本文基于SpringBoot 2.5.0-M2講解Spring中Lifecycle和SmartLifecycle的作用和區(qū)別，以及如何控制SmartLifecycle的優(yōu)先級。

并講解SpringBoot中如何通過SmartLifecycle來啟動/停止web容器.

SmartLifecycle & Lifecycle作用和區(qū)別

SmartLifecycle和Lifecycle作用

都是讓開發(fā)者可以在所有的bean都創(chuàng)建完成(getBean) 之后執(zhí)行自己的初始化工作，或者在退出時執(zhí)行資源銷毀工作。

SmartLifecycle和Lifecycle區(qū)別

SmartLifecycle接口繼承Lifecycle接口，同時繼承了
org.springframework.context.Phased接口用于控制多個SmartLifecycle實現(xiàn)之間的優(yōu)先級。

在SpringBoot應(yīng)用中，或在Spring應(yīng)用中沒有調(diào)用
AbstractApplicationContext#start方法，如果一個Bean只是實現(xiàn)了Lifecycle接口的情況下:

不會執(zhí)行Lifecycle接口中的啟動方法，包括Lifecycle#isRunning方法也不會被執(zhí)行。

但是在應(yīng)用 退出時 會執(zhí)行Lifecycle#isRunning方法判斷該Lifecycle是否已經(jīng)啟動，如果返回true則調(diào)用Lifecycle#stop()停止方法。

如果一個Bean實現(xiàn)了SmartLifecycle接口，則會執(zhí)行啟動方法。先會被根據(jù)Phased接口優(yōu)先級分組，封裝在LifecycleGroup，然后循環(huán)調(diào)用LifecycleGroup#start()方法，SmartLifecycle#isRunning判斷是否已經(jīng)執(zhí)行，返回false表示還未執(zhí)行，則調(diào)用SmartLifecycle#start()執(zhí)行。Phased返回值越小，優(yōu)先級越高。

SmartLifecycle中還有個isAutoStartup方法，如果返回false，在啟動時也不會執(zhí)行start方法，默認(rèn)返回true

源碼分析

SmartLifecycle和Lifecycle都是在
org.springframework.context.support.DefaultLifecycleProcessor中被調(diào)用，

DefaultLifecycleProcessor#onRefresh方法在執(zhí)行
AbstractApplicationContext#finishRefresh時會被調(diào)用，調(diào)用棧如下:

startBeans:142, DefaultLifecycleProcessor (org.springframework.context.support)
onRefresh:123, DefaultLifecycleProcessor (org.springframework.context.support)
finishRefresh:934, AbstractApplicationContext (org.springframework.context.support)
refresh:585, AbstractApplicationContext (org.springframework.context.support)
refresh:144, ServletWebServerApplicationContext (org.springframework.boot.web.servlet.context)
refresh:755, SpringApplication (org.springframework.boot)
refreshContext:426, SpringApplication (org.springframework.boot)
run:326, SpringApplication (org.springframework.boot)
run:1299, SpringApplication (org.springframework.boot)
run:1288, SpringApplication (org.springframework.boot)
main:31, DemoApplication (com.example.demo)

DefaultLifecycleProcessor#onRefresh源碼:

@Override
public void onRefresh() {
	startBeans(true);  //autoStartupOnly = true
	this.running = true;
}

DefaultLifecycleProcessor#startBeans源碼如下:

autoStartupOnly 在onRefresh時傳入的是true，表示只執(zhí)行可以自動啟動的bean，即為：SmartLifecycle的實現(xiàn)類，并且SmartLifecycle#isAutoStartup返回值必須為true。

private void startBeans(boolean autoStartupOnly) {
	Map<String, Lifecycle> lifecycleBeans = getLifecycleBeans();
	Map<Integer, LifecycleGroup> phases = new TreeMap<>();

	lifecycleBeans.forEach((beanName, bean) -> {
		if (!autoStartupOnly || (bean instanceof SmartLifecycle && ((SmartLifecycle) bean).isAutoStartup())) {
			int phase = getPhase(bean);
			phases.computeIfAbsent(phase, p -> 
			    new LifecycleGroup(phase, this.timeoutPerShutdownPhase, lifecycleBeans, autoStartupOnly)
			).add(beanName, bean);
		}
	});
	if (!phases.isEmpty()) {
		phases.values().forEach(LifecycleGroup::start);
	}
}

而Spring
AbstractApplicationContext#doClose退出時，無論是SmartLifecycle或Lifecycle都會執(zhí)行isRunning方法，判斷是否已經(jīng)啟動，返回true表示已經(jīng)啟動，則執(zhí)行SmartLifecycle或Lifecycle的stop方法。

源碼見
:org.springframework.context.support.DefaultLifecycleProcessor#doStop方法。

而執(zhí)行
AbstractApplicationContext#doClose一般是應(yīng)用進程退出，通過jvm注冊的鉤子方法，或者應(yīng)用程序編碼調(diào)用。

AbstractApplicationContext#registerShutdownHook源碼

@Override
public void registerShutdownHook() {
	if (this.shutdownHook == null) {
		// No shutdown hook registered yet.
		this.shutdownHook = new Thread(SHUTDOWN_HOOK_THREAD_NAME) {
			@Override
			public void run() {
				synchronized (startupShutdownMonitor) {
					doClose();
				}
			}
		};
		Runtime.getRuntime().addShutdownHook(this.shutdownHook);
	}
}

自定義LifecycleProcessor處理Lifecycle

在源碼分析中提到了DefaultLifecycleProcessor，其實現(xiàn)了LifecycleProcessor接口。然而我們自己也可以實現(xiàn)該接口，替換默認(rèn)的DefaultLifecycleProcessor。SpringBoot中則是自己配置了DefaultLifecycleProcessor，我們可以按照同樣的方式，覆蓋默認(rèn)的實現(xiàn)。例如可以讓Lifecycle中的start()方法在onRefresh()時也能被執(zhí)行。

org.springframework.boot.autoconfigure.context.LifecycleAutoConfiguration源碼:

/**
 * {@link EnableAutoConfiguration Auto-configuration} relating to the application
 * context's lifecycle.
 *
 * @author Andy Wilkinson
 * @since 2.3.0
 */
@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(LifecycleProperties.class)
public class LifecycleAutoConfiguration {

	@Bean(name = AbstractApplicationContext.LIFECYCLE_PROCESSOR_BEAN_NAME)
	@ConditionalOnMissingBean(name = AbstractApplicationContext.LIFECYCLE_PROCESSOR_BEAN_NAME,
			search = SearchStrategy.CURRENT)
	public DefaultLifecycleProcessor defaultLifecycleProcessor(LifecycleProperties properties) {
		DefaultLifecycleProcessor lifecycleProcessor = new DefaultLifecycleProcessor();
		lifecycleProcessor.setTimeoutPerShutdownPhase(properties.getTimeoutPerShutdownPhase().toMillis());
		return lifecycleProcessor;
	}
}

SpringBoot中內(nèi)嵌web容器啟動時機

SpringBoo中就是通過實現(xiàn)SmartLifecycle來啟動內(nèi)嵌的web容器，實現(xiàn)類為
WebServerStartStopLifecycle。

ServletWebServerApplicationContext在onRefresh方法中調(diào)用createWebServer，createWebServer方法中創(chuàng)建org.springframework.boot.web.server.WebServer實例，該對象則包含了控制web容器(tomcat、jetty)的啟動與停止方法。

@Override
protected void onRefresh() {
	super.onRefresh();
	try {
		createWebServer();
	}catch (Throwable ex) {
		throw new ApplicationContextException("Unable to start web server", ex);
	}
}

ServletWebServerApplicationContext#createWebServer源碼:

private void createWebServer() {
	WebServer webServer = this.webServer;
	ServletContext servletContext = getServletContext();
	if (webServer == null && servletContext == null) {
		StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
		ServletWebServerFactory factory = getWebServerFactory();
		createWebServer.tag("factory", factory.getClass().toString());
		this.webServer = factory.getWebServer(getSelfInitializer());
		createWebServer.end();
		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();
}

createWebServer方法會將創(chuàng)建的webServer封裝在
WebServerStartStopLifecycle對象中，并注冊到Spring容器中。

org.springframework.boot.web.servlet.context.WebServerStartStopLifecycle源碼如下:

class WebServerStartStopLifecycle implements SmartLifecycle {

	private final ServletWebServerApplicationContext applicationContext;
	private final WebServer webServer;
	private volatile boolean running;

	WebServerStartStopLifecycle(ServletWebServerApplicationContext applicationContext, WebServer webServer) {
		this.applicationContext = applicationContext;
		this.webServer = webServer;
	}

	@Override
	public void start() {
		this.webServer.start();
		this.running = true;
		this.applicationContext
				.publishEvent(new ServletWebServerInitializedEvent(this.webServer, this.applicationContext));
	}

	@Override
	public void stop() {  this.webServer.stop();   }

	@Override
	public boolean isRunning() {	return this.running;  }

	@Override
	public int getPhase() {    return Integer.MAX_VALUE - 1;  }
}

WebServerStartStopLifecycle則實現(xiàn)了SmartLifecycle接口，當(dāng)Spring回調(diào)到SmartLifecycle接口方法時則調(diào)用this.webServer.start();啟動web容器，web容器啟動完成之后會通過applicationContext發(fā)布ServletWebServerInitializedEvent事件，表示web容器啟動成功，可以接收http請求。

和
SmartInitializingSingleton區(qū)別

相同點：
SmartInitializingSingleton和Lifecycle、SmartLifecycle都是在所有的單實例bean創(chuàng)建(getBean方法)之后執(zhí)行。

不同點：

SmartInitializingSingleton優(yōu)先于Lifecycle、SmartLifecycle執(zhí)行。

SmartInitializingSingleton只有一個afterSingletonsInstantiated方法。而Lifecycle有start,stop,isRunning等方法。

多個
SmartInitializingSingleton實現(xiàn)之間無法排序控制執(zhí)行的順序，而SmartLifecycle實現(xiàn)了Phased接口，可以通過int getPhase()控制執(zhí)行循序。

SmartInitializingSingleton之間可以通過@DependsOn來控制執(zhí)行順序，但這是由Spring中@DependsOn注解的作用及原理來實現(xiàn)的. 并不是對SmartInitializingSingleton做了排序。

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版權(quán)聲明：本文為CSDN博主「brucelwl」的原創(chuàng)文章，遵循CC 4.0 BY-SA版權(quán)協(xié)議，轉(zhuǎn)載請附上原文出處鏈接及本聲明。

原文鏈接：
https://blog.csdn.net/u013202238/article/details/114489001