spring boot 2.0 源码分析（二）

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 = 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。