Question

How can we resolve JSF EL expressions programmatically on Seam managed beans? I have tried the following, but it does not work:

private String resolveExpression(String expression){

        if(expression == null)
        {
            return null;
        }

        javax.faces.context.FacesContext facesCtx = FacesContext.getCurrentInstance();
        Application app = facesCtx.getApplication();

        try 
        {
            // Here we bind a value expression into the item, 
            // so it can dynamically change its language
            ELContext elCtx = facesCtx.getELContext();
            ExpressionFactory ef = app.getExpressionFactory();
            ValueExpression ve = ef.createValueExpression(elCtx, expression, String.class);

            return (String) ve.getValue(elCtx);
        } 
        catch (ELException ex) {
        }

        return expression;
    }

In my application I have a bean named User that in session scope of Seam sits and this bean has a property name. The EL expression is #{usr.name}, but this expression returns empty while it works fine in a Facelet file.

Answer 1

Seam provides two utility components for dealing with EL expressions: Expressions and Interpolator.

For your particular use case, I believe Interpolator is the correct choice, since you don’t really need a valid ValueExpression. Interpolator parses the whole string for EL expressions and converts all of them to their evaluated values.

Beware that Interpolator#interpolate() also accepts arguments (see the documentation).

Example:

@Name("bean")
@Scope(ScopeType.SESSION)
class MyBean {
     ...

     public String getValue() {
          return value;
     }

     ...
}

@Name("otherBean")
@Scope(ScopeType.EVENT)
class MyBean {
     ...

     public String getInterpolatedValue() {
          return Interpolator.instance().interpolate("#{bean.value}");
     }

     ...

}