在学习spring boot 2.0源码之前，我们先利用spring initializr快速地创建一个基本的简单的示例：
1.先从创建示例中的main函数开始读起：
package com.example;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;/** * spring boot 2.0 源码分析 * author lizongshen * date 2018/06/02 */ @SpringBootApplicationpublic class DemoApplication { public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
}2.在这里我们可以看到，spring boot是通过SpringApplication.run这个函数来进行启动的，其中args可以传递启动时需要的个性化参数。跳转到源码中继续一探究竟：
/** * Static helper that can be used to run a {@link SpringApplication} from the * specified source using default settings. * @param primarySource the primary source to load * @param args the application arguments (usually passed from a Java main method) * @return the running {@link ApplicationContext} */
public static ConfigurableApplicationContext run(Class<?> primarySource,
String... args) { return run(new Class<?>[] { primarySource }, args);
}3.在这里我们看到，其把primarySource这个参数包装成数组，跳转到了另外一个同样的方法中。
小发现：SpringApplication.run()函数是允许同时启动多个Application的。
接着往下读
/**
* Static helper that can be used to run a {@link SpringApplication} from the
* specified sources using default settings and user supplied arguments.
* @param primarySources the primary sources to load
* @param args the application arguments (usually passed from a Java main method)
* @return the running {@link ApplicationContext}
*/
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) { return new SpringApplication(primarySources).run(args);
}4.在这段代码中，我们可以看到spring boot把我们所使用的静态方法，创建出了一个SpringApplication的实例，并启动了实例中的run方法。
小知识：根据这个发现，我们也可以在main函数中，自己创建SpringApplication的实例，然后调用实例方法run。
我们来看一下SpringApplication的构造函数期间都干了些什么事：
/** * Create a new {@link SpringApplication} instance. The application context will load * beans from the specified primary sources (see {@link SpringApplication class-level} * documentation for details. The instance can be customized before calling * {@link #run(String...)}. * @param primarySources the primary bean sources * @see #run(Class, String[]) * @see #SpringApplication(ResourceLoader, Class...) * @see #setSources(Set) */
public SpringApplication(Class<?>... primarySources) { this(null, primarySources);
} /** * Create a new {@link SpringApplication} instance. The application context will load * beans from the specified primary sources (see {@link SpringApplication class-level} * documentation for details. The instance can be customized before calling * {@link #run(String...)}. * @param resourceLoader the resource loader to use * @param primarySources the primary bean sources * @see #run(Class, String[]) * @see #setSources(Set) */
@SuppressWarnings({ "unchecked", "rawtypes" }) public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) { this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null"); this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources)); this.webApplicationType = deduceWebApplicationType(); setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class)); setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)); this.mainApplicationClass = deduceMainApplicationClass();
}在这里主要是初始化了SpringApplication的私有属性，在构造的过程中，调用了另外一个构造函数，并传递了ResourceLoader的参数。
小知识：通过ResourceLoader这个参数，我们看到如果自定义SpringApplication在初始化过程中，是可以通过ResourceLoader来引入自定义资源的。
接着往下看，来看看run函数的真面目
/** * 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;
}这段代码很长，我们先来一点一点地分析。
1.通过configureHeadlessProperty();这行代码，配置属性：
private void configureHeadlessProperty() {
System.setProperty(SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, System.getProperty(
SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, Boolean.toString(this.headless)));
}2.通过SpringApplicationRunListeners listeners = getRunListeners(args);这行代码获取了监听器：
private SpringApplicationRunListeners getRunListeners(String[] args) {
Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class }; return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances(
SpringApplicationRunListener.class, types, this, args));
}3.通过listeners.starting();这行代码启动监听器：
public void starting() {
Iterator var1 = this.listeners.iterator(); while(var1.hasNext()) {
SpringApplicationRunListener listener = (SpringApplicationRunListener)var1.next();
listener.starting();
}
}4.通过context = this.createApplicationContext();创建了容器
/** * Strategy method used to create the {@link ApplicationContext}. By default this * method will respect any explicitly set application context or application context * class before falling back to a suitable default. * @return the application context (not yet refreshed) * @see #setApplicationContextClass(Class) */
protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass; if (contextClass == null) { try { switch (this.webApplicationType) { case SERVLET:
contextClass = Class.forName(DEFAULT_WEB_CONTEXT_CLASS); break; case REACTIVE:
contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS); break; default:
contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
}
} catch (ClassNotFoundException ex) { throw new IllegalStateException( "Unable create a default ApplicationContext, "
+ "please specify an ApplicationContextClass",
ex);
}
} return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}5.通过prepareContext(context, environment, listeners, applicationArguments,printedBanner);这行代码来准备容器：
private void prepareContext(ConfigurableApplicationContext context,
ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
ApplicationArguments applicationArguments, Banner printedBanner) {
context.setEnvironment(environment); postProcessApplicationContext(context); applyInitializers(context);
listeners.contextPrepared(context); if (this.logStartupInfo) { logStartupInfo(context.getParent() == null); logStartupProfileInfo(context);
} // Add boot specific singleton beans
context.getBeanFactory().registerSingleton("springApplicationArguments",
applicationArguments); if (printedBanner != null) {
context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
} // Load the sources
Set<Object> sources = getAllSources();
Assert.notEmpty(sources, "Sources must not be empty"); load(context, sources.toArray(new Object[0]));
listeners.contextLoaded(context);
}6.通过this.refreshContext(context);刷新容器：
private void refreshContext(ConfigurableApplicationContext context) { this.refresh(context); if(this.registerShutdownHook) { try {
context.registerShutdownHook();
} catch (AccessControlException var3) {
;
}
}
}在刷新容器完成之后，调用afterRefresh函数，跟踪过去发现是空的，可能是预留了一个扩展点。
通过阅读发现spring boot 2.0的源码和1.x的源码对比起来，细节方面还是变化很大的。当然，这些还只是其中的冰山一角，后面我会继续与大家分享在源码阅读中的一些心得体会，
作者：Lizongshen
出处：http://www.cnblogs.com/lizongshen/
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