springboot异常处理之404

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源码分析

在springboot中默认有一个异常处理器接口ErrorContorller，该接口提供了getErrorPath()方法，此接口的BasicErrorController实现类实现了getErrorPath()方法，如下：

/*
* AbstractErrorController是ErrorContoller的实现类
*/
@Controller
@RequestMapping("${server.error.path:${error.path:/error}}")
public class BasicErrorController extends AbstractErrorController {

    private final ErrorProperties errorProperties;

    ...
	@Override
	public String getErrorPath() {
		return this.errorProperties.getPath();
	}

	@RequestMapping(produces = MediaType.TEXT_HTML_VALUE)
	public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {
		HttpStatus status = getStatus(request);
		Map<String, Object> model = Collections
				.unmodifiableMap(getErrorAttributes(request, isIncludeStackTrace(request, MediaType.TEXT_HTML)));
		response.setStatus(status.value());
		ModelAndView modelAndView = resolveErrorView(request, response, status, model);
		return (modelAndView != null) ? modelAndView : new ModelAndView("error", model);
	}

	@RequestMapping
	public ResponseEntity<Map<String, Object>> error(HttpServletRequest request) {
		HttpStatus status = getStatus(request);
		if (status == HttpStatus.NO_CONTENT) {
			return new ResponseEntity<>(status);
		}
		Map<String, Object> body = getErrorAttributes(request, isIncludeStackTrace(request, MediaType.ALL));
		return new ResponseEntity<>(body, status);
	}
	....
}

errorProperties类定义如下：

public class ErrorProperties {

	/**
	 * Path of the error controller.
	 */
	@Value("${error.path:/error}")
	private String path = "/error";
	...
}

由此可见，springboot中默认有一个处理/error映射的控制器，有error和errorHtml两个方法的存在，它可以处理来自浏览器页面和来自机器客户端（app应用）的请求。

当用户请求不存在的url时，dispatcherServlet会交由ResourceHttpRequestHandler映射处理器来处理该请求，并在handlerRequest方法中，重定向至/error映射，代码如下：

	@Override
	public void handleRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		// For very general mappings (e.g. "/") we need to check 404 first
		Resource resource = getResource(request);
		if (resource == null) {
			logger.debug("Resource not found");
			response.sendError(HttpServletResponse.SC_NOT_FOUND); // 404
			return;
		}
		...
	}

getResource(request)会调用this.resolverChain.resolveResource(request, path, getLocations())方法，getLocations()方法返回结果如下：

接着程序会执行到getResource(pathToUse, location)方法如下：

	@Nullable
	protected Resource getResource(String resourcePath, Resource location) throws IOException {
	    // 新建一个resource对象，url为  location + resourcePath，判断此对象（文件）是否存在
		Resource resource = location.createRelative(resourcePath);
		if (resource.isReadable()) {
			if (checkResource(resource, location)) {
				return resource;
			}
			else if (logger.isWarnEnabled()) {
				Resource[] allowedLocations = getAllowedLocations();
				logger.warn("Resource path \"" + resourcePath + "\" was successfully resolved " +
						"but resource \"" +	resource.getURL() + "\" is neither under the " +
						"current location \"" + location.getURL() + "\" nor under any of the " +
						"allowed locations " + (allowedLocations != null ? Arrays.asList(allowedLocations) : "[]"));
			}
		}
		return null;
	}

在resource.isReadable()中，程序会在locations目录中寻找path目录下文件，由于不存在，返回null。

最终也就导致程序重定向至/error映射，如果是来自浏览器的请求，也就会返回/template/error/404.html页面,所以对于404请求，只需要在template目录下新建error目录，放入404页面即可。

使用注意

1. 在springboot4.x中我们可以自定义ControllerAdvice注解 + ExceptionHandler注解来处理不同错误类型的异常，但在springboot中404异常和拦截器异常由spring自己处理。