EL是否会自动转换/转换类型? $ {a.name}实际如何工作? [英] Does EL automatically convert/cast the type? How does ${a.name} actually work?

问题描述

我有一个声明为类型Object a的变量，它实际上引用了类型A的实例.

I have a variable declared as type Object a which actually refers an instance of type A.

在EL中，我可以直接使用以下表达式来打印类型为A的name属性:

In EL, I can directly use the following expression to print the name property of type A:

${a.name}

它如何工作?

推荐答案

EL使用反省，通常通过 javax.beans.Introspector API .

EL uses reflection under the hoods, usually via javax.beans.Introspector API.

这是${a.name}底下的大致内容.

// EL will breakdown the expression.
String base = "a";
String property = "name";

// Then EL will find the object and getter and invoke it.
Object object = pageContext.findAttribute(base);
String getter = "get" + property.substring(0, 1).toUpperCase() + property.substring(1);
Method method = object.getClass().getMethod(getter, new Class[0]);
Object result = method.invoke(object);

// Now EL will print it (only when not null).
out.println(result);

它不会以任何方式转换/转换类型.

It does not convert/cast the type in any way.

• 我们的EL Wiki页面
• 如何使用EL表达式语言$ {}访问对象
• 遍历Java中的对象

• Our EL wiki page
• How to access objects in EL expression language ${}
• Travesring through an object in java

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