SpringBoot源碼解析:創建SpringApplication對象實例

上篇文章SpringBoot自動裝配原理解析中，我們分析了SpringBoot的自動裝配原理以及@SpringBootApplication註解的原理，本篇文章則繼續基於上篇文章中的main方法來分析SpringApplication這個類

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

點擊run方法一路跟蹤下來，發現首先做的是實例化SpringApplication對象實例

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();
    }

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;
        }
    }

大抵意思就是根據當前項目中是否存在上方的幾個類來推斷出當前的web環境，這裡因為SpringBoot預設使用的web框架是SpringMVC，所以最後返回結果為WebApplicationType.SERVLET

1. 載入所有的ApplicationContextInitializer和ApplicationListener的實現類

 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;
    }

可以看到主要還是用的SpringFactoriesLoader這個類去載入這兩個介面的實現類，載入到類以後使用反射的方式構造出這些類的實例，然後根據這些實現類上的Order註解的值進行排序

關於這些實現類的具體作用請關註後續的文章

1. 最後一行的意義是找到入口方法main所在的類，賦值給全局變數mainApplicationClass