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.
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。
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
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);
}
}
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;
}
}
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。
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。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。
