SpringBoot启动流程简析（一）

た入场券 2023-07-01 12:55 83阅读 0赞

最近在业务中碰到SpringBoot启动流程相关问题，故梳理记录下：

**要点：本章主要梳理SpringBoot启动流程中是如何创建SpringApplication实例的。**

启动主类代码如下：

@Slf4j
    @SpringBootApplication
    public class ItemFootprintApplication {
        public static void main(String[] args) {
            ApplicationContext ctx = SpringApplication.run(ItemFootprintApplication.class, args);
            log.info("spring boot 启动环境和配置信息如下：" + ctx.getEnvironment());
        }
    }

首先进入到SpringApplication的run方法中看一下这个方法的内容：

/**
    	 * Static helper that can be used to run a {@link SpringApplication} from the
    	 * specified source using default settings.
    	 * @param primarySource the primary source to load
    	 * @param args the application arguments (usually passed from a Java main method)
    	 * @return the running {@link ApplicationContext}
    	 */
    	public static ConfigurableApplicationContext run(Class<?> primarySource,
    			String... args) {
    		return run(new Class<?>[] { primarySource }, args);
    	}
    
    	/**
    	 * Static helper that can be used to run a {@link SpringApplication} from the
    	 * specified sources using default settings and user supplied arguments.
    	 * @param primarySources the primary sources to load
    	 * @param args the application arguments (usually passed from a Java main method)
    	 * @return the running {@link ApplicationContext}
    	 */
    	public static ConfigurableApplicationContext run(Class<?>[] primarySources,
    			String[] args) {
    		return new SpringApplication(primarySources).run(args);
    	}

(1) 创建SpringApplication对象；  
(2) 调用SpringApplication的run方法，返回一个Spring上下文 ConfigurableApplicationContext的实例。

我们先去看看SpringApplication的构造函数的内容：

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));
    		
    		//(z1) 判断是否是web环境
    		this.webApplicationType = deduceWebApplicationType();
    		
    		//(z2) 从spring.factories中获取ApplicationContextInitializer集合并赋值
    		setInitializers((Collection) getSpringFactoriesInstances(
    				ApplicationContextInitializer.class));
    		
    		//(z3) 从spring.factories中获取ApplicationListener集合并赋值		
    		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    	
    		this.mainApplicationClass = deduceMainApplicationClass();
    	}

**(z1) 判断是否是web环境**

private WebApplicationType deduceWebApplicationType() {
    		if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null)
    				&& !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)
    				&& !ClassUtils.isPresent(JERSEY_WEB_ENVIRONMENT_CLASS, null)) {
    			return WebApplicationType.REACTIVE;
    		}
    		for (String className : WEB_ENVIRONMENT_CLASSES) {
    			if (!ClassUtils.isPresent(className, null)) {
    				return WebApplicationType.NONE;
    			}
    		}
    		return WebApplicationType.SERVLET;
    	}
    	
    private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",
    			"org.springframework.web.context.ConfigurableWebApplicationContext" };

这里判断是不是web开发环境也很简单，就是看类路径下是能加载到javax.servlet.Servlet和org.springframework.web.context.ConfigurableWebApplicationContext这两个两类，如果能加载到则是web环境，否则非web环境。

**(z2) 从spring.factories中获取ApplicationContextInitializer集合**

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) {
    		//获取当前线程上下文加载器
    		ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    		
    		// Use names and ensure unique to protect against duplicates
    		Set<String> names = new LinkedHashSet<>(
    		        //META-INF/spring.factories中加载ApplicationContextInitializer类型的类的全路径名
    				SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    		//根据上一步获取到的类名，创建实例对象		
    		List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
    				classLoader, args, names);
    		//排序		
    		AnnotationAwareOrderComparator.sort(instances);
    		return instances;
    	}

**(z3) 从spring.factories中获取ApplicationListener集合**  
与(z2) 类似

**本章总结：**  
在启动SpringBoot的过程中，创建SpringApplication的实例的主要流程：  
(1) deduceWebApplicationType()方法判断是不是web环境；  
(2) 从spring.factories中加载org.springframework.context.ApplicationContextInitializer和org.springframework.context.ApplicationListener类型的对象并赋值到SpringApplication实例中；