使用“请选择"f:selectItem 在 p:selectOneMenu 中具有 null/空值 [英] Using a "Please select" f:selectItem with null/empty value inside a p:selectOneMenu
问题描述
我正在从数据库中填充一个
I'm populating a <p:selectOneMenu/> from database as follows.
<p:selectOneMenu id="cmbCountry"
value="#{bean.country}"
required="true"
converter="#{countryConverter}">
<f:selectItem itemLabel="Select" itemValue="#{null}"/>
<f:selectItems var="country"
value="#{bean.countries}"
itemLabel="#{country.countryName}"
itemValue="#{country}"/>
<p:ajax update="anotherMenu" listener=/>
</p:selectOneMenu>
<p:message for="cmbCountry"/>
默认选择的选项，当这个页面被加载时，
The default selected option, when this page is loaded is,
<f:selectItem itemLabel="Select" itemValue="#{null}"/>
转换器:
@ManagedBean
@ApplicationScoped
public final class CountryConverter implements Converter {
@EJB
private final Service service = null;
@Override
public Object getAsObject(FacesContext context, UIComponent component, String value) {
try {
//Returns the item label of <f:selectItem>
System.out.println("value = " + value);
if (!StringUtils.isNotBlank(value)) {
return null;
} // Makes no difference, if removed.
long parsedValue = Long.parseLong(value);
if (parsedValue <= 0) {
throw new ConverterException(new FacesMessage(FacesMessage.SEVERITY_ERROR, "", "Message"));
}
Country entity = service.findCountryById(parsedValue);
if (entity == null) {
throw new ConverterException(new FacesMessage(FacesMessage.SEVERITY_WARN, "", "Message"));
}
return entity;
} catch (NumberFormatException e) {
throw new ConverterException(new FacesMessage(FacesMessage.SEVERITY_ERROR, "", "Message"), e);
}
}
@Override
public String getAsString(FacesContext context, UIComponent component, Object value) {
return value instanceof Country ? ((Country) value).getCountryId().toString() : null;
}
}
当来自 < f:selectItem> 表示的菜单中的第一个项目，然后提交表单，在 getasobject中获取的 方法是 值()Select ，它是
When the first item from the menu represented by <f:selectItem> is selected and the form is submitted then, the value obtained in the getAsObject() method is Select which is the label of <f:selectItem> - the first item in the list which is intuitively not expected at all.
当 itemValue 属性设置为空字符串时，它会抛出 java.lang.NumberFormatException: For input string: "" 在 getAsObject() 方法中，即使异常被精确捕获并注册为 ConverterException.
When the itemValue attribute of <f:selectItem> is set to an empty string then, it throws java.lang.NumberFormatException: For input string: "" in the getAsObject() method even though the exception is precisely caught and registered for ConverterException.
当 getAsString() 的 return 语句从
return value instanceof Country?((Country)value).getCountryId().toString():null;
到
return value instanceof Country?((Country)value).getCountryId().toString():"";
null 被一个空字符串替换，但当所讨论的对象为 null 时返回一个空字符串，进而导致另一个问题，如此处.
null is replaced by an empty string but returning an empty string when the object in question is null, in turn incurs another problem as demonstrated here.
如何使此类转换器正常工作?
How to make such converters work properly?
也尝试过 org.omnifaces.converter.SelectItemsConverter 但没有任何区别.
Also tried with org.omnifaces.converter.SelectItemsConverter but it made no difference.
推荐答案
When the select item value is null, then JSF won't render <option value>，但只有 .因此，浏览器将改为提交选项的标签.这在 HTML 规范(强调我的):
When the select item value is null, then JSF won't render <option value>, but only <option>. As consequence, browsers will submit the option's label instead. This is clearly specified in HTML specification (emphasis mine):
value = cdata [CS]
该属性指定控件的初始值.如果未设置该属性，则将初始值设置为 OPTION 元素的内容.
This attribute specifies the initial value of the control. If this attribute is not set, the initial value is set to the contents of the OPTION element.
您也可以通过查看 HTTP 流量监视器来确认这一点.您应该会看到正在提交的选项标签.
You can also confirm this by looking at HTTP traffic monitor. You should see the option label being submitted.
您需要将选择项值设置为空字符串.然后 JSF 将呈现
You need to set the select item value to an empty string instead. JSF will then render a <option value="">. If you're using a converter, then you should actually be returning an empty string "" from the converter when the value is null. This is also clearly specified in Converter#getAsString() javadoc (emphasis mine):
getAsString
...
返回:如果值为空，则返回一个零长度的字符串，否则为转换结果
因此，如果您将 <f:selectItem itemValue="#{null}>> 与这样的转换器结合使用，则 <option value=""> 将被渲染，浏览器将只提交一个空字符串而不是选项标签.
So if you use <f:selectItem itemValue="#{null}"> in combination with such a converter, then a <option value=""> will be rendered and the browser will submit just an empty string instead of the option label.
至于处理提交的空字符串值(或 null)，您实际上应该让您的转换器将此责任委托给 required="true" 属性.因此，当传入的 value 为 null 或空字符串时，您应该立即返回 null.基本上你的实体转换器应该如下实现:
As to dealing with the empty string submitted value (or null), you should actually let your converter delegate this responsibility to the required="true" attribute. So, when the incoming value is null or an empty string, then you should return null immediately. Basically your entity converter should be implemented like follows:
@Override
public String getAsString(FacesContext context, UIComponent component, Object value) {
if (value == null) {
return ""; // Required by spec.
}
if (!(value instanceof SomeEntity)) {
throw new ConverterException("Value is not a valid instance of SomeEntity.");
}
Long id = ((SomeEntity) value).getId();
return (id != null) ? id.toString() : "";
}
@Override
public Object getAsObject(FacesContext context, UIComponent component, String value) {
if (value == null || value.isEmpty()) {
return null; // Let required="true" do its job on this.
}
if (!Utils.isNumber(value)) {
throw new ConverterException("Value is not a valid ID of SomeEntity.");
}
Long id = Long.valueOf(value);
return someService.find(id);
}
至于您对此的特殊问题，
As to your particular problem with this,
但是当有问题的对象为空时返回一个空字符串，反过来又会引发另一个问题，如此处.
正如那边的回答，这是 Mojarra 中的一个错误，并且从 OmniFaces 1.8 开始在
As answered over there, this is a bug in Mojarra and bypassed in <o:viewParam> since OmniFaces 1.8. So if you upgrade to at least OmniFaces 1.8.3 and use its <o:viewParam> instead of <f:viewParam>, then you shouldn't be affected anymore by this bug.
OmniFaces SelectItemsConverter 在这种情况下也应该能正常工作.它为 null 返回一个空字符串.
The OmniFaces SelectItemsConverter should also work as good in this circumstance. It returns an empty string for null.
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