jsp:param 与 Java 类
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jsp:param with Java class
提问by John Hendrik
I have a JSP file that includes another JSP file. The first JSP should pass an instance of a Java class (widget) to the second JSP file.
我有一个包含另一个 JSP 文件的 JSP 文件。第一个 JSP 应该将 Java 类（小部件）的实例传递给第二个 JSP 文件。
This is what I have:
这就是我所拥有的：
The first JSP:
第一个JSP：
<jsp:include page="/container/SpecialWidget.jsp">
<jsp:param name="widget" value="${widget}"/> // widget is a .Java POJO
</jsp:include>
The second JSP:
第二个JSP：
${param.widget.id}
The problem is that this code gives an error (it says it doesn't know ID). If I omit the ".id" part, the page prints the Java code for the Java class, which means the class has been transferred correctly. If I change the ${widget} rule of the first page in, for example, ${widget.id} and I try to print ${param.widget}, everything works fine.
问题是这段代码给出了一个错误（它说它不知道 ID）。如果我省略“.id”部分，页面会打印 Java 类的 Java 代码，这意味着该类已正确传输。如果我更改第一页的 ${widget} 规则，例如，${widget.id} 并尝试打印 ${param.widget}，一切正常。
My question: Why can't I pass a Java class and directly call upon its attributes? What am I doing wrong?
我的问题：为什么我不能传递一个 Java 类并直接调用它的属性？我究竟做错了什么？
Edit: error message: Caused by: javax.el.PropertyNotFoundException: Property 'id' not found on type java.lang.String
编辑：错误消息：由：javax.el.PropertyNotFoundException：在类型 java.lang.String 上找不到属性“id”
采纳答案by Rafael Borja
When you pass the variable ${widget}it is translated at request time to a string (widget.toString()). This value is then passed to the second JSP as a String, not as the original java object.
当您传递变量时，${widget}它会在请求时转换为字符串 ( widget.toString())。然后将该值作为字符串传递给第二个 JSP，而不是作为原始 java 对象。
One approach to access the object's values is setting the parameter's value with the attribute's value:
访问对象值的一种方法是使用属性值设置参数值：
<jsp:param name="widgetId" value="${widget.id}"/>
Then use the code bellow on the second JSP:
然后在第二个 JSP 上使用下面的代码：
${param.widgetId}
You can also set widget as an request attributeand use it on the second page as ${widget.id}or ${request.widget.id}. I suggest you use the second approach.
您还可以将小部件设置为请求属性，并在第二页上使用它作为${widget.id}或 ${request.widget.id}。我建议你使用第二种方法。
回答by John Hendrik
I managed to fix my problem with the following code:
我设法用以下代码解决了我的问题：
<c:set var="widget" value="${widget}" scope="request" />
<jsp:include page="/SOMEWHERE/SpecialWidget.jsp"/>
Thank you both for your help:) It saved my day
谢谢你们的帮助：）它拯救了我的一天
回答by JB Nizet
<jsp:param>passes the parameter as an HTTP request parameter, which can only be a String. So toString()is called on your widget, and the result of this method is passed as parameter.
<jsp:param>参数作为HTTP请求参数传递，只能是String。SotoString()在您的小部件上调用，并且此方法的结果作为参数传递。
You should use a JSP tag, implemented as a tag file, instead of using a JSP include. See http://docs.oracle.com/javaee/1.4/tutorial/doc/JSPTags5.htmlfor how to define an use them.
您应该使用作为标记文件实现的 JSP 标记，而不是使用 JSP 包含。有关如何定义使用它们的信息，请参阅http://docs.oracle.com/javaee/1.4/tutorial/doc/JSPTags5.html。
For example:
例如：
Tag definintion, in /WEB-INF/tags/specialWidget.tag:
标签定义，在 /WEB-INF/tags/specialWidget.tag 中：
<%@ tag %>
<%@ attribute name="widget" required="true" type="the.fully.qualified.name.of.WidgetClass" %>
TODO: add the HTML markup that must be displayed, using ${widget} to access the passed in widget attribute
Tag usage, in any JSP:
标签用法，在任何 JSP 中：
<%@ taglib prefix="myTags" tagdir="/WEB-INF/tags" %>
...
Tada! I will use the specialWidget tag here, with widget as an attribute:
<myTags:specialWidget widget="${widget}"/>
