springmvc静态资源配置

  <servlet>
    <servlet-name>dispatcher</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <async-supported>false</async-supported>
  </servlet>
  <servlet-mapping>
    <servlet-name>dispatcher</servlet-name>
    <url-pattern>/</url-pattern>
  </servlet-mapping>

在javaweb项目中配置了DispatcherServlet的情况下，如果不进行额外配置的话，几乎所有的请求都会走这个servlet来处理，默认静态资源按路径是访问不到的会报404错误，下面讲一讲如何配置才能访问到静态资源，本文将介绍三种方法

1. 在java配置文件中配置DefaultServletHttpRequestHandler来进行处理

@Configuration
@EnableWebMvc
public class MyMvcConfigurer implements WebMvcConfigurer {
  @Override
  public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
    // tomcat默认处理静态资源的servlet名称为default,不指定也可以DefaultServletHttpRequestHandler.setServletContext会自动获取
//    configurer.enable("default");
    configurer.enable();
  }
}

上述配置完成后org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#defaultServletHandlerMapping 方法会生成一个类名为SimpleUrlHandlerMapping的bean，当其他handlerMapping无法处理请求时会接着调用SimpleUrlHandlerMapping对象进行处理

org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#defaultServletHandlerMapping
/**
 * Return a handler mapping ordered at Integer.MAX_VALUE with a mapped
 * default servlet handler. To configure "default" Servlet handling,
 * override {@link #configureDefaultServletHandling}.
 */
@Bean
public HandlerMapping defaultServletHandlerMapping() {
  Assert.state(this.servletContext != null, "No ServletContext set");
  DefaultServletHandlerConfigurer configurer = new DefaultServletHandlerConfigurer(this.servletContext);
  configureDefaultServletHandling(configurer);
  HandlerMapping handlerMapping = configurer.buildHandlerMapping();
  return (handlerMapping != null ? handlerMapping : new EmptyHandlerMapping());
}
org.springframework.web.servlet.config.annotation.DefaultServletHandlerConfigurer#buildHandlerMapping
@Nullable
protected SimpleUrlHandlerMapping buildHandlerMapping() {
  if (this.handler == null) {
    return null;
  }
  SimpleUrlHandlerMapping handlerMapping = new SimpleUrlHandlerMapping();
  handlerMapping.setUrlMap(Collections.singletonMap("/**", this.handler));
  handlerMapping.setOrder(Integer.MAX_VALUE);
  return handlerMapping;
}

SimpleUrlHandlerMapping中有一个urlMap属性，key为请求路径匹配模式串，'/**'能匹配所有的路径, value为handler匹配完成后会调用handler处理请求

下面这个方法主要用来匹配获取handler

org.springframework.web.servlet.handler.AbstractUrlHandlerMapping#getHandlerInternal

 	@Override
	@Nullable
	protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
		// 请求静态资源 path=/zxq/static/login.png
		// 处理完lookupPath=/static/login.png
		String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
		Object handler = lookupHandler(lookupPath, request);
		if (handler == null) {
			// We need to care for the default handler directly, since we need to
			// expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
			Object rawHandler = null;
			if ("/".equals(lookupPath)) {
				rawHandler = getRootHandler();
			}
			if (rawHandler == null) {
				rawHandler = getDefaultHandler();
			}
			if (rawHandler != null) {
				// Bean name or resolved handler?
				if (rawHandler instanceof String) {
					String handlerName = (String) rawHandler;
					rawHandler = obtainApplicationContext().getBean(handlerName);
				}
				validateHandler(rawHandler, request);
				handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
			}
		}
		if (handler != null && logger.isDebugEnabled()) {
			logger.debug("Mapping [" + lookupPath + "] to " + handler);
		}
		else if (handler == null && logger.isTraceEnabled()) {
			logger.trace("No handler mapping found for [" + lookupPath + "]");
		}
		return handler;
	}

org.springframework.web.servlet.handler.AbstractUrlHandlerMapping#lookupHandler

 	/**
	 * Look up a handler instance for the given URL path.
	 * <p>Supports direct matches, e.g. a registered "/test" matches "/test",
	 * and various Ant-style pattern matches, e.g. a registered "/t*" matches
	 * both "/test" and "/team". For details, see the AntPathMatcher class.
	 * <p>Looks for the most exact pattern, where most exact is defined as
	 * the longest path pattern.
	 * @param urlPath the URL the bean is mapped to
	 * @param request current HTTP request (to expose the path within the mapping to)
	 * @return the associated handler instance, or {@code null} if not found
	 * @see #exposePathWithinMapping
	 * @see org.springframework.util.AntPathMatcher
	 */
	@Nullable
	protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {
		// Direct match?
		// 精确匹配，是否有符合的handler
		// urlPath = /static/login.png
		Object handler = this.handlerMap.get(urlPath);
		if (handler != null) {
			// Bean name or resolved handler?
			if (handler instanceof String) {
				String handlerName = (String) handler;
				handler = obtainApplicationContext().getBean(handlerName);
			}
			validateHandler(handler, request);
			return buildPathExposingHandler(handler, urlPath, urlPath, null);
		}
		// Pattern match?
		// 路径匹配
		List<String> matchingPatterns = new ArrayList<>();
		for (String registeredPattern : this.handlerMap.keySet()) {
			if (getPathMatcher().match(registeredPattern, urlPath)) {
				matchingPatterns.add(registeredPattern);
			}
			else if (useTrailingSlashMatch()) {
				if (!registeredPattern.endsWith("/") && getPathMatcher().match(registeredPattern + "/", urlPath)) {
					matchingPatterns.add(registeredPattern + "/");
				}
			}
		}
		String bestMatch = null;
		Comparator<String> patternComparator = getPathMatcher().getPatternComparator(urlPath);
		if (!matchingPatterns.isEmpty()) {
			matchingPatterns.sort(patternComparator);
			if (logger.isDebugEnabled()) {
				logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);
			}
			bestMatch = matchingPatterns.get(0);
		}
		// bestMatch = /static/**
		if (bestMatch != null) {
			handler = this.handlerMap.get(bestMatch);
			if (handler == null) {
				if (bestMatch.endsWith("/")) {
					handler = this.handlerMap.get(bestMatch.substring(0, bestMatch.length() - 1));
				}
				if (handler == null) {
					throw new IllegalStateException(
							"Could not find handler for best pattern match [" + bestMatch + "]");
				}
			}
			// Bean name or resolved handler?
			if (handler instanceof String) {
				String handlerName = (String) handler;
				handler = obtainApplicationContext().getBean(handlerName);
			}
			validateHandler(handler, request);
			// login.png
			String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestMatch, urlPath);
			// There might be multiple 'best patterns', let's make sure we have the correct URI template variables
			// for all of them
			Map<String, String> uriTemplateVariables = new LinkedHashMap<>();
			for (String matchingPattern : matchingPatterns) {
				if (patternComparator.compare(bestMatch, matchingPattern) == 0) {
					Map<String, String> vars = getPathMatcher().extractUriTemplateVariables(matchingPattern, urlPath);
					Map<String, String> decodedVars = getUrlPathHelper().decodePathVariables(request, vars);
					uriTemplateVariables.putAll(decodedVars);
				}
			}
			if (logger.isDebugEnabled()) {
				logger.debug("URI Template variables for request [" + urlPath + "] are " + uriTemplateVariables);
			}
			// /static/**   login.png
			return buildPathExposingHandler(handler, bestMatch, pathWithinMapping, uriTemplateVariables);
		}
		// No handler found...
		return null;
	}

接着调用DefaultServletHttpRequestHandler的handleRequest方法处理请求，逻辑比较简单，获取请求转发器进行请求转发交给tomcat默认的servlet来进行处理

	@Override
	public void handleRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		Assert.state(this.servletContext != null, "No ServletContext set");
		RequestDispatcher rd = this.servletContext.getNamedDispatcher(this.defaultServletName);
		if (rd == null) {
			throw new IllegalStateException("A RequestDispatcher could not be located for the default servlet '" +
					this.defaultServletName + "'");
		}
		rd.forward(request, response);
	}

2. 在java配置文件中配置ResourceHttpRequestHandler来进行处理

@Configuration
@EnableWebMvc
public class MyMvcConfigurer implements WebMvcConfigurer {
  @Override
  public void addResourceHandlers(ResourceHandlerRegistry registry) {
    registry.addResourceHandler("/static/**").addResourceLocations("/static/");
  }
}

和第一种配置几乎一样，其实只是换了一个handler类型来处理请求罢了

上述配置完成后org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#resourceHandlerMapping 方法会生成一个类名为SimpleUrlHandlerMapping的bean，当其他handlerMapping无法处理请求时会接着调用SimpleUrlHandlerMapping对象进行处理

ResourceHttpRequestHandler比DefaultServletHttpRequestHandler的构建稍微复杂一点

org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#resourceHandlerMapping
/**
 * Return a handler mapping ordered at Integer.MAX_VALUE-1 with mapped
 * resource handlers. To configure resource handling, override
 * {@link #addResourceHandlers}.
 */
@Bean
public HandlerMapping resourceHandlerMapping() {
  Assert.state(this.applicationContext != null, "No ApplicationContext set");
  Assert.state(this.servletContext != null, "No ServletContext set");
  ResourceHandlerRegistry registry = new ResourceHandlerRegistry(this.applicationContext,
      this.servletContext, mvcContentNegotiationManager(), mvcUrlPathHelper());
  addResourceHandlers(registry);
  AbstractHandlerMapping handlerMapping = registry.getHandlerMapping();
  if (handlerMapping != null) {
    handlerMapping.setPathMatcher(mvcPathMatcher());
    handlerMapping.setUrlPathHelper(mvcUrlPathHelper());
    handlerMapping.setInterceptors(getInterceptors());
    handlerMapping.setCorsConfigurations(getCorsConfigurations());
  }
  else {
    handlerMapping = new EmptyHandlerMapping();
  }
  return handlerMapping;
}
org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry#getHandlerMapping
/**
 * Return a handler mapping with the mapped resource handlers; or {@code null} in case
 * of no registrations.
 */
@Nullable
protected AbstractHandlerMapping getHandlerMapping() {
  if (this.registrations.isEmpty()) {
    return null;
  }
  Map<String, HttpRequestHandler> urlMap = new LinkedHashMap<>();
  for (ResourceHandlerRegistration registration : this.registrations) {
    for (String pathPattern : registration.getPathPatterns()) {
      ResourceHttpRequestHandler handler = registration.getRequestHandler();
      if (this.pathHelper != null) {
        handler.setUrlPathHelper(this.pathHelper);
      }
      if (this.contentNegotiationManager != null) {
        handler.setContentNegotiationManager(this.contentNegotiationManager);
      }
      handler.setServletContext(this.servletContext);
      handler.setApplicationContext(this.applicationContext);
      try {
        handler.afterPropertiesSet();
      }
      catch (Throwable ex) {
        throw new BeanInitializationException("Failed to init ResourceHttpRequestHandler", ex);
      }
      urlMap.put(pathPattern, handler);
    }
  }
  SimpleUrlHandlerMapping handlerMapping = new SimpleUrlHandlerMapping();
  handlerMapping.setOrder(order);
  handlerMapping.setUrlMap(urlMap);
  return handlerMapping;
}

之后也是调用SimpleUrlHandlerMapping相同的逻辑先根据请求路径匹配找到对应处理的handler，这里对应的是ResourceHttpRequestHandler之后调用handleRequest方法，原理是先根据请求的路径找到对应的资源文件，再获取资源文件的输入流写入到response响应中，源码如下：

org.springframework.web.servlet.resource.ResourceHttpRequestHandler#handleRequest

 /**
 * Processes a resource request.
 * <p>Checks for the existence of the requested resource in the configured list of locations.
 * If the resource does not exist, a {@code 404} response will be returned to the client.
 * If the resource exists, the request will be checked for the presence of the
 * {@code Last-Modified} header, and its value will be compared against the last-modified
 * timestamp of the given resource, returning a {@code 304} status code if the
 * {@code Last-Modified} value  is greater. If the resource is newer than the
 * {@code Last-Modified} value, or the header is not present, the content resource
 * of the resource will be written to the response with caching headers
 * set to expire one year in the future.
 */
@Override
public void handleRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
  // For very general mappings (e.g. "/") we need to check 404 first
  // 根据请求的文件路径找到对应的资源文件
  Resource resource = getResource(request);
  if (resource == null) {
    logger.trace("No matching resource found - returning 404");
    response.sendError(HttpServletResponse.SC_NOT_FOUND);
    return;
  }
  if (HttpMethod.OPTIONS.matches(request.getMethod())) {
    response.setHeader("Allow", getAllowHeader());
    return;
  }
  // Supported methods and required session
  // 校验支持的方法GET和HEAD 以及验证session是否必须
  checkRequest(request);
  // Header phase
  if (new ServletWebRequest(request, response).checkNotModified(resource.lastModified())) {
    logger.trace("Resource not modified - returning 304");
    return;
  }
  // Apply cache settings, if any
  // 可以根据设置的秒数设置缓存时间 cache-control:max-age=xxx
  prepareResponse(response);
  // Check the media type for the resource
  // 根据文件后缀去寻找 png -> image/png
  MediaType mediaType = getMediaType(request, resource);
  if (mediaType != null) {
    if (logger.isTraceEnabled()) {
      logger.trace("Determined media type '" + mediaType + "' for " + resource);
    }
  }
  else {
    if (logger.isTraceEnabled()) {
      logger.trace("No media type found for " + resource + " - not sending a content-type header");
    }
  }
  // Content phase
  if (METHOD_HEAD.equals(request.getMethod())) {
    setHeaders(response, resource, mediaType);
    logger.trace("HEAD request - skipping content");
    return;
  }
  ServletServerHttpResponse outputMessage = new ServletServerHttpResponse(response);
  if (request.getHeader(HttpHeaders.RANGE) == null) {
    Assert.state(this.resourceHttpMessageConverter != null, "Not initialized");
    // 设置content-type、content-length等响应头
    setHeaders(response, resource, mediaType);
    // 将文件流写入到response响应中
    this.resourceHttpMessageConverter.write(resource, mediaType, outputMessage);
  }
  else {
    Assert.state(this.resourceRegionHttpMessageConverter != null, "Not initialized");
    response.setHeader(HttpHeaders.ACCEPT_RANGES, "bytes");
    ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(request);
    try {
      List<HttpRange> httpRanges = inputMessage.getHeaders().getRange();
      response.setStatus(HttpServletResponse.SC_PARTIAL_CONTENT);
      this.resourceRegionHttpMessageConverter.write(
          HttpRange.toResourceRegions(httpRanges, resource), mediaType, outputMessage);
    }
    catch (IllegalArgumentException ex) {
      response.setHeader("Content-Range", "bytes */" + resource.contentLength());
      response.sendError(HttpServletResponse.SC_REQUESTED_RANGE_NOT_SATISFIABLE);
    }
  }
}

3.  web.xml配置servlet映射

原理可以参考上一篇文章

  <servlet-mapping>
    <servlet-name>default</servlet-name>
    <url-pattern>/static/*</url-pattern>
  </servlet-mapping>

将带有/static/xxx 路径的请求直接交给tomcat默认的servlet去进行处理

最后

完成上述的一种配置后就能访问到我们的静态资源了，请求路径http://localhost:8082/zxq/static/login.png