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；

springboot源码学习笔记之springboot启动流程

原文：https://www.cnblogs.com/qlsy/p/9580409.html