spring boot 2.0 源码分析（三）

通过上一章的源码分析，我们知道了spring boot里面的listeners到底是什么（META-INF/spring.factories定义的资源的实例），以及它是创建和启动的，今天我们继续深入分析一下SpringApplication实例变量中的run函数中的其他内容。还是先把run函数的代码贴出来：

    /**
     * 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;
    }

**在listeners启动了以后，我们来看一下ApplicationArguments applicationArguments **

= new DefaultApplicationArguments(args); 在DefaultApplicationArguments的构造函数里，我们跟踪过去发现其最终调用的SimpleCommandLineArgsParser.parse函数：

public CommandLineArgs parse(String... args) {
        CommandLineArgs commandLineArgs = new CommandLineArgs();
        String[] var3 = args;
        int var4 = args.length;

        for(int var5 = 0; var5 < var4; ++var5) {
            String arg = var3[var5];
            if(arg.startsWith("--")) {
                String optionText = arg.substring(2, arg.length());
                String optionValue = null;
                String optionName;
                if(optionText.contains("=")) {
                    optionName = optionText.substring(0, optionText.indexOf(61));
                    optionValue = optionText.substring(optionText.indexOf(61) + 1,
                    optionText.length());
                } else {
                    optionName = optionText;
                }

                if(optionName.isEmpty() || optionValue != null && optionValue.isEmpty()) {
                    throw new IllegalArgumentException("Invalid argument syntax: " + arg);
                }

                commandLineArgs.addOptionArg(optionName, optionValue);
            } else {
                commandLineArgs.addNonOptionArg(arg);
            }
        }

        return commandLineArgs;
    }

从这段代码中我们看到DefaultApplicationArguments其实是读取了命令行的参数。

小发现：通过分析这个函数的定义，你是不是想起了spring boot启动的时候，用命令行参数自定义端口号的情景？

java -jar MySpringBoot.jar --server.port=8000

接着往下看：ConfigurableEnvironment environment = this.prepareEnvironment(listeners, ex);

通过这行代码我们可以看到spring boot把前面创建出来的listeners和命令行参数，传递到prepareEnvironment函数中来准备运行环境。来看一下prepareEnvironment函数的真面目：

	private ConfigurableEnvironment prepareEnvironment(
			SpringApplicationRunListeners listeners,
			ApplicationArguments applicationArguments) {
		// Create and configure the environment
		ConfigurableEnvironment environment = getOrCreateEnvironment();
		configureEnvironment(environment, applicationArguments.getSourceArgs());
		listeners.environmentPrepared(environment);
		bindToSpringApplication(environment);
		if (this.webApplicationType == WebApplicationType.NONE) {
			environment = new EnvironmentConverter(getClassLoader())
					.convertToStandardEnvironmentIfNecessary(environment);
		}
		ConfigurationPropertySources.attach(environment);
		return environment;
	}

在这里我们看到了环境是通过getOrCreateEnvironment创建出来的，再深挖一下getOrCreateEnvironment的源码：

	private ConfigurableEnvironment getOrCreateEnvironment() {
		if (this.environment != null) {
			return this.environment;
		}
		if (this.webApplicationType == WebApplicationType.SERVLET) {
			return new StandardServletEnvironment();
		}
		return new StandardEnvironment();
	}

通过这段代码我们看到了如果environment 已经存在，则直接返回当前的环境。

小思考：在什么情况下会出现environment 已经存在的情况？提示：我们前面讲过，可以自己初始化SpringApplication，然后调用run函数，在初始化SpringApplication和调用run函数之间，是不是可以发生点什么？

下面的代码判断了webApplicationType是不是SERVLET，如果是，则创建Servlet的环境，否则创建基本环境。我们来挖一挖webApplicationType是在哪里初始化的：

	private static final String REACTIVE_WEB_ENVIRONMENT_CLASS = "org.springframework."
			+ "web.reactive.DispatcherHandler";

	private static final String MVC_WEB_ENVIRONMENT_CLASS = "org.springframework."
			+ "web.servlet.DispatcherServlet";
	/**
	 * 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();
	}

	private WebApplicationType deduceWebApplicationType() {
		if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null)
				&& !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)) {
			return WebApplicationType.REACTIVE;
		}
		for (String className : WEB_ENVIRONMENT_CLASSES) {
			if (!ClassUtils.isPresent(className, null)) {
				return WebApplicationType.NONE;
			}
		}
		return WebApplicationType.SERVLET;
	}

通过这段代码，我们发现了原来spring boot是通过检查当前环境中是否存在

org.springframework.web.servlet.DispatcherServlet类来判断当前是否是web环境的。

接着往下看，获得了ConfigurableEnvironment环境以后，通过后面的代码对环境进行“微调”。

通过this.configureIgnoreBeanInfo(environment);如果System中的spring.beaninfo.ignore属性为空，就把当前环境中的属性覆盖上去：

    private void configureIgnoreBeanInfo(ConfigurableEnvironment environment) {
        if(System.getProperty("spring.beaninfo.ignore") == null) {
            Boolean ignore = (Boolean)environment.getProperty("spring.beaninfo.ignore",
            Boolean.class, Boolean.TRUE);
            System.setProperty("spring.beaninfo.ignore", ignore.toString());
        }

    }

通过Banner printedBanner = this.printBanner(environment);这行代码打印出spring boot的Banner。还记得spring boot启动的时候，在控制台显示的那个图片吗？这里不作深究，继续往下看：

context = this.createApplicationContext();创建了应用上下文：

	public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."
			+ "annotation.AnnotationConfigApplicationContext";

	public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework.boot."
			+ "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";

	public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework."
			+ "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";

	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);
	}

通过这里我们看到，spring boot是根据不同的webApplicationType的类型，来创建不同的ApplicationContext的。

总结：通过上面的各种深挖，我们知道了spring boot 2.0中的环境是如何区分普通环境和web环境的，以及如何准备运行时环境和应用上下文。时间不早了，今天就跟大家分享到这里，下一篇文章会继续跟大家分享spring boot 2.0源码的实现。