java - 读取 servlet 中的自定义 header

Question

我需要在 http response 中设置自定义 header ，并在需要时读取它们。我使用以下代码来读取 header 。

servlet1:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  response.addHeader("cust-header", "cust-value");
  RequestDispatcher rd = request.getRequestDispatcher("servlet2");
  rd.include(request, response);
}

servlet2:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  System.out.println(request.getHeader("cust-header"));
}

当我尝试读取自定义 header 值时，控制台显示“null”。为什么会这样？如何在需要时读取响应中设置的自定义 header ？

Answer 1

来自RequestDipatcher include method API doc :

[...] The ServletResponse object has its path elements and parameters remain unchanged from the caller's. The included servlet cannot change the response status code or set headers; any attempt to make a change is ignored. [...]

因此，如果您查看您的代码，您会在响应对象中设置 header ，但会尝试从请求中获取它。由于它们保持不变，因此无法正常工作。

在转发或包含重定向中将值从一个 servlet 传递到另一个的最常见方法是将其作为请求属性传递:

servlet1:

//set a request attribute
request.setAttribute("cust-header", "cust-value");
RequestDispatcher rd = request.getRequestDispatcher("servlet2");
rd.include(request, response);

servlet2:

System.out.println(request.getAttribute("cust-header"));