What is the difference between #{expr} and ${expr}

问题:

This question already has an answer here:

• Difference between JSP EL, JSF EL and Unified EL [closed] 2 answers

I've read some time ago about the difference in 'Core JSF' but now I can't find that place.

Nevertheless I don't remember that there was a word about cases when we should use ${expr} in jsf. So I'm just curious what is the difference (in a chestnut) and if there a case to use ${expr} in JSF application?

回答1:

To summarize in clear language: ${expression} does only get, while #{expression} can do both get and set. This is because the ${expression} is evaluated only once (immediate), while the #{expression} is evaluated on every access (deferred).

In JSF on JSP 2.0 or Facelets 1.x, when you put something like this as first expression of the page

${bean.property}

where bean is a request scoped managed bean, you will see nothing. But if bean is a session scoped managed bean and already been created before, then you will see the property value being printed. This also applies if the request scoped managed bean is created before by #{bean.xxx} in the same page.

If you instead do as first expression of the page

#{bean.property}

then EL will test if bean is null and if so, then it will set (create) a new one. If the property is set during bean construction, then you will see the property being displayed by this expression.

This all is mandatory to get among others JSF UIInput components such as <h:inputText> to work. When you submit the form, the #{expression} will set the values in the bean.

回答2:

From JavaEE tutorial:

All expressions using the ${} syntax are evaluated immediately. These expressions can only be used within template text or as the value of a JSP tag attribute that can accept runtime expressions. [...] Immediate evaluation expressions are always read-only value expressions. The expression shown above can only get the total price from the cart bean; it cannot set the total price.

Deferred evaluation expressions take the form #{expr} and can be evaluated at other phases of a page life cycle as defined by whatever technology is using the expression. In the case of JavaServer Faces technology, its controller can evaluate the expression at different phases of the life cycle depending on how the expression is being used in the page.