前言
我们知道SpringBoot给我们带来了一个全新的开发体验,我们可以直接把web程序达成jar包,直接启动,这就得益于SpringBoot内置了容器,可以直接启动,本文将以Tomcat为例,来看看SpringBoot是如何启动Tomcat的,同时也将展开学习下Tomcat的源码,了解Tomcat的设计。
从 Main 方法说起
用过SpringBoot的人都知道,首先要写一个main方法来启动
- @SpringBootApplication
- public class TomcatdebugApplication {
- public static void main(String[] args) {
- SpringApplication.run(TomcatdebugApplication.class, args);
- }
- }
我们直接点击run方法的源码,跟踪下来,发下最终 的run方法是调用ConfigurableApplicationContext方法,源码如下:
- public ConfigurableApplicationContext run(String... args) {
- StopWatch stopWatch = new StopWatch();
- stopWatch.start();
- ConfigurableApplicationContext context = null;
- Collection<springbootexceptionreporter> exceptionReporters = new ArrayList<>();
- //设置系统属性『java.awt.headless』,为true则启用headless模式支持
- configureHeadlessProperty();
- //通过*SpringFactoriesLoader*检索*META-INF/spring.factories*,
- //找到声明的所有SpringApplicationRunListener的实现类并将其实例化,
- //之后逐个调用其started()方法,广播SpringBoot要开始执行了
- SpringApplicationRunListeners listeners = getRunListeners(args);
- //发布应用开始启动事件
- listeners.starting();
- try {
- //初始化参数
- ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
- //创建并配置当前SpringBoot应用将要使用的Environment(包括配置要使用的PropertySource以及Profile),
- //并遍历调用所有的SpringApplicationRunListener的environmentPrepared()方法,广播Environment准备完毕。
- ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
- configureIgnoreBeanInfo(environment);
- //打印banner
- Banner printedBanner = printBanner(environment);
- //创建应用上下文
- context = createApplicationContext();
- //通过*SpringFactoriesLoader*检索*META-INF/spring.factories*,获取并实例化异常分析器
- exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
- new Class[] { ConfigurableApplicationContext.class }, context);
- //为ApplicationContext加载environment,之后逐个执行ApplicationContextInitializer的initialize()方法来进一步封装ApplicationContext,
- //并调用所有的SpringApplicationRunListener的contextPrepared()方法,【EventPublishingRunListener只提供了一个空的contextPrepared()方法】,
- //之后初始化IoC容器,并调用SpringApplicationRunListener的contextLoaded()方法,广播ApplicationContext的IoC加载完成,
- //这里就包括通过**@EnableAutoConfiguration**导入的各种自动配置类。
- prepareContext(context, environment, listeners, applicationArguments, printedBanner);
- //刷新上下文
- refreshContext(context);
- //再一次刷新上下文,其实是空方法,可能是为了后续扩展。
- afterRefresh(context, applicationArguments);
- stopWatch.stop();
- if (this.logStartupInfo) {
- new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
- }
- //发布应用已经启动的事件
- listeners.started(context);
- //遍历所有注册的ApplicationRunner和CommandLineRunner,并执行其run()方法。
- //我们可以实现自己的ApplicationRunner或者CommandLineRunner,来对SpringBoot的启动过程进行扩展。
- callRunners(context, applicationArguments);
- }
- catch (Throwable ex) {
- handleRunFailure(context, ex, exceptionReporters, listeners);
- throw new IllegalStateException(ex);
- }
- try {
- //应用已经启动完成的监听事件
- listeners.running(context);
- }
- catch (Throwable ex) {
- handleRunFailure(context, ex, exceptionReporters, null);
- throw new IllegalStateException(ex);
- }
- return context;
- }
其实这个方法我们可以简单的总结下步骤为 > 1. 配置属性 > 2. 获取监听器,发布应用开始启动事件 > 3. 初始化输入参数 > 4. 配置环境,输出banner > 5. 创建上下文 > 6. 预处理上下文 > 7. 刷新上下文 > 8. 再刷新上下文 > 9. 发布应用已经启动事件 > 10. 发布应用启动完成事件
其实上面这段代码,如果只要分析tomcat内容的话,只需要关注两个内容即可,上下文是如何创建的,上下文是如何刷新的,分别对应的方法就是createApplicationContext() 和refreshContext(context),接下来我们来看看这两个方法做了什么。
- protected ConfigurableApplicationContext createApplicationContext() {
- Class<!--?--> contextClass = this.applicationContextClass;
- if (contextClass == null) {
- try {
- switch (this.webApplicationType) {
- case SERVLET:
- contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
- break;
- case REACTIVE:
- contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
- break;
- default:
- 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);
- }
这里就是根据我们的webApplicationType 来判断创建哪种类型的Servlet,代码中分别对应着Web类型(SERVLET),响应式Web类型(REACTIVE),非Web类型(default),我们建立的是Web类型,所以肯定实例化 DEFAULT_SERVLET_WEB_CONTEXT_CLASS指定的类,也就是AnnotationConfigServletWebServerApplicationContext类,我们来用图来说明下这个类的关系
通过这个类图我们可以知道,这个类继承的是ServletWebServerApplicationContext,这就是我们真正的主角,而这个类最终是继承了AbstractApplicationContext,了解完创建上下文的情况后,我们再来看看刷新上下文,相关代码如下:
- //类:SpringApplication.java
- private void refreshContext(ConfigurableApplicationContext context) {
- //直接调用刷新方法
- refresh(context);
- if (this.registerShutdownHook) {
- try {
- context.registerShutdownHook();
- }
- catch (AccessControlException ex) {
- // Not allowed in some environments.
- }
- }
- }
- //类:SpringApplication.java
- protected void refresh(ApplicationContext applicationContext) {
- Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
- ((AbstractApplicationContext) applicationContext).refresh();
- }
这里还是直接传递调用本类的refresh(context)方法,最后是强转成父类AbstractApplicationContext调用其refresh()方法,该代码如下:
- // 类:AbstractApplicationContext
- public void refresh() throws BeansException, IllegalStateException {
- synchronized (this.startupShutdownMonitor) {
- // Prepare this context for refreshing.
- prepareRefresh();
- // Tell the subclass to refresh the internal bean factory.
- ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
- // Prepare the bean factory for use in this context.
- prepareBeanFactory(beanFactory);
- try {
- // Allows post-processing of the bean factory in context subclasses.
- postProcessBeanFactory(beanFactory);
- // Invoke factory processors registered as beans in the context.
- invokeBeanFactoryPostProcessors(beanFactory);
- // Register bean processors that intercept bean creation.
- registerBeanPostProcessors(beanFactory);
- // Initialize message source for this context.
- initMessageSource();
- // Initialize event multicaster for this context.
- initApplicationEventMulticaster();
- // Initialize other special beans in specific context subclasses.这里的意思就是调用各个子类的onRefresh()
- onRefresh();
- // Check for listener beans and register them.
- registerListeners();
- // Instantiate all remaining (non-lazy-init) singletons.
- finishBeanFactoryInitialization(beanFactory);
- // Last step: publish corresponding event.
- finishRefresh();
- }
- catch (BeansException ex) {
- if (logger.isWarnEnabled()) {
- logger.warn("Exception encountered during context initialization - " +
- "cancelling refresh attempt: " + ex);
- }
- // Destroy already created singletons to avoid dangling resources.
- destroyBeans();
- // Reset 'active' flag.
- cancelRefresh(ex);
- // Propagate exception to caller.
- throw ex;
- }
- finally {
- // Reset common introspection caches in Spring's core, since we
- // might not ever need metadata for singleton beans anymore...
- resetCommonCaches();
- }
- }
- }
这里我们看到onRefresh()方法是调用其子类的实现,根据我们上文的分析,我们这里的子类是ServletWebServerApplicationContext。
- //类:ServletWebServerApplicationContext
- protected void onRefresh() {
- super.onRefresh();
- try {
- createWebServer();
- }
- catch (Throwable ex) {
- throw new ApplicationContextException("Unable to start web server", ex);
- }
- }
- private void createWebServer() {
- WebServer webServer = this.webServer;
- ServletContext servletContext = getServletContext();
- if (webServer == null && servletContext == null) {
- ServletWebServerFactory factory = getWebServerFactory();
- this.webServer = factory.getWebServer(getSelfInitializer());
- }
- else if (servletContext != null) {
- try {
- getSelfInitializer().onStartup(servletContext);
- }
- catch (ServletException ex) {
- throw new ApplicationContextException("Cannot initialize servlet context", ex);
- }
- }
- initPropertySources();
- }
到这里,其实庐山真面目已经出来了,createWebServer()就是启动web服务,但是还没有真正启动Tomcat,既然webServer是通过ServletWebServerFactory来获取的,我们就来看看这个工厂的真面目。
走进Tomcat内部
根据上图我们发现,工厂类是一个接口,各个具体服务的实现是由各个子类来实现的,所以我们就去看看TomcatServletWebServerFactory.getWebServer()的实现。
- @Override
- 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);
- }
- prepareContext(tomcat.getHost(), initializers);
- return getTomcatWebServer(tomcat);
- }
根据上面的代码,我们发现其主要做了两件事情,第一件事就是把Connnctor(我们称之为连接器)对象添加到Tomcat中,第二件事就是configureEngine,这连接器我们勉强能理解(不理解后面会述说),那这个Engine是什么呢?我们查看tomcat.getEngine()的源码:
- 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;
- }
根据上面的源码,我们发现,原来这个Engine是容器,我们继续跟踪源码,找到Container接口
上图中,我们看到了4个子接口,分别是Engine,Host,Context,Wrapper。我们从继承关系上可以知道他们都是容器,那么他们到底有啥区别呢?我看看他们的注释是怎么说的。
- /**
- If used, an Engine is always the top level Container in a Catalina
- * hierarchy. Therefore, the implementation's <code>setParent()</code> method
- * should throw <code>IllegalArgumentException</code>.
- *
- * @author Craig R. McClanahan
- */
- public interface Engine extends Container {
- //省略代码
- }
- /**
- * <p>
- * The parent Container attached to a Host is generally an Engine, but may
- * be some other implementation, or may be omitted if it is not necessary.
- * </p><p>
- * The child containers attached to a Host are generally implementations
- * of Context (representing an individual servlet context).
- *
- * @author Craig R. McClanahan
- */
- public interface Host extends Container {
- //省略代码
- }
- /*** </p><p>
- * The parent Container attached to a Context is generally a Host, but may
- * be some other implementation, or may be omitted if it is not necessary.
- * </p><p>
- * The child containers attached to a Context are generally implementations
- * of Wrapper (representing individual servlet definitions).
- * </p><p>
- *
- * @author Craig R. McClanahan
- */
- public interface Context extends Container, ContextBind {
- //省略代码
- }
- /**</p><p>
- * The parent Container attached to a Wrapper will generally be an
- * implementation of Context, representing the servlet context (and
- * therefore the web application) within which this servlet executes.
- * </p><p>
- * Child Containers are not allowed on Wrapper implementations, so the
- * <code>addChild()</code> method should throw an
- * <code>IllegalArgumentException</code>.
- *
- * @author Craig R. McClanahan
- */
- public interface Wrapper extends Container {
- //省略代码
- }
上面的注释翻译过来就是,Engine是最高级别的容器,其子容器是Host,Host的子容器是Context,Wrapper是Context的子容器,所以这4个容器的关系就是父子关系,也就是Engine>Host>Context>Wrapper。 我们再看看Tomcat类的源码:
- //部分源码,其余部分省略。
- public class Tomcat {
- //设置连接器
- public void setConnector(Connector connector) {
- Service service = getService();
- boolean found = false;
- for (Connector serviceConnector : service.findConnectors()) {
- if (connector == serviceConnector) {
- found = true;
- }
- }
- if (!found) {
- service.addConnector(connector);
- }
- }
- //获取service
- public Service getService() {
- return getServer().findServices()[0];
- }
- //设置Host容器
- public void setHost(Host host) {
- Engine engine = getEngine();
- boolean found = false;
- for (Container engineHost : engine.findChildren()) {
- if (engineHost == host) {
- found = true;
- }
- }
- if (!found) {
- engine.addChild(host);
- }
- }
- //获取Engine容器
- 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;
- }
- //获取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;
- }
- //添加Context容器
- public Context addContext(Host host, String contextPath, String contextName,
- String dir) {
- silence(host, contextName);
- Context ctx = createContext(host, contextPath);
- ctx.setName(contextName);
- ctx.setPath(contextPath);
- ctx.setDocBase(dir);
- ctx.addLifecycleListener(new FixContextListener());
- if (host == null) {
- getHost().addChild(ctx);
- } else {
- host.addChild(ctx);
- }
- //添加Wrapper容器
- public static Wrapper addServlet(Context ctx,
- String servletName,
- Servlet servlet) {
- // will do class for name and set init params
- Wrapper sw = new ExistingStandardWrapper(servlet);
- sw.setName(servletName);
- ctx.addChild(sw);
- return sw;
- }
- }
阅读Tomcat的getServer()我们可以知道,Tomcat的最顶层是Server,Server就是Tomcat的实例,一个Tomcat一个Server;通过getEngine()我们可以了解到Server下面是Service,而且是多个,一个Service代表我们部署的一个应用,而且我们还可以知道,Engine容器,一个service只有一个;根据父子关系,我们看setHost()源码可以知道,host容器有多个;同理,我们发现addContext()源码下,Context也是多个;addServlet()表明Wrapper容器也是多个,而且这段代码也暗示了,其实Wrapper和Servlet是一层意思。另外我们根据setConnector源码可以知道,连接器(Connector)是设置在service下的,而且是可以设置多个连接器(Connector)。
根据上面分析,我们可以小结下: Tomcat主要包含了2个核心组件,连接器(Connector)和容器(Container),用图表示如下:
一个Tomcat是一个Server,一个Server下有多个service,也就是我们部署的多个应用,一个应用下有多个连接器(Connector)和一个容器(Container),容器下有多个子容器,关系用图表示如下:
Engine下有多个Host子容器,Host下有多个Context子容器,Context下有多个Wrapper子容器。
总结
SpringBoot的启动是通过new SpringApplication()实例来启动的,启动过程主要做如下几件事情: > 1. 配置属性 > 2. 获取监听器,发布应用开始启动事件 > 3. 初始化输入参数 > 4. 配置环境,输出banner > 5. 创建上下文 > 6. 预处理上下文 > 7. 刷新上下文 > 8. 再刷新上下文 > 9. 发布应用已经启动事件 > 10. 发布应用启动完成事件
而启动Tomcat就是在第7步中“刷新上下文”;Tomcat的启动主要是初始化2个核心组件,连接器(Connector)和容器(Container),一个Tomcat实例就是一个Server,一个Server包含多个Service,也就是多个应用程序,每个Service包含多个连接器(Connetor)和一个容器(Container),而容器下又有多个子容器,按照父子关系分别为:Engine,Host,Context,Wrapper,其中除了Engine外,其余的容器都是可以有多个。
下期展望
本期文章通过SpringBoot的启动来窥探了Tomcat的内部结构,下一期,我们来分析下本次文章中的连接器(Connetor)和容器(Container)的作用,敬请期待。