以编程方式获取facelets参数的表达式值(变量) [英] Programmatically get expression value of facelets parameter (variable)

问题描述

以下Java代码允许从Faces上下文访问任何对象或变量:

Following java code allows to access any object or variable from faces context:

ELContext elCtx = facesContext.getELContext();
ExpressionFactory exprFac = facesContext.getApplication().getExpressionFactory();
MyProperty myProperty = (MyProperty) exprFac.createValueExpression(elCtx, "#{somebean.someattr.someproperty}", MyProperty.class).getValue(elCtx);

我使用自定义转换器中的代码从上下文中读取其他转换参数.

I use the code from within my custom converter to read additional converting parameters from context.

如果在JSF上下文中将#{somebean}定义为普通后备bean，则代码可以正常工作.

The code works correctly if #{somebean} is defined as normal backing bean within JSF context.

Facelets允许为JSF表达式创建快捷方式".示例:

Facelets allow to create 'shortcut' to JSF expressions. Example:

<ui:param name="shortcut" value="#{somebean.someattr.someproperty}" />
<div>#{somebean.someattr.someproperty} equals #{shortcut}</div>

在这种情况下，#{somebean.someattr.someproperty}和#{shortcut}具有相同的值.

In this case both #{somebean.someattr.someproperty} and #{shortcut} have the same value.

但是，使用上面的Java代码无法访问这些快捷方式"名称.例如:

However these 'shortcut' names are not accessible using java code above. For example:

MyProperty myProperty1 = (MyProperty) exprFac.createValueExpression(elCtx, "#{somebean.someattr.someproperty}", MyProperty.class).getValue(elCtx);
// myProperty1 has expected value

MyProperty myProperty2 = (MyProperty) exprFac.createValueExpression(elCtx, "#{shortcut}", MyProperty.class).getValue(elCtx);
// myProperty2 is null

是否有一种方法可以访问facelets上下文并读取在当前JSF页面上定义的快捷方式"参数值?

Is there a way to access a facelets context and to read 'shortcut' parameter values, defined on the current JSF page?

推荐答案

我遇到了同样的问题，并选择了以下方法:

I had the same problem and have chosen the following approach:

/**
     * Führt eine Expression im aktuellen Faces EL Context 
     * UND im Facelets El Context aus.
     * 
     * @param facesContext
     * @param expression
     * @return object
     */
    private static Object executeExpressionInUIContext (final FacesContext facesContext, final String expression) {
        final ELContext elContext = facesContext.getELContext();
        final Application application = facesContext.getApplication();

        Object result = executeExpressionInElContext(application, elContext, expression);
        if (null == result) {
            FaceletContext faceletElContext = (FaceletContext) FacesContext.getCurrentInstance().getAttributes().get(FaceletContext.FACELET_CONTEXT_KEY);
            result = executeExpressionInElContext(application, faceletElContext, expression);
        }
        return result;
    }

    private static Object executeExpressionInElContext (Application application, ELContext elContext, String expression) {
        ExpressionFactory expressionFactory = application.getExpressionFactory();
        ValueExpression exp = expressionFactory.createValueExpression(elContext, expression, Object.class);
        return exp.getValue(elContext);
    }

"ui:param"是Facelet视图处理技术的一部分. Facelets扩展了JSF. 两种技术在存储变量时都使用它们自己的 Context . 在Faces El Context旁边有一个Facelet El Context(FaceletContext).

"ui:param" is part of the Facelet view handling technology. Facelets extends JSF. Both technologies use their own Context when storing variables. Beside the Faces El Context there is a Facelet El Context (FaceletContext).

声明的方法在两种情况下都对表达式求值.请注意，如果在每个上下文中以相同的名称存储两个值，则此操作将无效.

The stated method evaluates expressions in both contexts. Be aware that this will not work if two values are stored under the same name in each context.

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