为什么我不能使用< jsp:getProperty>没有< jsp:useBean>吗? [英] Why I can't use <jsp:getProperty> without <jsp:useBean>?

问题描述

说有一个具有代码的servlet:

Say there is servlet that has code:

protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    foo.Person p = new foo.Person("Evan");
    req.setAttribute("person", p);

    RequestDispatcher view = req.getRequestDispatcher("/result.jsp");
    view.forward(req, resp);
}

，然后转到result.jsp以打印给定名称(Evan).这是它的外观图(源Head First Servlet和JSP):

, that goes to result.jsp to print given name (Evan). Here is a picture of how it would look (source Head First Servlets and JSP):

我知道<jsp:useBean>通过调用getAttribute()-返回相同的Person对象，因为它们在相同的请求范围中.另一方面，<jsp:getProperty>会调用findAttribute()来逐字地尝试查找值"person" ..的属性，并最终打印 Evan .

I know that <jsp:useBean> returns same Person object by calling getAttribute()- since they are in same request scope. While on the other side, <jsp:getProperty> will call findAttribute() to literally try to find attribute of value "person".. and eventually print Evan.

但是，如果我不使用<jsp:useBean>怎么办?这是否意味着我无法访问个人"信息?范围 request 的属性?我的意思是，即使我不使用<jsp:useBean>.，它仍然会存在，那为什么我在两个 id 中的 id 中必须具有相同的 value ("person") c1>和 name 在<jsp:getProperty>里面?简单删除<jsp:useBean>会破坏我的程序.

But what if I didn't use <jsp:useBean>? Does that mean I couldn't access "person" attribute at scope request? I mean it would still be there, even if I didn't use <jsp:useBean>. So why I must have same value("person") inside both id in <jsp:useBean> and name inside <jsp:getProperty>? Simple removing <jsp:useBean> breaks my program.

知道<jsp:getProperty>会调用findAttribute()，如果只有一个属性(如 attribute-name )会被用作查找属性的参数，那么逻辑会更合逻辑吗?在范围 page> request> session> application 中?为什么我必须绑"这两个标签:<jsp:useBean>和<jsp:getProperty>?

Knowing that <jsp:getProperty> calls findAttribute(), wouldn't it be more logical if there was a single attribute (like attribute-name), that will be used as an argument to find attributes in scopes page>request>session>application? Why I must "tie" those two tags: <jsp:useBean> and <jsp:getProperty>?

推荐答案

您如何看待以下代码?

public class Main {
    public static void main(String[] args) {
        System.out.println(person);
    }
}

您必须已经正确地猜到它不会成功编译.

You must have already correctly guessed that it won't be compiled successfully.

现在，下面的代码呢?

public class Main {
    public static void main(String[] args) {
        Person person = new Person();// Declaring person
        System.out.println(person);
    }
}

当然，它将成功编译 ¹ ，因为现在编译器可以理解person是什么.

Of course, it will be compiled successfully¹ because now the compiler understands what person is.

类似地，使用

<jsp:getProperty name="person" property="name">

没有将person声明为

<!-- Declaring person -->
<jsp:useBean id="person" class="foo.Person" scope="request" />

不会成功编译.

^{1假设存在Person.class.}

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