SpringBoot 获取上下文，获取bean的几种中方式

Posted 2022-01-18 mb6107c14696a87

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  ApplicationContext applicationContext;

  @Override
  public void setApplicationContext(ApplicationContext applicationContext) throws BeansException 
    this.applicationContext = applicationContext;
  

  public Object getBean(String beanName) 
    return applicationContext.getBean(beanName);
  

  public 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