Does EL automatically convert/cast the type? How d

问题:

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

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

${a.name}

How does it work?

回答1:

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

This is what it roughly does under the covers on ${a.name}.

// 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);

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

See also:

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

回答2:

It's because name is a property of the object a, and probably the object is also a JavaBean (not to be confused with Enterprise JavaBean).

See here for Expression Language Documentation and here for a short tutorial.