死磕源码系列【springcloud之BootstrapApplicationListener初始化bootstrap上下文及bootstrap.properties配置文件源码解析】

亦凉 2022-12-27 07:26 128阅读 0赞

> BootstrapApplicationListener是ApplicationListener监听器接口的子类，其主要作用是初始化parent上下文、bootstrap.properties配置文件及应用程序上下文设置父上下文；

##### BootstrapApplicationListener监听器源码 #####

public class BootstrapApplicationListener
    		implements ApplicationListener<ApplicationEnvironmentPreparedEvent>, Ordered { 
    
    	/** * 属性源名称是bootstrap */
    	public static final String BOOTSTRAP_PROPERTY_SOURCE_NAME = "bootstrap";
    
    	/** * 监听器的默认顺序 */
    	public static final int DEFAULT_ORDER = Ordered.HIGHEST_PRECEDENCE + 5;
    
    	/** * 默认属性名称 */
    	public static final String DEFAULT_PROPERTIES = "springCloudDefaultProperties";
    
    	private int order = DEFAULT_ORDER;
    	/** * ApplicationEnvironmentPreparedEvent是SpringApplication启动且Environment首次可供检查和修改时发布的事件 **/
    	@Override
    	public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) { 
    		ConfigurableEnvironment environment = event.getEnvironment();
        // 判定bootstrap引导配置是否开启
    		if (!environment.getProperty("spring.cloud.bootstrap.enabled", Boolean.class,
    				true)) { 
    			return;
    		}
    		// 不监听bootstrap引导上下文
    		if (environment.getPropertySources().contains(BOOTSTRAP_PROPERTY_SOURCE_NAME)) { 
    			return;
    		}
    		ConfigurableApplicationContext context = null;
    		String configName = environment
    				.resolvePlaceholders("${spring.cloud.bootstrap.name:bootstrap}");
        //轮询上下文中配置的初始化器
    		for (ApplicationContextInitializer<?> initializer : event.getSpringApplication()
    				.getInitializers()) { 
          // 如果初始化器是ParentContextApplicationContextInitializer类型实例，则调用其初始化方法
    			if (initializer instanceof ParentContextApplicationContextInitializer) { 
    				context = findBootstrapContext(
    						(ParentContextApplicationContextInitializer) initializer,
    						configName);
    			}
    		}
        //在springcloud中此处context为null
    		if (context == null) { 
          //核心 获取bootstrap上下文及初始化environment
    			context = bootstrapServiceContext(environment, event.getSpringApplication(),
    					configName);
          //添加监听器
    			event.getSpringApplication()
    					.addListeners(new CloseContextOnFailureApplicationListener(context));
    		}
    
    		apply(context, event.getSpringApplication(), environment);
    	}

##### bootstrapServiceContext方法获取应用上下文及bootstrap配置文件 #####

private ConfigurableApplicationContext bootstrapServiceContext(
    			ConfigurableEnvironment environment, final SpringApplication application,
    			String configName) { 
        //新建environment上下文对象
    		StandardEnvironment bootstrapEnvironment = new StandardEnvironment();
    	
        ...
        //此处会将BootstrapImportSelectorConfiguration类家属资源集合，在初始化上下文的过程中会将其加入IOC容器
        builder.sources(BootstrapImportSelectorConfiguration.class);
        // 核心-此处调用初始化上下文的方法，并且不会创建tomcat，会通过回调设置应用程序的父上下文为bootstrap上下文 
    		final ConfigurableApplicationContext context = builder.run();
    		// 设置上下文ID
    		context.setId("bootstrap");
    		// 设置上下文的父上下文
    		addAncestorInitializer(application, context);
    		// It only has properties in it now that we don't want in the parent so remove
    		// it (and it will be added back later)
    		bootstrapProperties.remove(BOOTSTRAP_PROPERTY_SOURCE_NAME);
    		mergeDefaultProperties(environment.getPropertySources(), bootstrapProperties);
    		return context;
    	}

上述源码中提到了会初始化上下文，但是不会创建tomcat，这又是为什么呢？看下SpringApplication\#run方法中的createApplicationContext方法：

public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
    			+ "annotation.AnnotationConfigApplicationContext";
    
    	public static final String DEFAULT_SERVLET_WEB_CONTEXT_CLASS = "org.springframework.boot."
    			+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";
    
    	public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework."
    			+ "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";
    
    protected ConfigurableApplicationContext createApplicationContext() { 
    		Class<?> contextClass = this.applicationContextClass;
    		if (contextClass == null) { 
    			try { 
    				switch (this.webApplicationType) { 
    				case SERVLET:
    					contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
    					break;
    				case REACTIVE:
    					contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
    					break;
    				default:
    					contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
    				}
    			}
    			catch (ClassNotFoundException ex) { 
    				throw new IllegalStateException(
    						"Unable create a default ApplicationContext, please specify an ApplicationContextClass", ex);
    			}
    		}
    		return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
    	}

此方法通过反射的方式创建应用程序上下文，并且根据容器的类型创建不同的上下文，servlet容器创建的是AnnotationConfigServletWebServerApplicationContext上下文，bootstrap引导上下文是使用默认的AnnotationConfigApplicationContext上下文创建，到这里还是不明白不同的上下文跟tomcat创建有什么关系？那我们继续往下看，创建tomcat容器是在AbstractApplicationContext\#refresh方法中通过onRefresh实现的，看下onRefresh方法：

protected void onRefresh() throws BeansException { 
    		// For subclasses: do nothing by default.
    	}

这是一个受保护的空方法，具体是由其子类实现,AnnotationConfigServletWebServerApplicationContext上下文的父类ServletWebServerApplicationContext是AbstractApplicationContext类的子类，并且实现了onRefresh方法；AnnotationConfigApplicationContext上下文虽然也是AbstractApplicationContext的子类，但是并未实现onRefresh方法，所以就可以很好的解释为什么创建了上下文，但是并没有创建tomcat的原因。

另外应用程序会通过ContextIdApplicationContextInitializer初始化器设置其自身的父上下文ID;

GitHub地址：[https://github.com/mingyang66/spring-parent][https_github.com_mingyang66_spring-parent]

[https_github.com_mingyang66_spring-parent]: https://github.com/mingyang66/spring-parent