【SpringBoot】启动原理（二）：构造 SpringApplication

淩亂°似流年 2022-11-04 01:40 188阅读 0赞

# 进入SpringApplication #

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

我们根据DemoApplication跟进代码，发现其调用的SpringApplication类的run方法。这个方法就干了2件事：

*  一是创建SpringApplication对象
 *  二是启动SpringApplication。

# SpringApplication 构造器分析 #

## **1.构造器** ##

public SpringApplication(Class<?>... primarySources) { 
        this(null, primarySources);
    }
    
    /** * Create a new {@link SpringApplication} instance. The application context will load * beans from the specified primary sources */
    public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) { 
       this.resourceLoader = resourceLoader;
       Assert.notNull(primarySources, "PrimarySources must not be null");
       this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        //根据应用是否存在某些类推断应用类型，分为响应式web应用，servlet类型web应用和非web应用，
        // 在后面用于确定实例化applicationContext的类型
       this.webApplicationType = WebApplicationType.deduceFromClasspath();
        //设置初始化器，读取spring.factories文件key ApplicationContextInitializer对应的value并实例化
        //ApplicationContextInitializer接口用于在Spring上下文被刷新之前进行初始化的操作
       setInitializers((Collection) getSpringFactoriesInstances(
             ApplicationContextInitializer.class));
    
        //设置监听器，读取spring.factories文件key ApplicationListener对应的value并实例化
        // interface ApplicationListener<E extends ApplicationEvent> extends EventListener
        //ApplicationListener继承EventListener，实现了观察者模式。对于Spring框架的观察者模式实现，它限定感兴趣的事件类型需要是ApplicationEvent类型事件
       setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        //没啥特别作用，仅用于获取入口类class对象
       this.mainApplicationClass = deduceMainApplicationClass();
    }

在构造器里主要干了件事

*  设置主类
 *  推断应用类型
 *  一个设置初始化器
 *  二是设置监听器。

## 推断应用类型 ##

static WebApplicationType deduceFromClasspath() { 
    		if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
    				&& !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) { 
    			return WebApplicationType.REACTIVE;
    		}
    		for (String className : SERVLET_INDICATOR_CLASSES) { 
    			if (!ClassUtils.isPresent(className, null)) { 
    				return WebApplicationType.NONE;
    			}
    		}
    		return WebApplicationType.SERVLET;
    	}

## 设置初始化器 ##

setInitializers((Collection) getSpringFactoriesInstances(
          ApplicationContextInitializer.class));
    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) { 
       return getSpringFactoriesInstances(type, new Class<?>[] { });
    }
    
    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
          Class<?>[] parameterTypes, Object... args) { 
       ClassLoader classLoader = getClassLoader();
       // Use names and ensure unique to protect against duplicates
       Set<String> names = new LinkedHashSet<>(
        //从类路径的META-INF处读取相应配置文件spring.factories，然后进行遍历，读取配置文件中Key(type)对应的value
             SpringFactoriesLoader.loadFactoryNames(type, classLoader));
       //将names的对象实例化
       List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
             classLoader, args, names);
       AnnotationAwareOrderComparator.sort(instances);
       return instances;
    }

根据入参type类型ApplicationContextInitializer.class从类路径的META-INF处读取相应配置文件spring.factories并实例化对应Initializer。上面这2个函数后面会反复用到。

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

## 设置监听器 ##

setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));

和设置初始化器一个套路，通过getSpringFactoriesInstances函数实例化监听器。

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