spring boot 2.0 源码分析（一）

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，