SpringApplication Classes that can be used to bootstrap and launch a Spring application from a Java main method
在java的main方法中使用该类用于引导和启动一个spring应用程序
By default class will perform(执行) the following steps to bootstrap your application

1. Create an appropriate(适当的) {ApplicationContext} instance(实例) (depending on your classpath)
2. Register(注册) a {CommandLinePropertySource} to expose (暴露) command line arguments as Spring properties
3. Refresh(刷新) the application context, loading(加载) all singleton beans
4. Trigger(触发) any {CommandLineRunner} beans

创建 ApplicationContext
注册 CommandLinePropertySource
刷新 应用程序上下文环境,加载 all singleton beans
触发 CommandLineRunner beans

new SpringApplication(sources).run(args)
1. new SpringApplication(sources) 
  1.1 initialize(sources)
2. run(args)

private final Set<Object> sources = new LinkedHashSet<Object>()
private void initialize(Object[] sources) {
        if (sources != null && sources.length > 0) {
            this.sources.addAll(Arrays.asList(sources)); // 1 name = com.xst.springboot01.SpringBoot01Application
        }
        this.webEnvironment = deduceWebEnvironment(); // 2 true
        setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));// 3
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)); // 4
        this.mainApplicationClass = deduceMainApplicationClass(); // 5
    }

//1 
private final Set<Object> sources = new LinkedHashSet<Object>();

// 2
private boolean webEnvironment; 

//3
private List<ApplicationContextInitializer<?>> initializers;  // size:7

// 4
private List<ApplicationListener<?>> listeners; //size:11

// 5
private Class<?> mainApplicationClass; 
// mainApplicationClass:com.xst.springboot01.SpringBoot01Application

private <T> Collection<? extends 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<String>(
                SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
                classLoader, args, names);
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }

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

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        try {
            Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            List<String> result = new ArrayList<String>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
                String factoryClassNames = properties.getProperty(factoryClassName);
                result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
            }
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                    "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }