Spring Boot 项目启动报错: Unable to start embedded

相关版本

Spring Boot 2.1.5.RELEASE

Spring AOP 1.5.7.RELEASE

CGLIB原理: 动态生成一个要代理类的子类，子类重写要代理的类的所有不是final的方法。在子类中采用方法拦截的技术拦截所有父类方法的调用，顺势织入横切逻辑。它比使用java反射的JDK动态代理要快。

• 主要原因由于org.springframework.web.filter.GenericFilterBean的子类A的代理对象的logger属性为空引发的空指针异常

• 分析

  1. AOP 注解@Aspect。对应切点配置的路径下类会由CGLIB(SpringBoot默认)动态代理生成.

  @Pointcut("execution(public * com.example.handler..*.*(..))")
  

  2. 包com.example.handler下的类A继承了OncePerRequestFilter

  3. 由于CGLIB原理, 生成类A的代理对象时, final声明的属性将为空

   public abstract class GenericFilterBean implements Filter, BeanNameAware, EnvironmentAware,
   	EnvironmentCapable, ServletContextAware, InitializingBean, DisposableBean {
  
   /** Logger available to subclasses. */
   # ======> logger 属性声明为 final <======
   protected final Log logger = LogFactory.getLog(getClass());
  
   @Nullable
   private String beanName;
  
   @Nullable
   private Environment environment;
  
   @Nullable
   private ServletContext servletContext;
  
   @Nullable
   private FilterConfig filterConfig;
  
   private final Set<String> requiredProperties = new HashSet<>(4);
  
   略......
  

  4. GenericFilterBean执行init()方法到类A时,由于类A的代理对象的logger属性为空, 导致logger.isDebugEnabled()报空指针异常

  // org.springframework.web.filter.GenericFilterBean
  
  /**
   * Standard way of initializing this filter.
   * Map config parameters onto bean properties of this filter, and
   * invoke subclass initialization.
   * @param filterConfig the configuration for this filter
   * @throws ServletException if bean properties are invalid (or required
   * properties are missing), or if subclass initialization fails.
   * @see #initFilterBean
   */
  @Override
  public final void init(FilterConfig filterConfig) throws ServletException {
     Assert.notNull(filterConfig, "FilterConfig must not be null");
  
     this.filterConfig = filterConfig;
  
     // Set bean properties from init parameters.
     PropertyValues pvs = new FilterConfigPropertyValues(filterConfig, this.requiredProperties);
     if (!pvs.isEmpty()) {
        try {
           BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
           ResourceLoader resourceLoader = new ServletContextResourceLoader(filterConfig.getServletContext());
           Environment env = this.environment;
           if (env == null) {
              env = new StandardServletEnvironment();
           }
           bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, env));
           initBeanWrapper(bw);
           bw.setPropertyValues(pvs, true);
        }
        catch (BeansException ex) {
           String msg = "Failed to set bean properties on filter '" +
              filterConfig.getFilterName() + "': " + ex.getMessage();
           logger.error(msg, ex);
           throw new NestedServletException(msg, ex);
        }
     }
  
     // Let subclasses do whatever initialization they like.
     initFilterBean();
  
     # ========> 由于 logger 为空, 此处出现空指针异常 <========
     if (logger.isDebugEnabled()) {
        logger.debug("Filter '" + filterConfig.getFilterName() + "' configured for use");
     }
  }
  

• 解决方案

  • 将继承OncePerRequestFilter的类A移出对应包路径com.example.handler

  • 将对应 Filter 功能移到其他包下执行