Springboot 启动简析

入口程序

首先，我们回到到梦开始的地方

@SpringBootApplication
public class DemoApplication {

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

大道极简，上面的的代码非常简洁，背后蕴藏着何种玄机，我们一步一步来揭开。
上面的两处地方，让我们既熟悉又陌生，分别是@SpringBootApplication 这个注解，和SpringApplication的run 方法。

注解@SpringBootApplication浅析

首先，我们看下这个注解的源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

}

通过上面的代码，我们可以看到，springbootApplication 这个注解是由多个注解组合而成的一个注解，接下来，我们分开看下上面的注解

• @SpringbootConfigruation

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
    @AliasFor(  // 继承 configuration 这个注解
        annotation = Configuration.class
    )
    boolean proxyBeanMethods() default true;
}

这个注解的作用就是标注当前类是配置类，并会将当前类内声明的一个或多个以@Bean注解标记的方法的实例纳入到spring容器中，并且实例名就是方法名。

• @EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)   
 // enable 类型的注解 主要就是用来Import 一些配置文件的注解，enable某个组件，便于我们理解
public @interface EnableAutoConfiguration {

	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

	/**
	 * 排除特定的自动配置类
	 * @return the classes to exclude
	 */
	Class<?>[] exclude() default {};

	/**
	 * 排除特定的自动配置类名称，使它们永远不会加载
	 * applied.
	 * @return the class names to exclude
	 * @since 1.3.0
	 */
	String[] excludeName() default {};

}

• @ComponentScan

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Documented
@Repeatable(ComponentScans.class)
public @interface ComponentScan {
    @AliasFor("basePackages")
    String[] value() default {};    

    @AliasFor("value")
    String[] basePackages() default {};   

    Class<?>[] basePackageClasses() default {};

    Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;

    Class<? extends ScopeMetadataResolver> scopeResolver() default AnnotationScopeMetadataResolver.class;

    ScopedProxyMode scopedProxy() default ScopedProxyMode.DEFAULT;
}

定义了---

浅析SpringApplication的run方法

         // 1. 以使用默认设置和用户提供的参数从指定的类运行
	public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
		return new SpringApplication(primarySources).run(args);
	}

        //2.  真正的run 方法
       	public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
                
		DefaultBootstrapContext bootstrapContext = createBootstrapContext();
		ConfigurableApplicationContext context = null;
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting(bootstrapContext, this.mainApplicationClass);
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
			configureIgnoreBeanInfo(environment);
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			context.setApplicationStartup(this.applicationStartup);
			prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
			}
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, listeners);
			throw new IllegalStateException(ex);
		}

		try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}