springboot源码学习笔记之springboot启动流程

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springboot启动代码如下：

SpringApplication.run(SpringBootApp.class, args);

SpringApplication静态方法run分析

实际调用的是SpringApplication实例方法run因此我们分两步分别分析SpringApplication构造函数和实例方法run

1.springApplication构造方法

    public SpringApplication(Class<?>... primarySources) {
		this(null, primarySources);
	}

    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 = deduceWebApplicationType();
		setInitializers((Collection) getSpringFactoriesInstances(
				ApplicationContextInitializer.class));
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}

可以看到真正的调用方法是第二个构造函数；

• 第一步：设置 primarySources
• 第二步：设置webApplicationType 也就是web类型

    public enum WebApplicationType {
	/**
	 * The application should not run as a web application and should not start an
	 * embedded web server.
	 */
	NONE,

	/**
	 * The application should run as a servlet-based web application and should start an
	 * embedded servlet web server.
	 */
	SERVLET,

	/**
	 * The application should run as a reactive web application and should start an
	 * embedded reactive web server.
	 */
	REACTIVE
    }
    

默认的web类型是SERVLET 也就是基础servlet类型web,会启动一个嵌入式servlet web服务器

• 第三步：设置初始化器
  通过读取“META-INF/spring.factories” 文件获取 ApplicationContextInitializer子类实例；看到这里我们可以通过了解这个特性实现自己的ApplicationContextInitializer初始化器，在此基础上做自己的事情
  通过spring-boot的jar包中的spring.factories文件可以看到其中有下面内容

# 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.web.context.ServerPortInfoApplicationContextInitializer

由此可以看出spring boot程序默认至少设置了四个初始化器

• 第四步： 设置监听器ApplicationListener
  也是通过读取“META-INF/spring.factories” 文件获取ApplicationListener，我们再去spring-boot的源码中可以看到spring.factories中配置的listerner 达到9个

# 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.context.logging.ClasspathLoggingApplicationListener,
org.springframework.boot.context.logging.LoggingApplicationListener,
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

宗上所述，再构建我的项目时候可以通过spring.factories的方式做很多有意义的事情

2.springApplication的实例方法run

首先看下面源码

public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
		configureHeadlessProperty();//设置headless模式，设置SystemProperty java.awt.headless默认设置为true
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(
					args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners,
					applicationArguments);//准备环境
			configureIgnoreBeanInfo(environment);
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			exceptionReporters = getSpringFactoriesInstances(
					SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			prepareContext(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, exceptionReporters, listeners);
			throw new IllegalStateException(ex);
		}

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

执行流程

以下是重点说明

• 1.SpringApplicationRunListeners获取

SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();

通过从spring.factories中获取实现了SpringApplicationRunListener接口的类实例，此处我们知道实现类是EventPublishingRunListener

• 2.启动过程

prepareContext(context, environment, listeners, applicationArguments,printedBanner);//准备Context
refreshContext(context);//Context
afterRefresh(context, applicationArguments);//完成Context刷新后

以上代码体现context的处理过程;充分理解这个过程能很好的利用springboot的特性和context的初始化过程

• 2.1prepareContext准备
  此方法中准备context加载环境，例如资源Resources和日志配置
• 2.2refreshContext刷新
  此方法刷新context通过各种途径去加载bean(扫描包通过注解加载bean……)；查看源码我们能知道根据webtype 使用的是AnnotationConfigServletWebServerApplicationContext 作为context的；