【Spring Boot】（19）、Spring Boot嵌入式Servlet容器自动配置原理
EmbeddedServletContainerAutoConfiguration是嵌入式Servlet容器的自动配置类，该类在spring-boot-autoconfigure-xxx.jar中的web模块可以找到。
@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();
}
}
//other code...
}
TomcatEmbeddedServletContainerFactory
JettyEmbeddedServletContainerFactory
UndertowEmbeddedServletContainerFactory
这里以大家熟悉的Tomcat为例，首先Spring Boot会判断当前环境中是否引入了Servlet和Tomcat依赖，并且当前容器中没有自定义的EmbeddedServletContainerFactory的情况下，则创建Tomcat容器工厂。其他Servlet容器工厂也是同样的道理。
EmbeddedServletContainerFactory：嵌入式Servlet容器工厂
public interface EmbeddedServletContainerFactory {
EmbeddedServletContainer getEmbeddedServletContainer(
ServletContextInitializer... initializers);
}
该工厂接口主要有三个实现类，分别对应三种嵌入式Servlet容器的工厂类，如图所示：
EmbeddedServletContainer：嵌入式Servlet容器
同样道理，对应三种嵌入式Servlet容器，如图所示：
TomcatEmbeddedServletContainerFactory类为例：
public class TomcatEmbeddedServletContainerFactory
extends AbstractEmbeddedServletContainerFactory implements ResourceLoaderAware {
//other code...
@Override
public EmbeddedServletContainer getEmbeddedServletContainer(
ServletContextInitializer... initializers) {
//创建一个Tomcat
Tomcat tomcat = new Tomcat();
//配置Tomcat的基本环节
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);
}
prepareContext(tomcat.getHost(), initializers);
//包装tomcat对象，返回一个嵌入式Tomcat容器，内部会启动该tomcat容器
return getTomcatEmbeddedServletContainer(tomcat);
}
}TomcatEmbeddedServletContainerFactory#getTomcatEmbeddedServletContainer函数：
protected TomcatEmbeddedServletContainer getTomcatEmbeddedServletContainer(
Tomcat tomcat) {
return new TomcatEmbeddedServletContainer(tomcat, getPort() >= 0);
}该函数很简单，就是来创建Tomcat容器并返回。
TomcatEmbeddedServletContainer类定义：
public class TomcatEmbeddedServletContainer implements EmbeddedServletContainer {
public TomcatEmbeddedServletContainer(Tomcat tomcat, boolean autoStart) {
Assert.notNull(tomcat, "Tomcat Server must not be null");
this.tomcat = tomcat;
this.autoStart = autoStart;
//初始化嵌入式Tomcat容器，并启动Tomcat
initialize();
}
private void initialize() throws EmbeddedServletContainerException {
TomcatEmbeddedServletContainer.logger
.info("Tomcat initialized with port(s): " + getPortsDescription(false));
synchronized (this.monitor) {
try {
addInstanceIdToEngineName();
try {
final Context context = findContext();
context.addLifecycleListener(new LifecycleListener() {
@Override
public void lifecycleEvent(LifecycleEvent event) {
if (context.equals(event.getSource())
&& Lifecycle.START_EVENT.equals(event.getType())) {
// Remove service connectors so that protocol
// binding doesn't happen when the service is
// started.
removeServiceConnectors();
}
}
});
// Start the server to trigger initialization listeners
//启动tomcat
this.tomcat.start();
// We can re-throw failure exception directly in the main thread
rethrowDeferredStartupExceptions();
try {
ContextBindings.bindClassLoader(context, getNamingToken(context),
getClass().getClassLoader());
}
catch (NamingException ex) {
// Naming is not enabled. Continue
}
// Unlike Jetty, all Tomcat threads are daemon threads. We create a
// blocking non-daemon to stop immediate shutdown
startDaemonAwaitThread();
}
catch (Exception ex) {
containerCounter.decrementAndGet();
throw ex;
}
}
catch (Exception ex) {
stopSilently();
throw new EmbeddedServletContainerException(
"Unable to start embedded Tomcat", ex);
}
}
}
}
那么问题来了，我们对嵌入式容器的修改配置是如何生效的？
之前讲过可以通过修改ServerProperties相关配置或者自定义EmbeddedServletContainerCustomizer定制器两种方式来修改默认配置。而ServerProperties其实就是EmbeddedServletContainerCustomizer的子类，所以说到底还是EmbeddedServletContainerCustomizer起了修改的作用。
其实在EmbeddedServletContainerAutoConfiguration类上导入了一个BeanPostProcessorsRegistrar类：
@Import(BeanPostProcessorsRegistrar.class)该类主要用于给容器导入组件，看定义：
public static class BeanPostProcessorsRegistrar
implements ImportBeanDefinitionRegistrar, BeanFactoryAware {
private ConfigurableListableBeanFactory beanFactory;
@Override
public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
if (beanFactory instanceof ConfigurableListableBeanFactory) {
this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;
}
}
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
BeanDefinitionRegistry registry) {
if (this.beanFactory == null) {
return;
}
//注册了一个EmbeddedServletContainerCustomizerBeanPostProcessor后置处理器的Bean
registerSyntheticBeanIfMissing(registry,
"embeddedServletContainerCustomizerBeanPostProcessor",
EmbeddedServletContainerCustomizerBeanPostProcessor.class);
registerSyntheticBeanIfMissing(registry,
"errorPageRegistrarBeanPostProcessor",
ErrorPageRegistrarBeanPostProcessor.class);
}
private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,
String name, Class beanClass) {
if (ObjectUtils.isEmpty(
this.beanFactory.getBeanNamesForType(beanClass, true, false))) {
RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);
beanDefinition.setSynthetic(true);
registry.registerBeanDefinition(name, beanDefinition);
}
}
}后置处理器：在bean初始化前(创建完成，还未属性赋值)，会执行初始化工作。
registerBeanDefinitions方法中向容器中导入了EmbeddedServletContainerCustomizerBeanPostProcessor类型的Bean：
public class EmbeddedServletContainerCustomizerBeanPostProcessor
implements BeanPostProcessor, BeanFactoryAware {
//初始化之前
@Override
public Object postProcessBeforeInitialization(Object bean, String beanName)
throws BeansException {
if (bean instanceof ConfigurableEmbeddedServletContainer) {
postProcessBeforeInitialization((ConfigurableEmbeddedServletContainer) bean);
}
return bean;
}
private void postProcessBeforeInitialization(
ConfigurableEmbeddedServletContainer bean) {
//获取所有的定制器，调用每个定制器的customize方法，给Servlet容器进行属性赋值
for (EmbeddedServletContainerCustomizer customizer : getCustomizers()) {
customizer.customize(bean);
}
}
//从IOC容器中获取所有类型为EmbeddedServletContainerCustomizer的定制器
private Collection getCustomizers() {
if (this.customizers == null) {
// Look up does not include the parent context
this.customizers = new ArrayList(
//类型为EmbeddedServletContainerCustomizer的Bean
this.beanFactory
.getBeansOfType(EmbeddedServletContainerCustomizer.class,
false, false)
.values());
Collections.sort(this.customizers, AnnotationAwareOrderComparator.INSTANCE);
this.customizers = Collections.unmodifiableList(this.customizers);
}
return this.customizers;
}
//other code...
} EmbeddedServletContainerCustomizer类型的组件，用于自定义属性配置，然后在导入的后置处理器中获取到该组件，并调用该自定义组件的customize方法，来修改默认的属性配置。
总结：
（1）、Spring Boot根据导入容器类型的依赖情况，会给容器中添加相应的EmbeddedServletContainerFactory嵌入式Servlet容器工厂；
（2）、应用程序一旦导入了嵌入式Servlet容器依赖，就会触动后置处理器EmbeddedServletContainerCustomizerBeanPostProcessor；
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