springboot 启动过程中，web环境的判断

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springboot在初始化过程中，会校验当前项目是否满足web环境，是如何做到的？很简单

在SpringApplication类initialize(Object[] sources)初始化方法里面，第6行是向webEnvironment赋值，调用的是13行的deduceWebEnvironment()方法，

@SuppressWarnings({ "unchecked", "rawtypes" })
	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();
	}

	private boolean deduceWebEnvironment() {
		for (String className : WEB_ENVIRONMENT_CLASSES) {
			if (!ClassUtils.isPresent(className, null)) {
				return false;
			}
		}
		return true;
	}

这个deduceWebEnvironment会依次遍历String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",
            "org.springframework.web.context.ConfigurableWebApplicationContext" }

数组，再调用ClassUtils.isPresent(String className, ClassLoader classLoader)方法

public static boolean isPresent(String className, ClassLoader classLoader) {
		try {
			forName(className, classLoader);
			return true;
		}
		catch (Throwable ex) {
			// Class or one of its dependencies is not present...
			return false;
		}
	}

再依次调用forName(String name, ClassLoader classLoader)方法，

public static Class<?> forName(String name, ClassLoader classLoader) throws ClassNotFoundException, LinkageError {
		Assert.notNull(name, "Name must not be null");

		Class<?> clazz = resolvePrimitiveClassName(name);
		if (clazz == null) {
			clazz = commonClassCache.get(name);
		}
		if (clazz != null) {
			return clazz;
		}

		// "java.lang.String[]" style arrays
		if (name.endsWith(ARRAY_SUFFIX)) {
			String elementClassName = name.substring(0, name.length() - ARRAY_SUFFIX.length());
			Class<?> elementClass = forName(elementClassName, classLoader);
			return Array.newInstance(elementClass, 0).getClass();
		}

		// "[Ljava.lang.String;" style arrays
		if (name.startsWith(NON_PRIMITIVE_ARRAY_PREFIX) && name.endsWith(";")) {
			String elementName = name.substring(NON_PRIMITIVE_ARRAY_PREFIX.length(), name.length() - 1);
			Class<?> elementClass = forName(elementName, classLoader);
			return Array.newInstance(elementClass, 0).getClass();
		}

		// "[[I" or "[[Ljava.lang.String;" style arrays
		if (name.startsWith(INTERNAL_ARRAY_PREFIX)) {
			String elementName = name.substring(INTERNAL_ARRAY_PREFIX.length());
			Class<?> elementClass = forName(elementName, classLoader);
			return Array.newInstance(elementClass, 0).getClass();
		}

		ClassLoader clToUse = classLoader;
		if (clToUse == null) {
			clToUse = getDefaultClassLoader();
		}
		try {
			return (clToUse != null ? clToUse.loadClass(name) : Class.forName(name));
		}
		catch (ClassNotFoundException ex) {
			int lastDotIndex = name.lastIndexOf(PACKAGE_SEPARATOR);
			if (lastDotIndex != -1) {
				String innerClassName =
						name.substring(0, lastDotIndex) + INNER_CLASS_SEPARATOR + name.substring(lastDotIndex + 1);
				try {
					return (clToUse != null ? clToUse.loadClass(innerClassName) : Class.forName(innerClassName));
				}
				catch (ClassNotFoundException ex2) {
					// Swallow - let original exception get through
				}
			}
			throw ex;
		}
	}

这个forName方法，最终执行的是ClassLoader.loadClass(String name)方法，简单的说，

分别去加载javax.servlet.Servlet和org.springframework.web.context.ConfigurableWebApplicationContext，如果有一个加载失败，则返回false，即不是web环境