在上一章学习了spring boot 2.0启动的大概流程以后，今天我们来深挖一下SpringApplication实例变量的run函数。
先把这段run函数的代码贴出来：
/** * Run the Spring application, creating and refreshing a new * {@link ApplicationContext}. * @param args the application arguments (usually passed from a Java main method) * @return a running {@link ApplicationContext} */ public ConfigurableApplicationContext run(String... args) { StopWatch stopWatch = new StopWatch(); stopWatch.start(); ConfigurableApplicationContext context = null; Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>(); configureHeadlessProperty(); SpringApplicationRunListeners listeners = getRunListeners(args); listeners.starting(); try { ApplicationArguments applicationArguments = new DefaultApplicationArguments( args); ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments); configureIgnoreBeanInfo(environment); Banner printedBanner = printBanner(environment); context = createApplicationContext(); exceptionReporters = getSpringFactoriesInstances( SpringBootExceptionReporter.class, new Class[] { ConfigurableApplicationContext.class }, context); prepareContext(context, environment, listeners, applicationArguments, printedBanner); refreshContext(context); afterRefresh(context, applicationArguments); stopWatch.stop(); if (this.logStartupInfo) { new StartupInfoLogger(this.mainApplicationClass) .logStarted(getApplicationLog(), stopWatch); } listeners.started(context); callRunners(context, applicationArguments); } catch (Throwable ex) { handleRunFailure(context, ex, exceptionReporters, listeners); throw new IllegalStateException(ex); } try { listeners.running(context); } catch (Throwable ex) { handleRunFailure(context, ex, exceptionReporters, null); throw new IllegalStateException(ex); } return context; }我们先来分析其中的第一个关键代码：SpringApplicationRunListeners listeners = getRunListeners(args);
这行代码是获取监听器，我们先跳转到getRunListeners中看一下：
private SpringApplicationRunListeners getRunListeners(String[] args) { Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class }; return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances( SpringApplicationRunListener.class, types, this, args)); }在这段代码中，我们看到获取监听器，是new出来了一个SpringApplicationRunListeners实例并返回。
再次跳转到SpringApplicationRunListeners的构造函数中，看到一下发生了什么：
SpringApplicationRunListeners(Log log, Collection<? extends SpringApplicationRunListener> listeners) { this.log = log; this.listeners = new ArrayList(listeners); }在这个构造函数里，我们看到其只是把接收到的listeners参数，保存到实例变量里，没有过多的操作。
所以，重点是在listeners参数这里，而listeners是通过getSpringFactoriesInstances创建出来的，来看一下getSpringFactoriesInstances函数做了什么事情吧：
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) { ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); // Use names and ensure unique to protect against duplicates Set<String> names =
