重定向到同一页面会导致空白页面

Question

我正在创建一个 java servlet web 应用程序，我在 index.jsp 处接收用户输入并在 POST 操作后在 result.jsp 页面显示结果。在提交表单之前，我验证用户输入。如果发现任何验证错误，我希望 redirect 用户访问包含错误消息的同一 index.jsp 页面。但是 redirect 操作会导致空白页。这是我到目前为止所做的 -

Servlet class doPost 方法 -

protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    String name = req.getParameter("name");

    //this is just a dto class
    CovidQa covidQa = new CovidQa(name);

    CovidTestValidator validator = new CovidTestValidator();
    boolean hasError = validator.validate(covidQa, req);

    if (hasError) {
        req.getRequestDispatcher("/index.jsp").forward(req, resp);
    }

    req.getRequestDispatcher("/result.jsp").forward(req, resp);
}

验证者的 validate 方法-

public boolean validate(CovidQa covidQa, HttpServletRequest request) {
    boolean hasError = false;

    if (covidQa.getName() == null || covidQa.getName().equals("")) {
        request.setAttribute("nameErr", "Name can not be null");
        hasError = true;
    }

    return hasError;
}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <servlet>
        <servlet-name>Covid-19</servlet-name>
        <servlet-class>CovidTest</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>Covid-19</servlet-name>
        <url-pattern>/Cov</url-pattern>
    </servlet-mapping>

</web-app>

index.jsp -

<form action="Cov" method="post">
    //input fields
    <label>Name</label>
    <input type="text"
           id="name"
           name="name">
    <span>${nameErr}</span>

    <button type="submit">Submit</button>
</form>

Answer 1

req.getRequestDispatcher() 不会从方法中隐式 return。如果您没有明确提及 return，编译器也会执行直接行。
正如@BalusC 评论的那样，系统通过连续执行两个 req.getRequestDispatcher() 语句而感到困惑。

要么您需要 return 明确 -

if (hasError) {
    req.getRequestDispatcher("/index.jsp").forward(req, resp);
    return;
}

req.getRequestDispatcher("/result.jsp").forward(req, resp);

或者，将后一个放在 else 块中 -

if (hasError) {
    req.getRequestDispatcher("/index.jsp").forward(req, resp);
} else {
    req.getRequestDispatcher("/result.jsp").forward(req, resp);
}

重定向到同一页面会导致空白页面

Redirecting to the same page results in blank page

java

jsp

tomcat

servlets

jstl