jsp:forward 在 Java 中不使用 JSP 标签？
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jsp:forward in Java without using JSP tag?
提问by Chris Carruthers
Is there a pure-Java equivalent to <jsp:forward page="..." /> that I can use within a <% ... %> block?
我可以在 <% ... %> 块中使用与 <jsp:forward page="..." /> 等效的纯 Java 吗？
For example, I currently have a JSP page something like this:
例如，我目前有一个 JSP 页面，如下所示：
<%
String errorMessage = SomeClass.getInstance().doSomething();
if (errorMessage != null) {
session.setAttribute("error", errorMessage);
%>
<jsp:forward page="error.jsp" />
<%
} else {
String url = response.encodeRedirectURL("index.jsp");
response.sendRedirect(url);
}
%>
Having to break the <% ... %> block to use the jsp:forward is ugly and makes it harder to read due to indentation, among other things.
必须打破 <% ... %> 块才能使用 jsp:forward 是丑陋的，并且由于缩进等原因使阅读变得更加困难。
So, can I do the forward in the Java code without use the JSP tag?
那么，我可以在不使用 JSP 标签的情况下在 Java 代码中进行转发吗？
Something like this would be ideal:
这样的事情将是理想的：
<%
String errorMessage = SomeClass.getInstance().doSomething();
if (errorMessage != null) {
session.setAttribute("error", errorMessage);
someObject.forward("error.jsp");
} else {
String url = response.encodeRedirectURL("index.jsp");
response.sendRedirect(url);
}
%>
采纳答案by Adam
The someObjectyou are looking for is pageContext.
将someObject你要找的是pageContext的。
This object is implicitly defined in JSP, so you can use it like this:
这个对象是在 JSP 中隐式定义的，所以你可以像这样使用它：
pageContext.forward("<some relative jsp>");
回答by Hyman Leow
You really should try and avoid scriplets if you can, and in your case, a lot of what you are doing can be replaced with JSTL code. The following replacement for your example is much cleaner, IMO:
如果可以，您真的应该尽量避免使用脚本，在您的情况下，您正在做的很多事情都可以用 JSTL 代码替换。您的示例的以下替换更干净，IMO：
<%
// Consider moving to a servlet or controller/action class
String errorMessage = SomeClass.getInstance().doSomething();
pageContext.setAttribute("errorMessage", errorMessage);
%>
<c:choose>
<c:when test="${not empty errorMessage}">
<c:set var="error" scope="session" value="${errorMessage}" />
<jsp:forward page="error.jsp" />
</c:when>
<c:otherwise>
<c:redirect url="index.jsp" />
</c:otherwise>
</c:choose>
Ideally, you'd modify error.jsp so that the error message doesn't even need to be set in the session, but I didn't want to change your design too much.
理想情况下，您应该修改 error.jsp 以便甚至不需要在会话中设置错误消息，但我不想对您的设计进行太多更改。
回答by Hyman Leow
A simple approach:
一个简单的方法：
<%@page errorPage="Error.jsp" %>
<%
String errorMessage = SomeClass.getInstance().doSomething();
if (errorMessage != null) {
throw new Exception(errorMessage); // Better throw the exception from doSomething()
}
pageContext.forward("index.jsp");
%>
Error.jsp
.........
<%@ page isErrorPage='true' %>
<%
out.print("Error!!!");
out.print(exception.getMessage());
%>
Better approach:
更好的方法：
Invoke the doSomething() from a servlet. Set your error page in web.xml
从 servlet 调用 doSomething()。在 web.xml 中设置您的错误页面
<error-page>
<exception-type>java.lang.Exception</exception-type>
<location>/WEB-INF/jsp/Error.jsp</location>
</error-page>
