传统Spring项目

在写传统的spring项目中，一般通过初始化抽象类AbstractXmlApplicationContext 的实现类，并传入spring.xml，来获取应用上下文，最终通过getBean方法获取bean，如下：

	ApplicationContext app1 = new FileSystemXmlApplicationContext("applicationContext.xml");
	app1.getBean("beanName");
	ApplicationContext app2 = new ClassPathXmlApplicationContext("applicationContext.xml");
	app2.getBean("beanName");

SpringBoot项目获取bean的几种方式

1. 通过启动类中返回的上下文获取

	ConfigurableApplicationContext app = SpringApplication.run(BeanDemoApplication.class, args);
	SpringUtil.setAppContext(app);
	public class SpringUtil {
	private static ApplicationContext appContext;	
	public static void setAppContext(ApplicationContext appContext) {
		SpringUtil.appContext = appContext;
	}	
	public static ApplicationContext getAppContext() {
		return appContext;
	}	
}

在第三方类中使用：

ApplicationContext appContext = SpringUtil.getAppContext();
appContext.getBean("beanName");

2. 通过工具类获取

RequestContextUtils.findWebApplicationContext(HttpServletRequest request),WebApplicationContextUtils.getWebApplicationContext(ServletContext sc)
a. 在controller中传入request，例如：

public String test(HttpServletRequest request,HttpServletRequest response) {
	WebApplicationContext wc = RequestContextUtils.findWebApplicationContext(request);
	wc.getBean("beanName");
	
	WebApplicationContext wc2 = WebApplicationContextUtils.getWebApplicationContext(request.getServletContext());
	wc2.getBean("beanName");
}

b. 在service中或者其他后端服务中获取：

HttpServletRequest request = ((ServletRequestAttributes)RequestContextHolder.getRequestAttributes()).getRequest();
WebApplicationContext wc = RequestContextUtils.findWebApplicationContext(request);
WebApplicationContext wc2 = WebApplicationContextUtils.getWebApplicationContext(request.getServletContext());

wc.getBean("beanName");
wc2.getBean("beanName");

3. 通过实现接口ApplicationContextAware

@Component
public class TestApplicationContextAware implements ApplicationContextAware {

	private static ApplicationContext applicationContext;

	@Override
	public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
		applicationContext = applicationContext;
	}
	
	public static Object getBean(String beanName) {
		return applicationContext.getBean(beanName);
	}
	
	public static ApplicationContext getApplicationContext() {
		return applicationContext;
	}

}

在其他类中调用

@Autowired
private TestApplicationContextAware app;
public void testMethod() {
	app.getBean("beanName");
}

4. 通过继承抽象类：ApplicationObjectSupport，WebApplicationObjectSupport

原理参考第3点

5. 其他方式

在网上看，发现也可以直接调用：ContextLoader.getCurrentWebApplicationContext()，或者 ContextLoaderListener.getCurrentWebApplicationContext() 其实都是调用同一段代码，如下：

@Nullable
public static WebApplicationContext getCurrentWebApplicationContext() {
	ClassLoader ccl = Thread.currentThread().getContextClassLoader();
	if (ccl != null) {
		WebApplicationContext ccpt = currentContextPerThread.get(ccl);
		if (ccpt != null) {
			return ccpt;
		}
	}
	return currentContext;
}

说明：目前通过这种方式获取上下文为null，从代码可以看出，上下文是通过currentContextPerThread.get(ccl)来获取的，而currentContextPerThread缓存是通过方法contextInitialized(ServletContextEvent event)来初始化的，至于为何获取为空.
参考：https://www.cnblogs.com/xysn/p/14863896.html