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 duplicatesSet<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));List<T> instances = createSpringFactoriesInstances(type, parameterTypes,classLoader, args, names);AnnotationAwareOrderComparator.sort(instances);return instances;}

在这里我们看到，首先创建了一个classloader，然后用SpringFactoriesLoader.loadFactoryNames(type, classLoader)，加载了SpringFactoriesLoader列表。我们来看一下loadFactoryNames里面的代码：

    public static List<String> loadFactoryNames(Class<?> factoryClass,@Nullable ClassLoader classLoader) {String factoryClassName = factoryClass.getName();return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());}private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {MultiValueMap result = (MultiValueMap)cache.get(classLoader);if(result != null) {return result;} else {try {Enumeration ex = classLoader != null?classLoader.getResources("META-INF/spring.factories"):ClassLoader.getSystemResources("META-INF/spring.factories");LinkedMultiValueMap result1 = new LinkedMultiValueMap();while(ex.hasMoreElements()) {URL url = (URL)ex.nextElement();UrlResource resource = new UrlResource(url);Properties properties = PropertiesLoaderUtils.loadProperties(resource);Iterator var6 = properties.entrySet().iterator();while(var6.hasNext()) {Entry entry = (Entry)var6.next();List factoryClassNames = Arrays.asList(StringUtils.commaDelimitedListToStringArray((String)entry.getValue()));result1.addAll((String)entry.getKey(), factoryClassNames);}}cache.put(classLoader, result1);return result1;} catch (IOException var9) {throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var9);}}}

通过这里我们看到了其首先加载了META-INF/spring.factories这个配置文件下的所有资源，并放入缓存，然后再获取了org.springframework.context.ApplicationListener定义的资源列表。

小发现：在这里我们发现spring boot自动装配文件的位置。

获取到META-INF/spring.factories这个配置文件下的资源名称列表以后，通过createSpringFactoriesInstances函数创建了SpringFactories的实例。

    private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args, Set<String> names) {ArrayList instances = new ArrayList(names.size());Iterator var7 = names.iterator();while(var7.hasNext()) {String name = (String)var7.next();try {Class ex = ClassUtils.forName(name, classLoader);Assert.isAssignable(type, ex);Constructor constructor = ex.getDeclaredConstructor(parameterTypes);Object instance = BeanUtils.instantiateClass(constructor, args);instances.add(instance);} catch (Throwable var12) {throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name,var12);}}return instances;}

通过上面的这些代码流转，我们大概搞清楚了listeners是怎么创建出来的。
然后调用了listeners的starting方法。我们先大概地看一下EventPublishingRunListener里面的starting的实现：

    public void starting() {this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(this.application,this.args));}

关键代码在这里：

    public void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType) {ResolvableType type = eventType != null?eventType:this.resolveDefaultEventType(event);Iterator var4 = this.getApplicationListeners(event, type).iterator();while(var4.hasNext()) {ApplicationListener listener = (ApplicationListener)var4.next();Executor executor = this.getTaskExecutor();if(executor != null) {executor.execute(() -> {this.invokeListener(listener, event);});} else {this.invokeListener(listener, event);}}}protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {ErrorHandler errorHandler = this.getErrorHandler();if(errorHandler != null) {try {this.doInvokeListener(listener, event);} catch (Throwable var5) {errorHandler.handleError(var5);}} else {this.doInvokeListener(listener, event);}}

在上面代码中我们看到，starting就是拿到META-INF/spring.factories中定义的资源的实例以后，然后再创建一个线程去启动起来。

通过上面的这些代码我们知道了spring boot会获取META-INF/spring.factories中的资源，并创建这些资源的实例（listeners监听器），然后为每一个监听器创建一个线程启动起来。

篇幅有限， 今天就写到这里吧。有希望一起学习spring boot 2.0源码的同学可以关注我，跟我一起分析spring boot 2.0 源码的实现方式。

作者：Lizongshen
出处：http://www.cnblogs.com/lizongshen/
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