servlet:前端向后台发送http请求报错误码405的解决方案。

项目场景：

在学习Servlet接受前端数据

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问题描述：

无论使用post请求还是get请求向后台发送数据，都会报错，错误代码405。

接收数据代码：

public class Sign extends HttpServlet {

    @Override
    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        super.service(req, res);

        UserService service = new UserService();
        List<User> users = service.showAllUsers();
        users.forEach(System.out::println);

        res.setContentType("text/html;charset=utf-8");
        PrintWriter writer = res.getWriter();
        users.forEach(val->writer.println(String.format("<h1>%s</h1>", val.toString())));

    }
}

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原因分析：

通过代码可以看出，service最终都会访问doPost()，doGet()等其他方法。

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        long lastModified;
        if (method.equals("GET")) {
            lastModified = this.getLastModified(req);
            if (lastModified == -1L) {
                this.doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
                    ifModifiedSince = req.getDateHeader("If-Modified-Since");
                } catch (IllegalArgumentException var9) {
                    ifModifiedSince = -1L;
                }

                if (ifModifiedSince < lastModified / 1000L * 1000L) {
                    this.maybeSetLastModified(resp, lastModified);
                    this.doGet(req, resp);
                } else {
                    resp.setStatus(304);
                }
            }
        } else if (method.equals("HEAD")) {
            lastModified = this.getLastModified(req);
            this.maybeSetLastModified(resp, lastModified);
            this.doHead(req, resp);
        } else if (method.equals("POST")) {
            this.doPost(req, resp);
        } else if (method.equals("PUT")) {
            this.doPut(req, resp);
        } else if (method.equals("DELETE")) {
            this.doDelete(req, resp);
        } else if (method.equals("OPTIONS")) {
            this.doOptions(req, resp);
        } else if (method.equals("TRACE")) {
            this.doTrace(req, resp);
        } else {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[]{method};
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(501, errMsg);
        }

    }

以doGet()方法为例:

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String msg = lStrings.getString("http.method_get_not_supported");
        this.sendMethodNotAllowed(req, resp, msg); //发送ERROR
    }

这里发送了一个Error，点进来后发现是resp.sendError(405,msg)这个方法在页面上写了错误码405:

    private void sendMethodNotAllowed(HttpServletRequest req, HttpServletResponse resp, String msg) throws IOException {
        String protocol = req.getProtocol();
        if (protocol.length() != 0 && !protocol.endsWith("0.9") && !protocol.endsWith("1.0")) {
            resp.sendError(405, msg);  //最终在页面上显示405
        } else {
            resp.sendError(400, msg);
        }
    }

错误最终原因找到，即调用父类的service()方法时会有可能产生错误码405。

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解决方案：

在自定义的servlet类中重写父类service()方法时，不要调用父类的service()方法，即不要写:

super.service(req, res);

本人初识servlet，才疏学浅，如有不当之处，还请各位批评指正。