使用了很长时间的springboot了，一直都知道它简单易用，简化了框架的搭建过程，但是还是不知道它是如何启动的，今天就跟着springboot的源码，去探探这其中的奥妙
以下是spring应用的启动：
@SpringBootApplication
public class Application extends WebMvcConfigurerAdapter {
public static void main(String[] args) {
//加载配置文件
System.setProperty("spring.config.location","classpath:db_config.properties,classpath:mq.properties");
SpringApplication.run(Application.class, args);
}
}
然后我们跟着Run方法进去
public ConfigurableApplicationContext run(String... args) {
//一开始就是一个StopWatch 类，这个类的主要作用是记录容器启动用时
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
FailureAnalyzers analyzers = null;
this.configureHeadlessProperty();
//加载SPI文件
SpringApplicationRunListeners listeners = this.getRunListeners(args);
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
Banner printedBanner = this.printBanner(environment);
context = this.createApplicationContext();
new FailureAnalyzers(context);
this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
this.refreshContext(context);
this.afterRefresh(context, applicationArguments);
listeners.finished(context, (Throwable)null);
stopWatch.stop();
if(this.logStartupInfo) {
(new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
}
return context;
} catch (Throwable var9) {
this.handleRunFailure(context, listeners, (FailureAnalyzers)analyzers, var9);
throw new IllegalStateException(var9);
}
}
第一步：可以看到，一开始是一个StopWatch类，该类的作用比较单一，就是记录springboot的启动用时，我们启动springboot完成后会在控制台springboot启动所花的时间，就是由它来完成的
//然后我们看看this.createApplicationContext()方法：
private void configureHeadlessProperty() {
System.setProperty("java.awt.headless", System.getProperty("java.awt.headless", Boolean.toString(this.headless)));
}
//主要是用来设置系统属性，具体这个属性有何作用，有待探究
第二步：创建SpringApplicationRunListeners
//接下来我们看看springboot是如何去初始化监听器SpringApplicationRunListeners的。this.getRunListeners(args)方法如下：
private SpringApplicationRunListeners getRunListeners(String[] args) {
Class<?>[] types = new Class[]{SpringApplication.class, String[].class};
return new SpringApplicationRunListeners(logger, this.getSpringFactoriesInstances(SpringApplicationRunListener.class, types, new Object[]{this, args}));
}
//利用传入的参数args去创建一个SpringApplicationRunListeners实例，此处会去获取一个SpringFactory的实例，下面我们重点看看是如何来获取该SpringFactory实例的
private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
Set<String> names = new LinkedHashSet(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
List<T> instances = this.createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
//首先获取当前线程的类加载器，然后通过SpringFactoriesLoader去加载SpringApplicationRunListener这个类，接下来看看在加载SpringApplicationRunListener这个spring应用的运行监听器时做了什么事情
public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
try {
Enumeration ex = classLoader != null?classLoader.getResources("META-INF/spring.factories"):ClassLoader.getSystemResources("META-INF/spring.factories");
ArrayList result = new ArrayList();
while(ex.hasMoreElements()) {
URL url = (URL)ex.nextElement();
Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
String factoryClassNames = properties.getProperty(factoryClassName);
result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
}
return result;
} catch (IOException var8) {
throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + "META-INF/spring.factories" + "]", var8);
}
}
//首先通过当前线程的类加载器获取Springboot项目META-INF下面的spring.factories文件，找到该文件，我们可以看到该文件中配置了PropertySource Loaders(属性来源加载)、运行监听器EventPublishingRunListener、运用上下文初始化工具、应用监听器等等一系列的自动化配置，都是从这里加载到应用中来的。加载完成之后，SpringFactory通过反射来获取这些类的实例，然后放入到SpringApplicationRunListeners中，至此，SpringApplicationRunListeners初始化完成。这里，我们看看SpringApplicationRunListeners中有些什么，源码如下：
class SpringApplicationRunListeners {
private final Log log;
private final List<SpringApplicationRunListener> listeners;
SpringApplicationRunListeners(Log log, Collection<? extends SpringApplicationRunListener> listeners) {
this.log = log;
this.listeners = new ArrayList(listeners);
}
}
//有一个log，用于记录日志，然后就是一个final的list，里面就是之前初始化时从配置中获取的各种listener
第三步：启动SpringApplicationRunListeners中所有的监听器
第四步：环境准备
第五步：打印Banner
第六步：创建上下文