationContext，注意图中可以看出是一个lambda表达式，这个ServletContextInitializer实际上是通过ServletWebServerApplicationContext的getSelfInitializer方法创建的匿名内部类实例：
private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
//返回一个ServletContextInitializer的匿名内部类实例
return this::selfInitialize;
}
private void selfInitialize(ServletContext servletContext) throws ServletException {
prepareWebApplicationContext(servletContext);
registerApplicationScope(servletContext);
WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
//核心在这
for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
//getServletContextInitializerBeans返回的是spring容器中所有的ServletContextInitializer的实例
//最终会调用每个ServletContextInitializer的onStartup方法对Servlet容器进行初始化
beans.onStartup(servletContext);
}
}
//先看getServletContextInitializerBeans方法：
protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
//可以看出ServletContextInitializerBeans本质是一个集合
//并且该集合的元素是ServletContextInitializer
//getBeanFactory获取spring容器，这里就可以猜到
//集合中的元素肯定是从Spring容器获取的
return new ServletContextInitializerBeans(getBeanFactory());
}
看一下构造，证实我们的想法：
//ServletContextInitializerBeans.java的构造
//包含的元素是ServletContextInitializer
public ServletContextInitializerBeans(ListableBeanFactory beanFactory,
Class<? extends ServletContextInitializer>... initializerTypes) {
this.initializers = new LinkedMultiValueMap<>();
//initializerTypes 默认是ServletContextInitializer.class
this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)
: Collections.singletonList(ServletContextInitializer.class);
//从spring容器中获取所有类型是ServletContextInitializer的实例
addServletContextInitializerBeans(beanFactory);
addAdaptableBeans(beanFactory);
List<ServletContextInitializer> sortedInitializers = this.initializers.values().stream()
.flatMap((value) -> value.stream().sorted(AnnotationAwareOrderComparator.INSTANCE))
.collect(Collectors.toList());
this.sortedList = Collections.unmodifiableList(sortedInitializers);
logMappings(this.initializers);
}
private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) {
//initializerTypes 默认只有ServletContextInitializer.class
for (Class<? extends ServletContextInitializer> initializerType : this.initializerTypes) {
//getOrderedBeansOfType 方法便是去容器中寻找注册过得 ServletContextInitializer
//其中 RegistrationBean 继承自 ServletContextInitializer 还实现了 Ordered 接口，在这儿用于排序
for (Entry<String, ? extends ServletContextInitializer> initializerBean : getOrderedBeansOfType(beanFactory,
initializerType)) {
// 找到以后遍历每个实例，下面这个方法会根据每个实例具体Class类型进行分类收集
addServletContextInitializerBean(initializerBean.getKey(), initializerBean.getValue(), beanFactory);
}
}
}
private void addServletContextInitializerBean(String beanName, ServletContextInitializer initializer,
ListableBeanFactory beanFactory) {
//根据不同的类型进行分类整理
if (initializer instanceof ServletRegistrationBean) {
//ServletRegistrationBean专门注册Servlet的
Servlet source = ((ServletRegistrationBean<?>) initializer).getServlet();
addServletContextInitializerBean(Servlet.class, beanName, initializer, beanFactory, source);
}
else if (initializer instanceof FilterRegistrationBean) {
//FilterRegistrationBean专门注册Servlet的Filter的
Filter source = ((FilterRegistrationBean<?>) initializer).getFilter();
addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source);
}
else if (initializer instanceof DelegatingFilterProxyRegistrationBean) {
String source = ((DelegatingFilterProxyRegistrationBean) initializer).getTargetBeanName();
addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source);
}
else if (initializer instanceof ServletListenerRegistrationBean) {
//ServletListenerRegistrationBean专门注册Servlet的Listener的
EventListener source = ((ServletListenerRegistrationBean<?>) initializer).getListener();
addServletContextInitializerBean(EventListener.class, beanName, initializer, beanFactory, source);
}
else {
addServletContextInitializerBean(ServletContextInitializer.class, beanName, initializer, beanFactory,
initializer);
}
}
private void addServletContextInitializerBean(Class<?> type, String beanName, ServletContextInitializer initializer,
ListableBeanFactory beanFactory, Object source) {
//最终存放到MultiValueMap，key是类型，value是一个集合
this.initializers.add(type, initializer);
if (source != null) {
// Mark the underlying source as seen in case it wraps an existing bean
this.seen.add(source);
}
if (logger.isTraceEnabled()) {
String resourceDescription = getResourceDescription(beanName, beanFactory);
int order = getOrder(initializer);
logger.trace("Added existing " + type.getSimpleName() + " initializer bean '" + beanName + "'; order="
+ order + ", resource=" + resourceDescription);
}
}
我们看一下ServletContextInitializer到底是什么：
有的人可能不知道RegistrationBean是什么，SpringBoot 加载 Servlet有两种方式：
- 注册方式一：
servlet3.0注解（@WebServlet，@WebFilter，@WebListener…）+ @ServletComponentScan@WebServlet("/hello") public class HelloWorldServlet extends HttpServlet{ ...} @SpringBootApplication @ServletComponentScan public class SpringBootServletApplication { public static void main(String[] args) { SpringApplication.run(SpringBootServletApplication.class, args); } }- 注册方式二：RegistrationBean
ServletRegistrationBean 和 FilterRegistrationBean 都集成自 RegistrationBean ，RegistrationBean 是 springboot 中广泛应用的一个注册类，负责把 servlet，filter，listener 给容器化，使他们被 spring 托管，并且完成自身对 web 容器的注册。@Bean public ServletRegistrationBean helloWorldServlet() { ServletRegistrationBean helloWorldServlet = new ServletRegistrationBean(); myServlet.addUrlMappings("/hello"); myServlet.setServlet(new HelloWorldServlet()); return helloWorldServlet; }
再回到selfInitialize方法，现在我们知道了这里遍历的就是Spring容器中的ServletContextInitializer
//ServletWebServerApplicationContext.java
private void selfInitialize(ServletContext servletContext) throws ServletException {
prepareWebApplicationContext(servletContext);
registerApplicationScope(servletContext);
WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
//核心在这
for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
//getServletContextInitializerBeans返回的是spring容器中所有的ServletContextInitializer的实例
//最终会调用每个ServletContextInitializer的onStartup方法对Servlet容器进行初始化
beans.onStartup(servletContext);
}
}
我们只看一个最常用的ServletContextInitializer的实现，ServletRegistrationBean的onStartup方法：
//RegistrationBean.java
//先调用父类RegistrationBean.onStartup
public final void onStartup(ServletContext servletContext) throws ServletException {
String description = getDescription();
if (!isEnabled()) {
logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
return;
}
//register实现在具体子类
register(description, servletContext);
}
//DynamicRegistrationBean.java
protected final void register(String description, ServletContext servletContext) {
//addRegistration实现在具体子类
D registration = addRegistration(description, servletContext);
if (registration == null) {
logger.info(
StringUtils.capitalize(description) + " was not registered " + "(possibly already registered?)");
return;
}
configure(registration);
}
//ServletRegistrationBean.java
protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
// 真正向Servlet容器添加Servlet的地方！！！
String name = getServletName();
return servletContext.addServlet(name, this.servlet);
}