使用try-catch资源在Servlet中提交响应后无法转发

Question

下面的方法抛出：servlet的Servlet.service（）DetailCakeServlet引发了异常 java.lang.IllegalStateException: Cannot forward after response has been committed.

码

 try (PrintWriter out = response.getWriter()) {
        /* TODO output your page here. You may use following sample code. */
        CakeDAO dao = new CakeDAO();
        String strID = request.getParameter("id");
        int id = Integer.parseInt(strID);
        request.setAttribute("cake", dao.getCakeById(id));
        request.getRequestDispatcher("/WEB-INF/detail.jsp").forward(request, response);
    }catch (Exception ex) {
        request.getRequestDispatcher("/WEB-INF/error.jsp").forward(request, response);
    }

但是当我删除资源输出对象时，它可以工作。可能是什么原因 ？

Answer 1

要么在没有输出的情况下进行转发：

try {
    CakeDAO dao = new CakeDAO();
    String strID = request.getParameter("id");
    int id = Integer.parseInt(strID);
    request.setAttribute("cake", dao.getCakeById(id));
    request.getRequestDispatcher("/WEB-INF/detail.jsp").forward(request, response);
} catch (Exception ex) {
    request.getRequestDispatcher("/WEB-INF/error.jsp").forward(request, response);
}

或做：

try {
    CakeDAO dao = new CakeDAO();
    String strID = request.getParameter("id");
    int id = Integer.parseInt(strID);
    PrintWriter out = response.getWriter();
    out.println("Cake: " + dao.getCakeById(id)); // Hereafter an exception would err.
    // NEVER forward: ... .forward(request, response);
} catch (Exception ex) {
    request.getRequestDispatcher("/WEB-INF/error.jsp").forward(request, response);
}

Try-with-resources自动关闭，不需要。转发时甚至会引发错误。

更重要的是，没有得到作家，你会更好。在实际写入输出（缓冲I / O！）之后，转发是错误的，因此catch中的转发是脏的。

Answer 2

如上所述，here（在规范中没有很快发现），在输入catch块之前，try-with-resources构造将尝试关闭try块内打开的资源 - 即你的out，从而response，您尝试在catch块中重定向时已经关闭。我建议通过以下方式处理可能的异常：

try (PrintWriter out = response.getWriter()) {
  try {
    CakeDAO dao = new CakeDAO();
    String strID = request.getParameter("id");
    int id = Integer.parseInt(strID);
    request.setAttribute("cake", dao.getCakeById(id));
    request.getRequestDispatcher("/WEB-INF/detail.jsp").forward(request, response);
  }catch (Exception ex) {
    request.getRequestDispatcher("/WEB-INF/error.jsp").forward(request, response);
  }
}

实际上，我没有在你的代码中看到使用out，所以整个try-with-resources块只是多余的。