How does Java expression language resolve boolean

问题:

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().

This got me thinking, what exactly does it do?

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.

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

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

回答1:

It's authoritatively documented in both the JavaBeans Spec and EL Specification.

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

8.3.2 Boolean properties

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

public boolean is<PropertyName>();

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.

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.

An example boolean property might be:

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

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

And in chapter 1.18.5 of EL spec:

1.18.5 Coerce A to Boolean

• 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

See also:

• javax.el.PropertyNotFoundException: Property 'foo' not readable on type java.lang.Boolean
• Java: how to name boolean properties

回答2:

Basically what you've stated is all there is to it. EL expects the object to follow regular java bean standards. These 2 should help:

• http://docstore.mik.ua/orelly/java-ent/jnut/ch06_02.htm
• http://docs.oracle.com/javaee/1.4/tutorial/doc/JSPIntro7.html#wp71019