精尽Spring MVC源码分析 - HandlerExceptionResolver 组件 (2)

如果未获得到，则获得默认配置的 HandlerExceptionResolver 类，调用 getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) 方法，就是从 DispatcherServlet.properties 文件中读取 HandlerExceptionResolver 的默认实现类，如下：

org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver,\ org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\ org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

在 Spring Boot 中，默认配置下会走上述 1 的逻辑，handlerExceptionResolvers 有两个元素：

org.springframework.boot.autoconfigure.web.DefaultErrorAttributes：在 Spring Boot 中，逻辑比较简单，暂时忽略

org.springframework.web.servlet.handler.HandlerExceptionResolverComposite：复合的 HandlerExceptionResolver 实现类

接下来会对 HandlerExceptionResolverComposite 中的这三种异常处理器进行分析

HandlerExceptionResolverComposite

org.springframework.web.servlet.handler.HandlerExceptionResolverComposite，实现 HandlerExceptionResolver、Ordered 接口，复合的 HandlerExceptionResolver 实现类

构造方法 public class HandlerExceptionResolverComposite implements HandlerExceptionResolver, Ordered { /** * 异常解析器数组 */ @Nullable private List<HandlerExceptionResolver> resolvers; /** * 优先级，默认最低 */ private int order = Ordered.LOWEST_PRECEDENCE; }

resolvers：HandlerExceptionResolver 实现类列表

order：优先级，默认最低

从上面的初始化过程中可以看到，Spring Boot 默认配置下 HandlerExceptionResolverComposite 包含三个实现类：

org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver

org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver

org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

resolveException

实现 resolveException(HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) 方法，遍历 HandlerExceptionResolver 数组，逐个处理异常 ex，如果成功，则返回 ModelAndView 对象，方法如下：

@Override @Nullable public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) { if (this.resolvers != null) { for (HandlerExceptionResolver handlerExceptionResolver : this.resolvers) { ModelAndView mav = handlerExceptionResolver.resolveException(request, response, handler, ex); if (mav != null) { return mav; } } } return null; } AbstractHandlerExceptionResolver

org.springframework.web.servlet.handler.AbstractHandlerExceptionResolver，实现 HandlerExceptionResolver、Ordered 接口，HandlerExceptionResolver 抽象类，作为所有 HandlerExceptionResolver 实现类的基类

构造方法 public abstract class AbstractHandlerExceptionResolver implements HandlerExceptionResolver, Ordered { private static final String HEADER_CACHE_CONTROL = "Cache-Control"; /** * 优先级，默认最低 */ private int order = Ordered.LOWEST_PRECEDENCE; /** * 匹配的处理器对象的集合 */ @Nullable private Set<?> mappedHandlers; /** * 匹配的处理器类型的数组 */ @Nullable private Class<?>[] mappedHandlerClasses; /** * 防止响应缓存 */ private boolean preventResponseCaching = false; }

上面的这些属性在后续方法中会讲到

shouldApplyTo

shouldApplyTo(HttpServletRequest request, Object handler) 方法，判断当前 HandlerExceptionResolver 是否能应用到传入的 handler 处理器，方法如下：

protected boolean shouldApplyTo(HttpServletRequest request, @Nullable Object handler) { if (handler != null) { // <1> 如果 mappedHandlers 包含 handler 对象，则返回 true if (this.mappedHandlers != null && this.mappedHandlers.contains(handler)) { return true; } // <2> 如果 mappedHandlerClasses 包含 handler 的类型，则返回 true if (this.mappedHandlerClasses != null) { for (Class<?> handlerClass : this.mappedHandlerClasses) { if (handlerClass.isInstance(handler)) { return true; } } } } // Else only apply if there are no explicit handler mappings. // <3> 如果 mappedHandlers 和 mappedHandlerClasses 都为空，说明直接匹配 return (this.mappedHandlers == null && this.mappedHandlerClasses == null); }

如果 mappedHandlers 包含该 handler 处理器对象，则返回 true

如果 mappedHandlerClasses 包含该 handler 处理器所在类，则返回 true

如果 mappedHandlers 和 mappedHandlerClasses 都为空，说明直接匹配

prepareResponse