Java 表达式语言如何解析布尔属性?(在 JSF 1.2 中) [英] How does Java expression language resolve boolean attributes? (in JSF 1.2)

问题描述

所以我们都知道#{someBean.value} 会尝试获取someBean 上某个名为value 的属性的内容.它将寻找 getValue().但是，如果这个属性是 boolean 呢?它将寻找 isValue().它不会寻找hasValue().

So we all know that #{someBean.value} will try and get the content of some property on someBean called value. It will look for getValue(). However, what if this property is boolean? It will look for isValue(). What it won't look for is hasValue().

这让我思考，它到底有什么作用?

This got me thinking, what exactly does it do?

Java EE 5 教程章节 - 统一表达式语言是指PageContext.FindAttribute().PageContext 将您发送到 JSPContext.他们都没有真正解释他们遵循的规则来确定他们正在寻找的方法的名称.

Java EE 5 tutorial chapter - Unified Expression Language refers to PageContext.FindAttribute(). PageContext sends you to JSPContext. None of them actually explain the rules they are following to determine the name of the method they are looking for.

也很容易找到说明方法名称必须以 get 开头的文档.但是，我知道 isValue() 有效.

It is also fairly easy to find documentation that says the method names must begin with get. However, I know that isValue() works.

任何人都可以指出我写下的文档.我不是在寻找教程或示例，而是在寻找参考.

Can anyone point me to documentation where this is written down. I'm not looking for tutorials or examples I'm looking for reference.

推荐答案

JavaBeans 规范 和 EL 规范.

以 boolean 属性为例，JavaBeans 规范的第 8.3.2 章对其进行了描述:

To take the boolean property as an example, it's described in chapter 8.3.2 of JavaBeans spec:

此外，对于布尔属性，我们允许使用 getter 方法来匹配模式:

8.3.2 Boolean properties

In addition, for boolean properties, we allow a getter method to match the pattern:

public boolean is();

这是PropertyName>"可以提供方法而不是get<PropertyName>"方法，或者它可以在get<PropertyName>"之外提供方法.

This "is<PropertyName>" method may be provided instead of a "get<PropertyName>" method, or it may be provided in addition to a "get<PropertyName>" method.

在任何一种情况下，如果是<PropertyName>"方法存在于布尔属性然后我们将使用is<PropertyName>"读取属性值的方法.

In either case, if the "is<PropertyName>" method is present for a boolean property then we will use the "is<PropertyName>" method to read the property value.

一个示例布尔属性可能是:

An example boolean property might be:

    public boolean isMarsupial();
    public void setMarsupial(boolean m);

所以，#{bean.marsupial} 完全期望上面的 getter/setter 对.

So, #{bean.marsupial} expects exactly the above getter/setter pair.

在 EL 规范的第 1.23.5 章中:

And in chapter 1.23.5 of EL spec:

• 如果A为null且目标类型不是原始类型boolean，则返回null
• 如果A为null或"，返回false
• 否则，如果A 是Boolean，则返回A
• 否则，如果 A 是一个 String，并且 Boolean.valueOf(A) 不会抛出一个异常，返回
• 否则，错误

1.23.5 Coerce A to Boolean or boolean

• If A is null and the target type is not the primitive type boolean, return null
• If A is null or "", return false
• Otherwise, if A is a Boolean, return A
• Otherwise, if A is a String, and Boolean.valueOf(A) does not throw an exception, return it
• Otherwise, error

另见:

• javax.el.PropertyNotFoundException:属性'foo' 在 java.lang.Boolean 类型上不可读
• Java:如何命名布尔属性

这篇关于Java 表达式语言如何解析布尔属性?(在 JSF 1.2 中)的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！