每天都不一样

引言

SpringBoot大大简化了Spring项目的配置，虽然很舒服，但是为了面试等，还是要知道里面的细节和原理呀，再此举个HelloWorld的例子

声明

使用Idea分析，并使用Idea创建SpringBoot的工程。
项目结构就不一一详细说了，大家肯定都知道每个文件是干什么的。
这里主要是负责细看其中的原理和细节。
版本：2.2.4.RELEASE

SpringApplication实例

先看源码：

public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {
      this.sources = new LinkedHashSet(); // 1. 
      this.bannerMode = Mode.CONSOLE;
      this.logStartupInfo = true;
      this.addCommandLineProperties = true;
      this.headless = true;
      this.registerShutdownHook = true;
      this.additionalProfiles = new HashSet();
      this.resourceLoader = resourceLoader;
      Assert.notNull(primarySources, "PrimarySources must not be null");
      this.primarySources = new LinkedHashSet(Arrays.asList(primarySources));
      this.webApplicationType = this.deduceWebApplicationType();
      this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class)); // 3. 
      this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class)); // 4.
      this.mainApplicationClass = this.deduceMainApplicationClass(); // 5.
  }

com.example.helloworld.HelloworldApplication放入到Set的集合中
判断是否为Web环境：存在（javax.servlet.Servlet && org.springframework.web.context.ConfigurableWebApplicationContext ）类
创建并初始化ApplicationInitializer列表（spring.factories）
创建并初始化ApplicationListener列表（spring.factories）
初始化主类mainApplicatioClass (DemoApplication)
总结：上面就是SpringApplication初始化的代码，new SpringApplication()没做啥事情，主要加载了META-INF/spring.factories 下面定义的事件监听器接口实现类

ConfigurableApplicationContext的run方法

看源码，都可以找的到

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch(); // 1.
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
        this.configureHeadlessProperty();
        SpringApplicationRunListeners listeners = this.getRunListeners(args); // 2.
        listeners.starting(); // 
 
        Collection exceptionReporters;
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args); // 
            ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments); // 
            this.configureIgnoreBeanInfo(environment); // 
            Banner printedBanner = this.printBanner(environment); // 
            context = this.createApplicationContext(); // 
            exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
            this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);// 
            this.refreshContext(context); // 
            this.afterRefresh(context, applicationArguments); //
            stopWatch.stop();// 
            if (this.logStartupInfo) {
                (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
            }
 
            listeners.started(context);
            this.callRunners(context, applicationArguments);
        } catch (Throwable var10) {
            this.handleRunFailure(context, var10, exceptionReporters, listeners);
            throw new IllegalStateException(var10);
        }
 
        try {
            listeners.running(context);
            return context;
        } catch (Throwable var9) {
            this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null);
            throw new IllegalStateException(var9);
        }
    }

创建计时器StopWatch
获取SpringApplicationRunListeners并启动
创建ApplicationArguments
创建并初始化ConfigurableEnvironment
打印Banner
创建ConfigurableApplicationContext
准备ConfigurableApplicationContext
刷新ConfigurableApplicationContext，这个refreshContext()加载了bean，还启动了内置web容器，需要细细的去看看
容器刷新后动作，啥都没做
计时器停止计时

refreshContext()

该源码中其实就是Spring源码的refresh()的源码

不过这里的refresh()是在AbstractApplicationContext抽象类上
其他就不提了，关注点在onrefresh()方法上，但是个空方法，毕竟是抽象类，去找其子类继承的它
debug调试可以找到ServletWebServerApplicationContext

ServletWebServerApplicationContext

先看个类图吧，很吊

onRefresh()->createWebServer()->getWebServerFactory()，此时已经加载了个web容器
可以返回刚才的createWebServer()，然后看factory.getWebServer

public WebServer getWebServer(ServletContextInitializer... initializers) {
        //tomcat这位大哥出现了
		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);
	}

内置的Tomcat就出现了
总结：run() 方法主要调用了spring容器启动方法扫描配置，加载bean到spring容器中；启动的内置Web容器

@SpringBootApplication

主要是三个注解

@SpringBootConfiguration:允许在上下文中注册额外的bean或导入其他配置类。
@EnableAutoConfiguration:启用 SpringBoot 的自动配置机制
@ComponentScan: 扫描常用的注解