SpringBoot启动过程：SpringApplication及SpringApplicationBuilder

SpringBoot应用的启动是基于SpringApplication类的，下面一步步地分析。

其构造方法有二，都调用了initialize方法，完成注解源的配置。

	public SpringApplication(Object... sources) {
		initialize(sources);
	}
    public SpringApplication(ResourceLoader resourceLoader, Object... sources) {
		this.resourceLoader = resourceLoader;
		initialize(sources);
	}

	private void initialize(Object[] sources) {
		if (sources != null && sources.length > 0) {
			this.sources.addAll(Arrays.asList(sources));
		}
		this.webEnvironment = deduceWebEnvironment();
		setInitializers((Collection) getSpringFactoriesInstances(
				ApplicationContextInitializer.class));
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}

• initialize方法首先将注解源设置到成员变量this.sources中，然后deduceWebEnvironment方法中根据类路径是否有javax.servlet.Servlet、org.springframework.web.context.ConfigurableWebApplicationContext这两个类判断是否是web环境this.webEnvironment。
• 利用spring.factories机制机制加载ApplicationContextInitializer和ApplicationListener。
• 然后找到main方法所在类mainApplicationClass。

run方法是程序运行的主要方法

	public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		FailureAnalyzers analyzers = null;
		configureHeadlessProperty();
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(
					args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners,
					applicationArguments);
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			analyzers = new FailureAnalyzers(context);
			prepareContext(context, environment, listeners, applicationArguments,
					printedBanner);
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			listeners.finished(context, null);
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass)
						.logStarted(getApplicationLog(), stopWatch);
			}
			return context;
		}
		catch (Throwable ex) {
			handleRunFailure(context, listeners, analyzers, ex);
			throw new IllegalStateException(ex);
		}
	}

• SpringApplicationRunListeners加载，利用spring.factories机制加载SpringApplicationRunListener，封装到SpringApplicationRunListeners，SpringApplicationRunListener定义了一些容器的生命周期方法，SpringApplicationRunListeners就是为了方便循环调用多个SpringApplicationRunListener的方法。在容器的各个生命周期时刻调用相应的方法。

public interface SpringApplicationRunListener {

	
	void starting();

	
	void environmentPrepared(ConfigurableEnvironment environment);

	
	void contextPrepared(ConfigurableApplicationContext context);

	
	void contextLoaded(ConfigurableApplicationContext context);

	
	void finished(ConfigurableApplicationContext context, Throwable exception);

}

• 将主方法入参args封装为ApplicationArguments(默认实现DefaultApplicationArguments)，方便获取命令行参数

public interface ApplicationArguments {

	
	String[] getSourceArgs();

	
	Set getOptionNames();

	
	boolean containsOption(String name);

	
	List getOptionValues(String name);

	
	List getNonOptionArgs();

}

• 准备ConfigurableEnvironment，其中configureEnvironment将命令行参数封装为SimpleCommandLinePropertySource添加到环境中，配置profile。

	private ConfigurableEnvironment prepareEnvironment(
			SpringApplicationRunListeners listeners,
			ApplicationArguments applicationArguments) {
		// Create and configure the environment
		ConfigurableEnvironment environment = getOrCreateEnvironment();
		configureEnvironment(environment, applicationArguments.getSourceArgs());
		listeners.environmentPrepared(environment);
		if (!this.webEnvironment) {
			environment = new EnvironmentConverter(getClassLoader())
					.convertToStandardEnvironmentIfNecessary(environment);
		}
		return environment;
	}
	protected void configureEnvironment(ConfigurableEnvironment environment,
			String[] args) {
		configurePropertySources(environment, args);
		configureProfiles(environment, args);
	}

	
	protected void configurePropertySources(ConfigurableEnvironment environment,
			String[] args) {
		MutablePropertySources sources = environment.getPropertySources();
		if (this.defaultProperties != null && !this.defaultProperties.isEmpty()) {
			sources.addLast(
					new MapPropertySource("defaultProperties", this.defaultProperties));
		}
		if (this.addCommandLineProperties && args.length > 0) {
			String name = CommandLinePropertySource.COMMAND_LINE_PROPERTY_SOURCE_NAME;
			if (sources.contains(name)) {
				PropertySource source = sources.get(name);
				CompositePropertySource composite = new CompositePropertySource(name);
				composite.addPropertySource(new SimpleCommandLinePropertySource(
						name + "-" + args.hashCode(), args));
				composite.addPropertySource(source);
				sources.replace(name, composite);
			}
			else {
				sources.addFirst(new SimpleCommandLinePropertySource(args));
			}
		}
	}

	
	protected void configureProfiles(ConfigurableEnvironment environment, String[] args) {
		environment.getActiveProfiles(); // ensure they are initialized
		// But these ones should go first (last wins in a property key clash)
		Set profiles = new LinkedHashSet(this.additionalProfiles);
		profiles.addAll(Arrays.asList(environment.getActiveProfiles()));
		environment.setActiveProfiles(profiles.toArray(new String[profiles.size()]));
	}

• createApplicationContext方法创建容器ConfigurableApplicationContext，根据签名的是否web环境决定是创建AnnotationConfigEmbeddedWebApplicationContext或者AnnotationConfigApplicationContext
• 创建失败分析器FailureAnalyzers
• prepareContext 准备环境

	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