springboot是如何工作的

一、前言

现在java后端开发框架比较多的使用springboot框架，springboot是在以前的springMVC进行封装和优化，最大的特点是简化了配置和内置Tomcat。本节通过阅读源码理解springboot是如何工作的。

二、springboot是如何工作的

1、从启动类开始

/**
  *服务启动类
  * @author: 
  * @Description:
  * @Company: 
  * @date: 
  */
@SpringBootApplication(scanBasePackages = {"cn.xxx.common","cn.xxx.admin.*","cn.xxx.interceptor.*"})
@EnableEurekaClient
@EnableApolloConfig
@EnableFeignClients(basePackages = {"cn.xxx.interceptor.*"})
@EnableDiscoveryClient
@MapperScan("cn.xxx.admin.dao")
@EnableSwagger2
@EnableTransactionManagement 
public class BaseAdminApplication {
	
	public static void main(String[] args) {
		SpringApplication.run(BaseAdminApplication.class,args);
	}

}

从代码上可以看出spring通过main方法启动，关键代码是这一句：SpringApplication.run(BaseAdminApplication.class,args);

和注解 @SpringBootApplication(scanBasePackages = {"cn.xxx.common","cn.xxx.admin.*","cn.xxx.interceptor.*"})

通过调用SpringApplication的run方法

	public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
		return run(new Class<?>[] { primarySource }, args);
	}

可以看出传入的参数是类 BaseAdminApplication.class

我们平时的方法很少参数传入一个类，这里传入一个类，就是为了获取这个类的属性，就是它上面的注解之类的

这里就要理解一下java的反射。

接着往下看 run(new Class<?>[] { primarySource }, args);

调用的run方法传入的是类的数组 Class<?>[] { primarySource }，数组中的元素是 BaseAdminApplication.class

继续往下调用

	public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
		return new SpringApplication(primarySources).run(args);
	}

这一段东西又怎么理解？

new SpringApplication(primarySources).run(args);

这个是构造函数SpringApplication(primarySources)，就是初始化出SpringApplication的一个实例对象，它传的值就是 BaseAdminApplication.class ，然后调用run的方法。

先看构成函数里都进行那些初始化

	@SuppressWarnings({ "unchecked", "rawtypes" })
	public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
		this.resourceLoader = resourceLoader;
		Assert.notNull(primarySources, "PrimarySources must not be null");
		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
		this.webApplicationType = WebApplicationType.deduceFromClasspath();
		this.bootstrapRegistryInitializers = new ArrayList<>(
				getSpringFactoriesInstances(BootstrapRegistryInitializer.class));
		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}

这里其实就是封装了配置信息

接下来看看如何运行配置文件和上下文信息

public ConfigurableApplicationContext run(String... args) {
		long startTime = System.nanoTime();
		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);
			Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);
			}
			listeners.started(context, timeTakenToStartup);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, listeners);
			throw new IllegalStateException(ex);
		}
		try {
			Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);
			listeners.ready(context, timeTakenToReady);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}

这段代码还是很复杂的，因为里面嵌套了很多模块和方法，它表示的就是如何处理配置信息和上下文。