Spring Boot整合Web容器的原理（以Tomcat为例）

在Spring Boot中可以不显式使用Tomcat一类的Web容器，Spring Boot会自动启用配置的容器，启动方法和直接启动容器有很大的不同，下面从源码角度分析一下容器在Spring Boot中是如何启动的。

首先是容器的注册，ServletWebServerFactoryConfiguration是Spring Boot提供的一个配置类，在这个类里默认注册了三种容器工厂TomcatServletWebServerFactory，JettyServletWebServerFactory，UndertowServletWebServerFactory，这三种都以Bean的形式加入到了Spring的Bean容器中，以供接下来的程序调用。

ServletWebServerFactoryConfiguration.java

@Configuration
class ServletWebServerFactoryConfiguration {

    @Configuration
    @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedTomcat {

        @Bean
        public TomcatServletWebServerFactory tomcatServletWebServerFactory() {
            return new TomcatServletWebServerFactory();
        }

    }

    /**
     * Nested configuration if Jetty is being used.
     */
    @Configuration
    @ConditionalOnClass({ Servlet.class, Server.class, Loader.class,
            WebAppContext.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedJetty {

        @Bean
        public JettyServletWebServerFactory JettyServletWebServerFactory() {
            return new JettyServletWebServerFactory();
        }

    }

    /**
     * Nested configuration if Undertow is being used.
     */
    @Configuration
    @ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedUndertow {

        @Bean
        public UndertowServletWebServerFactory undertowServletWebServerFactory() {
            return new UndertowServletWebServerFactory();
        }

    }

}

Spring Boot在ServletWebServerApplicationContext中重载了onRefresh方法，除了以前Spring默认的onRefresh方法外还增加了createWebServer方法，在这个方法中对Web容器进行了初始化工作。

ServletWebServerApplicationContext.java

    @Override
    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) {
		    // 获取设置的容器工厂
			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();
	}

    protected ServletWebServerFactory getWebServerFactory() {
        // Use bean names so that we don't consider the hierarchy
        // 从bean容器中获取所有的容器工厂类型
        String[] beanNames = getBeanFactory()
                .getBeanNamesForType(ServletWebServerFactory.class);
        // 如果发现没有注册有效的Web容器工厂就报错
        if (beanNames.length == 0) {
            throw new ApplicationContextException(
                    "Unable to start ServletWebServerApplicationContext due to missing "
                            + "ServletWebServerFactory bean.");
        }
        // 如果发现使用了多个Web容器工厂同样报错
        if (beanNames.length > 1) {
            throw new ApplicationContextException(
                    "Unable to start ServletWebServerApplicationContext due to multiple "
                            + "ServletWebServerFactory beans : "
                            + StringUtils.arrayToCommaDelimitedString(beanNames));
        }
        // 返回获取到的Web容器工厂
        return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);
    }

在ServletWebServerApplicationContext类中获取到相应的容器工厂后会调用容器工厂的getWebServer方法，下面以Tomcat为例说明容器工厂的作用。

TomcatServletWebServerFactory.java

    @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);
    }

    protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
        return new TomcatWebServer(tomcat, getPort() >= 0);
    }

通过getWebServer方法会new一个TomcatWebServer对象，new对象的方法会调用initialize方法，在这个方法中会对容器进行初始化并启动。

TomcatWebServer.java

    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 {
        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())) {
                        // Remove service connectors so that protocol binding doesn't
                        // happen when the service is started.
                        removeServiceConnectors();
                    }
                });

                // 启动tomcat容器
                this.tomcat.start();
                ...
            }
            catch (Exception ex) {
                stopSilently();
                throw new WebServerException("Unable to start embedded Tomcat", ex);
            }
        }
    }

至此tomcat容器便启动完毕。