Spring Boot学习笔记03--深入了解SpringBoot的启动过程

摘要

看完本文你将掌握如下知识点:

  1. SpringApplication的作用及运行过程
  2. SpringBootServletInitializer的作用及运行过程

PS:本节内容略显枯燥,如果对SpringBoot的启动过程不感兴趣,可以略过。


SpringBoot系列Spring Boot学习笔记


深入了解SpringApplication

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@SpringBootApplication
public class SpringBootWebDemoApplication {
public static void main(String[] args) {
SpringApplication.run(SpringBootWebDemoApplication.class, args);
}
}

这就是SpringBoot的启动入口,通过前面的学习我们大体上了解了**@SpringBootApplication的作用,接下来我们来认识一下SpringApplication**。
SpringApplication (Spring Boot Docs 1.4.2.RELEASE API)

SpringApplication.run(SpringBootWebDemoApplication.class, args);

通过源码我们来看一下**SpringApplication.run()**方法的执行过程
1.调用static方法

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//1
public static ConfigurableApplicationContext run(Object source, String... args) {return run(new Object[]{source}, args);}
public static ConfigurableApplicationContext run(Object[] sources, String[] args)
{return (new SpringApplication(sources)).run(args);}

2.创建SpringApplication对象

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//2
public SpringApplication(Object... sources) {
this.bannerMode = Mode.CONSOLE; //banner的打印模式,此时是控制台模式
this.logStartupInfo = true; //开启日志
this.addCommandLineProperties = true;//启用CommandLineProperties
this.headless = true;//开启headless模式支持
this.registerShutdownHook = true;//启用注册ShutdownHook,用于在非Web应用中关闭IoC容器和资源
this.additionalProfiles = new HashSet();
this.initialize(sources);//初始化
}

PS:Headless参考资料:在 Java SE 平台上使用 Headless 模式

3.初始化相关对象和属性

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//3
private void initialize(Object[] sources) {
if(sources != null && sources.length > 0) {
this.sources.addAll(Arrays.asList(sources));
}
//3.1判断是否是web运行环境,如果classpath中是否含有**WEB_ENVIRONMENT_CLASSES**指定的全部类,则返回true
this.webEnvironment = this.deduceWebEnvironment();
//3.2找到*META-INF/spring.factories*中声明的所有ApplicationContextInitializer的实现类并将其实例化
this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
//3.3找到*META-INF/spring.factories*中声明的所有ApplicationListener的实现类并将其实例化
this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
//3.4获得当前执行main方法的类对象,这里就是SpringBootWebDemoApplication的实例
this.mainApplicationClass = this.deduceMainApplicationClass();
}

3.1 判断是否是web运行环境
如果classpath中是否含有WEB_ENVIRONMENT_CLASSES指定的全部类,则返回true,用于创建指定类型的ApplicationContext对象。

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//3.1
private static final String[] WEB_ENVIRONMENT_CLASSES = new String[]{"javax.servlet.Servlet", "org.springframework.web.context.ConfigurableWebApplicationContext"};

3.2 大体的过程就是通过SpringFactoriesLoader检索META-INF/spring.factories,找到声明的所有ApplicationContextInitializer的实现类并将其实例化。
ApplicationContextInitializer是Spring框架中的接口,其作用可以理解为在ApplicationContext执行refresh之前,调用ApplicationContextInitializer的initialize()方法,对ApplicationContext做进一步的设置和处理。

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public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
void initialize(C var1);
}

spring-boot-1.4.2.RELEASE.jar中的META-INF/spring.factories包含的ApplicationContextInitializer

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# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.context.web.ServerPortInfoApplicationContextInitializer

spring-boot-autoconfigure-1.4.2.RELEASE.jar中的META-INF/spring.factories包含的ApplicationContextInitializer

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# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

3.3 大体的过程就是通过SpringFactoriesLoader检索META-INF/spring.factories,找到声明的所有ApplicationListener的实现类并将其实例化。
ApplicationListener是Spring框架中的接口,就是事件监听器,其作用可以理解为在SpringApplicationRunListener发布通知事件时,由ApplicationListener负责接收。

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public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
void onApplicationEvent(E var1);
}

SpringBoot只提供了一个SpringApplicationRunListener的实现类,就是EventPublishingRunListener,起作用就是在SpringBoot启动过程中,负责注册ApplicationListener监听器,在不同的时点发布不同的事件类型,如果有哪些ApplicationListener的实现类监听了这些事件,则可以接收并处理。

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public interface SpringApplicationRunListener {
//通知监听器,SpringBoot开始执行
void started();
//通知监听器,Environment准备完成
void environmentPrepared(ConfigurableEnvironment var1);
//通知监听器,ApplicationContext已经创建并初始化完成
void contextPrepared(ConfigurableApplicationContext var1);
//通知监听器,ApplicationContext已经完成IoC配置加载
void contextLoaded(ConfigurableApplicationContext var1);
//通知监听器,SpringBoot启动完成
void finished(ConfigurableApplicationContext var1, Throwable var2);
}

spring-boot-1.4.2.RELEASE.jar中的META-INF/spring.factories包含的ApplicationListener

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# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener,\
org.springframework.boot.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.logging.LoggingApplicationListener

spring-boot-autoconfigure-1.4.2.RELEASE.jar中的META-INF/spring.factories包含的ApplicationListener

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# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

spring-boot-1.4.2.RELEASE.jar中的META-INF/spring.factories包含的SpringApplicationRunListener

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# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

3.4 获得当前执行main方法的类对象,这里就是SpringBootWebDemoApplication的实例。

4.核心方法

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//4
public ConfigurableApplicationContext run(String... args) {
//开启任务执行时间监听器
StopWatch stopWatch = new StopWatch();
stopWatch.start();

ConfigurableApplicationContext context = null;
Object analyzers = null;

//设置系统属性『java.awt.headless』,为true则启用headless模式支持
this.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 = this.getRunListeners(args);
listeners.started();

try {
DefaultApplicationArguments ex = 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 = this.prepareEnvironment(listeners, ex);

//决定是否打印Banner
Banner printedBanner = this.printBanner(environment);

//根据webEnvironment的值来决定创建何种类型的ApplicationContext对象
//如果是web环境,则创建org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext
//否则创建org.springframework.context.annotation.AnnotationConfigApplicationContext
context = this.createApplicationContext();

//注册异常分析器
new FailureAnalyzers(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。
this.prepareContext(context, environment, listeners, ex, printedBanner);

//初始化所有自动配置类,调用ApplicationContext的refresh()方法
this.refreshContext(context);

//遍历所有注册的ApplicationRunner和CommandLineRunner,并执行其run()方法。
//该过程可以理解为是SpringBoot完成ApplicationContext初始化前的最后一步工作,
//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。
this.afterRefresh(context, ex);

//调用所有的SpringApplicationRunListener的finished()方法,广播SpringBoot已经完成了ApplicationContext初始化的全部过程。
listeners.finished(context, (Throwable)null);

//关闭任务执行时间监听器
stopWatch.stop();
//如果开启日志,则答应执行是时间
if(this.logStartupInfo) {
(new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
}

return context;
} catch (Throwable var9) {
//调用异常分析器打印报告,调用所有的SpringApplicationRunListener的finished()方法将异常信息发布出去
this.handleRunFailure(context, listeners, (FailureAnalyzers)analyzers, var9);
throw new IllegalStateException(var9);
}
}

spring-boot-1.4.2.RELEASE.jar中的META-INF/spring.factories包含的FailureAnalyzerFailureAnalysisReporters

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# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer

# FailureAnalysisReporters
org.springframework.boot.diagnostics.FailureAnalysisReporter=\
org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter

说明

  • SpringBoot的启动过程,实际上就是对ApplicationContext的初始化过程。
  • ApplicationContext创建后立刻为其设置Environmen,并由ApplicationContextInitializer对其进一步封装。
  • 通过SpringApplicationRunListener在ApplicationContext初始化过程中各个时点发布各种广播事件,并由ApplicationListener负责接收广播事件。
  • 初始化过程中完成IoC的注入,包括通过**@EnableAutoConfiguration**导入的各种自动配置类。
  • 初始化完成前调用ApplicationRunner和CommandLineRunner的实现类。

扩展SpringApplication

通过上面的学习,我们基本上了解了,如果要对SpringApplication进行扩展,我们可以选择如下三种方案:

  • 创建ApplicationContextInitializer的实现类
  • 创建ApplicationListener的实现类
  • 创建ApplicationRunner和CommandLineRunner的实现类

1.可以通过如下方式加载自定义的ApplicationContextInitializerApplicationListener

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@SpringBootApplication
public class SpringBootWebDemoApplication {
public static void main(String[] args) {
//SpringApplication.run(SpringBootWebDemoApplication.class, args);
SpringApplication springApplication = new SpringApplication(SpringBootWebDemoApplication.class);

springApplication.addInitializers(MyApplicationContextInitializer1,MyApplicationContextInitializer2);

springApplication.addListeners(MyApplicationListener1,MyApplicationListener2);

springApplication.run(args);
}
}

2.也可以在当前项目的类路径下创建META-INF/spring.factories文件,并声明相应的ApplicationContextInitializerApplicationListener

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org.springframework.context.ApplicationContextInitializer=\
xxx.xxx.MyApplicationContextInitializer1,\
xxx.xxx.MyApplicationContextInitializer2

# Application Listeners
org.springframework.context.ApplicationListener=\
xxx.xxx.MyApplicationListener1,\
xxx.xxx.MyApplicationListener2

3.至于ApplicationRunner和CommandLineRunner,只需要在其实现类上加上**@Component**注解或者在@Configuration配置类中通过@Bean注解注入。


深入了解SpringBootServletInitializer

熟悉了SpringApplication的原理之后,我们再来了解SpringBootServletInitializer的原理就比较容易了。

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public class ServletInitializer extends SpringBootServletInitializer {

@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
return application.sources(DemoWarApplication.class);
}
}

SpringBootServletInitializer就是一个org.springframework.web.WebApplicationInitializer,容器启动时会调用其onStartup(ServletContext servletContext)方法,接下来我么就来看一下这个方法:

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public void onStartup(ServletContext servletContext) throws ServletException {
this.logger = LogFactory.getLog(this.getClass());
final WebApplicationContext rootAppContext = this.createRootApplicationContext(servletContext);
if(rootAppContext != null) {
servletContext.addListener(new ContextLoaderListener(rootAppContext) {
public void contextInitialized(ServletContextEvent event) {
}
});
} else {
this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not return an application context");
}

}

这里的核心方法就是createRootApplicationContext(servletContext):

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protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {

//创建SpringApplicationBuilder,并用其生产出SpringApplication对象
SpringApplicationBuilder builder = this.createSpringApplicationBuilder();
builder.main(this.getClass());

ApplicationContext parent = this.getExistingRootWebApplicationContext(servletContext);
if(parent != null) {
this.logger.info("Root context already created (using as parent).");
servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, (Object)null);
builder.initializers(new ApplicationContextInitializer[]{new ParentContextApplicationContextInitializer(parent)});
}

//初始化并封装SpringApplicationBuilder对象,为SpringApplication对象增加ApplicationContextInitializer和ApplicationListener做准备
builder.initializers(new ApplicationContextInitializer[]{new ServletContextApplicationContextInitializer(servletContext)});
builder.listeners(new ApplicationListener[]{new ServletContextApplicationListener(servletContext)});
//指定创建的ApplicationContext类型
builder.contextClass(AnnotationConfigEmbeddedWebApplicationContext.class);

//传递入口类,并构建SpringApplication对象
//可以通过configure()方法对SpringBootServletInitializer进行扩展
builder = this.configure(builder);
SpringApplication application = builder.build();

if(application.getSources().isEmpty() && AnnotationUtils.findAnnotation(this.getClass(), Configuration.class) != null) {
application.getSources().add(this.getClass());
}

Assert.state(!application.getSources().isEmpty(), "No SpringApplication sources have been defined. Either override the configure method or add an @Configuration annotation");
if(this.registerErrorPageFilter) {
application.getSources().add(ErrorPageFilter.class);
}

//最后调用SpringApplication的run方法
return this.run(application);
}

说明
SpringBootServletInitializer的执行过程,简单来说就是通过SpringApplicationBuilder构建并封装SpringApplication对象,并最终调用SpringApplication的run方法的过程。


扩展SpringBootServletInitializer

与扩展SpringApplication类似,ApplicationContextInitializerApplicationListener可以基于SpringApplicationBuilder提供的public方法进行扩展

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public class ServletInitializer extends SpringBootServletInitializer {

@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
application.initializers(MyApplicationContextInitializer1,MyApplicationContextInitializer2);
application.listeners(MyApplicationListener1,MyApplicationListener2)
return application.sources(DemoWarApplication.class);
}

}