SpringBoot源码解析:建立SpringApplication对象实例

上篇文章SpringBoot自动装配原理解析中，咱们分析了SpringBoot的自动装配原理以及@SpringBootApplication注解的原理，本篇文章则继续基于上篇文章中的main方法来分析SpringApplication这个类

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}复制代码

点击run方法一路跟踪下来，发现首先作的是实例化SpringApplication对象实例

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

1. 首先看一下deduceWebApplicationType方法

private WebApplicationType deduceWebApplicationType() {
        if (ClassUtils.isPresent("org.springframework.web.reactive.DispatcherHandler", (ClassLoader)null) && !ClassUtils.isPresent("org.springframework.web.servlet.DispatcherServlet", (ClassLoader)null)) {
            return WebApplicationType.REACTIVE;
        } 
        for (String className : "javax.servlet.Servlet", org.springframework.web.context.ConfigurableWebApplicationContext") {
            if (!ClassUtils.isPresent(className, null)) {
                return WebApplicationType.NONE;
            }
    }
          return WebApplicationType.SERVLET;
        }
    }复制代码

大抵意思就是根据当前项目中是否存在上方的几个类来推断出当前的web环境，这里由于SpringBoot默认使用的web框架是SpringMVC，因此最后返回结果为WebApplicationType.SERVLET

1. 加载全部的ApplicationContextInitializer和ApplicationListener的实现类

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        Set<String> names = new LinkedHashSet(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        List<T> instances = this.createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }复制代码

能够看到主要仍是用的SpringFactoriesLoader这个类去加载这两个接口的实现类，加载到类之后使用反射的方式构造出这些类的实例，而后根据这些实现类上的Order注解的值进行排序

关于这些实现类的具体做用请关注后续的文章

1. 最后一行的意义是找到入口方法main所在的类，赋值给全局变量mainApplicationClass