Java Servlet header 隐式 object

Question

我一直在使用 Java servlet 并注意到以下代码中的一个特殊问题：

protected void doPost(HttpServletRequest request, HttpServletResponse 
    response) throws ServletException, IOException {

    response.setContentType("text/html");

    String url = "/Display.jsp";

    CSVFileOperations csvfo = new CSVFileOperations();

    String header = csvfo.getHeaders().remove();
    System.out.println(header);

    request.setAttribute("header", header);

    request.getServletContext().getRequestDispatcher(url).forward(request, 
    response);

特别是这一行：

request.setAttribute("header", header);

我把字符串标识符和变量名都设置成一样了。当我通过 ${header} 在我的 .jsp 文件中调用此变量时，我得到以下输出：

{accept-language=en-US, ua-cpu=AMD64, 
cookie=JSESSIONID=1E0C2784352A46D6EFDE0F8A522F4, host=localhost:8080, 
connection=Keep-Alive, cache-control=no-cache, accept-encoding=gzip, 
deflate, accept=image/gif, image/jpeg, image/pjpeg, application/x-ms- 
application, application/xaml+xml, application/x-ms-xbap, */*, user- 
agent=Mozilla/5.0 (Windows NT 10.0; Win64; x64; Trident/7.0; rv:11.0) like 
Gecko}

但是，当我将字符串标识符从 "header" 更改为 "head" 并在 .jsp 页面中调用 ${head} 时，我得到了预期的输出。

我的问题是，这是怎么回事？

Answer 1

JSP 定义了一些 implicit objects，以允许 JSP 访问 Servlet 代码具有的相同信息。

这些隐式对象在页面范围内分配，因此它们会取代任何同名的请求属性。

解决方案：

不要使用与隐式对象名称匹配的请求属性名称。
使用 requestScope 限定访问权限，即在您的 JSP.
中使用 ${requestScope.header}

我推荐选项 1。

The JSP expression language defines a set of implicit objects:

pageContext: The context for the JSP page. Provides access to various objects including:

servletContext: The context for the JSP page’s servlet and any web components contained in the same application. See Accessing the Web Context.

session: The session object for the client. See Maintaining Client State.

request: The request triggering the execution of the JSP page. See Getting Information from Requests.

response: The response returned by the JSP page. See Constructing Responses.

In addition, several implicit objects are available that allow easy access to the following objects:

param: Maps a request parameter name to a single value

paramValues: Maps a request parameter name to an array of values

header: Maps a request header name to a single value

headerValues: Maps a request header name to an array of values

cookie: Maps a cookie name to a single cookie

initParam: Maps a context initialization parameter name to a single value

Finally, there are objects that allow access to the various scoped variables described in Using Scope Objects.

pageScope: Maps page-scoped variable names to their values

requestScope: Maps request-scoped variable names to their values

sessionScope: Maps session-scoped variable names to their values

applicationScope: Maps application-scoped variable names to their values

Java Servlet header 隐式 object

Java Servlet header implicit object

java

servlets