如何解释EL表达式中方括号（[]）运算符的用法 [英] How to explain usages of square brackets ([]) operator in EL expressions

问题描述

当我阅读 Spring PetClinic 示例应用程序时，我发现他们总是将 add 和 modify 功能放入单个JSP文件中，并且使用 $ {owner ['new']} 表达式可自定义当前页面上的元素，例如标签的 New 所有者 或所有者 。

When I read Spring PetClinic sample application, I found they always put the add and modify functionalities into a single JSP file, and they use ${owner['new']} expression to customize elements on current page, for example "New Owner" or "Owner" for a label.

在JSP（春季）环境中， [] 运算符还有其他用法吗？

Are there any other usages of [] operator in JSP (Spring) environment?

Controller文件包含以下代码段：

The Controller file has the following snippet:

@RequestMapping(value = "/owners/new", method = RequestMethod.GET)
public String initCreationForm(Map<String, Object> model) {
    Owner owner = new Owner();
    model.put("owner", owner);
    return "owners/createOrUpdateOwnerForm";
}
@RequestMapping(value = "/owners/{ownerId}/edit", method = RequestMethod.GET)
public String initUpdateOwnerForm(@PathVariable("ownerId") int ownerId, Model model) {
    Owner owner = this.clinicService.findOwnerById(ownerId);
    model.addAttribute(owner);
    return "owners/createOrUpdateOwnerForm";
}

JSP文件包含以下代码段：

The JSP file has the following snippet:

<h2>
    <c:if test="${owner['new']}">New </c:if> Owner
</h2>

推荐答案

[] 将允许您：

${car['type']}

这将等效于 car.getType（）; （或 car.isType（）（如果 type 字段是布尔值）。

This will be equivalent to car.getType(); (or car.isType() if the type field is a boolean).

${carMap['Volvo']}

这等效于 carMap.get（'Volvo'）; 当 carMap 是 Map 。

${cars[1]}

如果，这等效于汽车[1] cars 是一个数组或等效于 cars.get（1）（如果汽车是列表。

This is equivalent to cars[1] if cars is an array or equivalent to cars.get(1) if cars is a List.

更多详细信息/来源： http://docs.oracle.com/javaee/6/tutorial/doc/bnahu .html

More details/source: http://docs.oracle.com/javaee/6/tutorial/doc/bnahu.html

编辑：

您的问题的表达方式（ $ {owner ['new']} ）属于第一种情况。在petclinick应用程序中， Owner 类是 Person 这是一个 BaseEntity 。并且 BaseEntity 具有方法 isNew（）（因此 Owner 也有这种方法。）

Your question's expression (${owner['new']}) falls into the first case. In the petclinick app, the Owner class is a subclass of Person which is a subclass of BaseEntity. And BaseEntity has a method isNew() (so Owner has that method as well).

这样，代码段 $ {owner ['new']} 等同于 owner.isNew（）。

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