Spring中SmartLifecycle和Lifecycle的作用和区别

本文基于SpringBoot 2.5.0-M2讲解Spring中Lifecycle和SmartLifecycle的作用和区别,以及如何控制SmartLifecycle的优先级。
并讲解SpringBoot中如何通过SmartLifecycle来启动/停止web容器.

 SmartLifecycle和Lifecycle作用

都是让开发者可以在所有的bean都创建完成(getBean) 之后执行自己的初始化工作,或者在退出时执行资源销毁工作

SmartLifecycle和Lifecycle区别

 1.SmartLifecycle接口继承Lifecycle接口,同时继承了org.springframework.context.Phased接口用于控制多个SmartLifecycle实现之间的优先级。

2.在SpringBoot应用中,或在Spring应用中没有调用AbstractApplicationContext#start方法,如果一个Bean只是实现了Lifecycle接口的情况下:

不会执行Lifecycle接口中的启动方法,包括Lifecycle#isRunning方法也不会被执行。

但是在应用 退出时 会执行Lifecycle#isRunning方法判断该Lifecycle是否已经启动,如果返回true则调用Lifecycle#stop()停止方法

3. 如果一个Bean实现了SmartLifecycle接口,则会执行启动方法。先会被根据Phased接口优先级分组,封装在LifecycleGroup,然后循环调用LifecycleGroup#start()方法,SmartLifecycle#isRunning判断是否已经执行,返回false表示还未执行,则调用SmartLifecycle#start()执行。Phased返回值越小,优先级越高。

4.SmartLifecycle中还有个isAutoStartup方法,如果返回false,在启动时也不会执行start方法,默认返回true

源码分析

SmartLifecycle和Lifecycle都是在org.springframework.context.support.DefaultLifecycleProcessor中被调用,
DefaultLifecycleProcessor#onRefresh方法在执行AbstractApplicationContext#finishRefresh时会被调用,调用栈如下:

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源码:

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

DefaultLifecycleProcessor#startBeans源码如下:
autoStartupOnly 在onRefresh时传入的是true,表示只执行可以自动启动的bean,即为:SmartLifecycle的实现类,并且SmartLifecycle#isAutoStartup返回值必须为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退出时,无论是SmartLifecycle或Lifecycle都会执行isRunning方法,判断是否已经启动,返回true表示已经启动,则执行SmartLifecycle或Lifecycle的stop方法。
源码见:org.springframework.context.support.DefaultLifecycleProcessor#doStop方法。

而执行AbstractApplicationContext#doClose一般是应用进程退出,通过jvm注册的钩子方法,或者应用程序编码调用。
AbstractApplicationContext#registerShutdownHook源码

@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);
	}
}

自定义LifecycleProcessor处理Lifecycle

在源码分析中提到了DefaultLifecycleProcessor,其实现了LifecycleProcessor接口。然而我们自己也可以实现该接口,替换默认的DefaultLifecycleProcessor。SpringBoot中则是自己配置了DefaultLifecycleProcessor,我们可以按照同样的方式,覆盖默认的实现。例如可以让Lifecycle中的start()方法在onRefresh()时也能被执行。

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

/**
 * {@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中内嵌web容器启动时机

SpringBoo中就是通过实现SmartLifecycle来启动内嵌的web容器,实现类为WebServerStartStopLifecycle。

ServletWebServerApplicationContext在onRefresh方法中调用createWebServer,createWebServer方法中创建org.springframework.boot.web.server.WebServer实例,该对象则包含了控制web容器(tomcat、jetty)的启动与停止方法。

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

ServletWebServerApplicationContext#createWebServer源码:

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方法会将创建的webServer封装在WebServerStartStopLifecycle对象中,并注册到Spring容器中。

org.springframework.boot.web.servlet.context.WebServerStartStopLifecycle源码如下:

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则实现了SmartLifecycle接口,当Spring回调到SmartLifecycle接口方法时则调用this.webServer.start();启动web容器,web容器启动完成之后会通过applicationContext发布ServletWebServerInitializedEvent事件,表示web容器启动成功,可以接收http请求。

和SmartInitializingSingleton区别

相同点:SmartInitializingSingleton和Lifecycle、SmartLifecycle都是在所有的单实例bean创建(getBean方法)之后执行。

不同点:

  • SmartInitializingSingleton优先于Lifecycle、SmartLifecycle执行。
  • SmartInitializingSingleton只有一个afterSingletonsInstantiated方法。而Lifecycle有start,stop,isRunning等方法。
  • 多个SmartInitializingSingleton实现之间无法排序控制执行的顺序,而SmartLifecycle实现了Phased接口,可以通过int getPhase()控制执行循序。
  • SmartInitializingSingleton之间可以通过@DependsOn来控制执行顺序,但这是由Spring中@DependsOn注解的作用及原理来实现的. 并不是对SmartInitializingSingleton做了排序。

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