spring boot 2.0 源码分析（一） - 好文

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 */ @SpringBootApplication public
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，

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