Java面试题 -源码解读

1. spring 启动refresh()：

public void refresh() throws BeansException, IllegalStateException {
       synchronized (this.startupShutdownMonitor) {
           // Prepare this context for refreshing.
           prepareRefresh();  // Tell the subclass to refresh the internal bean factory.
           //主要是创建beanFactory，同时加载配置⽂件.xml中的beanDefinition
		//通过String[] configLocations = getConfigLocations()获取资源路径，然后加载beanDefinition
           ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

           // Prepare the bean factory for use in this context.
           // 给beanFactory注册⼀些标准组件，如ClassLoader，BeanPostProcess
           prepareBeanFactory(beanFactory);

           try {
               // Allows post-processing of the bean factory in context subclasses.
               //提供给⼦类实现⼀些postProcess的注册，如AbstractRefreshableWebApplicationContext注册⼀些Servlet相关的
               //postProcess，真对web进⾏⽣命周期管理的Scope，通过registerResolvableDependency()⽅法注册指定ServletRequest，HttpSession，WebRequest对象的⼯⼚⽅法
               postProcessBeanFactory(beanFactory);

               // Invoke factory processors registered as beans in the context.
               //调⽤所有BeanFactoryProcessor的postProcessBeanFactory()⽅法
               invokeBeanFactoryPostProcessors(beanFactory);

               // Register bean processors that intercept bean creation.
               //注册BeanPostProcessor，BeanPostProcessor作⽤是⽤于拦截Bean的创建
               registerBeanPostProcessors(beanFactory);

               // Initialize message source for this context.
               //初始化消息Bean
               initMessageSource();

               // Initialize event multicaster for this context.
               //初始化上下⽂的事件多播组件，ApplicationEvent触发时由multicaster通知给ApplicationListener
               initApplicationEventMulticaster();

               // Initialize other special beans in specific context subclasses.
               //ApplicationContext初始化⼀些特殊的bean
               onRefresh();

               // Check for listener beans and register them.
               //注册事件监听器，事件监听Bean统⼀注册到multicaster⾥头，ApplicationEvent事件触发后会由multicaster⼴播
               registerListeners();

               // Instantiate all remaining (non-lazy-init) singletons.
               //⾮延迟加载的单例Bean实例化
               finishBeanFactoryInitialization(beanFactory);

               // Last step: publish corresponding event.
               finishRefresh();
           } catch (BeansException ex) {
               if (logger.isWarnEnabled()) {
                   logger.warn("Exception encountered during context initialization - " +
                           "cancelling refresh attempt: " + ex);
               }

               // Destroy already created singletons to avoid dangling resources.
               destroyBeans();

               // Reset 'active' flag.
               cancelRefresh(ex);

               // Propagate exception to caller.
               throw ex;
           } finally {
               // Reset common introspection caches in Spring's core, since we
               // might not ever need metadata for singleton beans anymore...
               resetCommonCaches();
           }
       }
   }

2. 声明式事务源码

a. @EnableTransactionManagement：利⽤TransactionManagementConfigurationSelector给容器中导⼊两个组件：

• i. AutoProxyRegistrar；
• ii. ProxyTransactionManagementConfiguration。

b. AutoProxyRegistrar：给容器注⼊InfrastructureAdvisorAutoProxyCreator组件。利⽤后置处理器在对象创建以后，包装对象，返回⼀个代理对象（增强器），代理对象执⾏⽬标⽅法时执⾏拦截器链。

c. ProxyTransactionManagementConfiguration：给容器注册增强器。

• i. 设置事务注解信息：AnnotationTransactionAttributeSource解析事务注解；
• ii. 设置事务拦截器：TransactionInterceptor（是⼀个MethodInterceptor），保存了事务属性信息和事务管理器。 在⽬标⽅法执⾏的时候，执⾏拦截器链

protected Object invokeWithinTransaction(Method method, Class targetClass, final InvocationCallback invocation
throws Throwable {
    // If the transaction attribute is null, the method is non-transactional.
    final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute
    final PlatformTransactionManager tm = determineTransactionManager(txAttr);
    final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
    if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
        // Standard transaction demarcation with getTransaction and commit/rollback calls.
        TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification
                Object retVal = null;
        try {
            // This is an around advice: Invoke the next interceptor in the chain.
            // This will normally result in a target object being invoked.
            retVal = invocation.proceedWithInvocation();
        } catch (Throwable ex) {
            // target invocation exception
            completeTransactionAfterThrowing(txInfo, ex);
            throw ex;
        } finally {
            cleanupTransactionInfo(txInfo);
        }
        commitTransactionAfterReturning(txInfo);
        return retVal;
    }
}

3. newrelic源码：

javaagent、asm操控字节码，类加载时在指定类前后增加拦截器。

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