JSF 2.0 - 当空白时，selectOneMenu默认为0 [英] JSF 2.0 - selectOneMenu defaults to 0 when empty
问题描述
对于我的项目，我想使用 h：selectOneMenu 实现组合框。我读了很多关于如何正确处理selectOneMenu的帖子以及如何实现自定义转换器。组合框应存储ID（ Long ），该ID位于限制条件的不同DB表（名为fk_ID）中。到目前为止，一切似乎按计划运作。现在我的问题：
For my project i want to implement a combobox with h:selectOneMenu. I read many posts about how to handle the selectOneMenu properly and how to implement a custom converter. The combobox shall store the ID (Long) from a different DB Table (named fk_ID) for which a constrain is in place. So far everything seems to function as planned. Now to my problem:
列fk_ID是可选的，因此组合框中的第一个选项是 f：selectItem ，没有标记为选择...。当我保存表单并更改值时，一切都可以，除非我将其设置为选择...。我得到的错误是约束完整性违规。
The column fk_ID is optional, so the first option in the combobox is a f:selectItem with no value labeled "choose ...". When I save the form and change the value everything is OK, except if I set it back to "choose ...". The error I get is a constraint integrity violation.
我也发现了问题：转换器返回 null 为意图，但豆接收长0 。
I found the problem too: The converter returns null as intended but the bean receives the Long 0.
XHTML：
<h:selectOneMenu value="#{userBean.selectedModel.fk_id}"
id="combobox">
<f:selectItem itemLabel="choose ..." />
<f:selectItems
value="#{userBean.items}"
var="item"
itemLabel="#{item.value}"
itemValue="#{item.id}"
/>
<f:converter converterId="userConverter" />
</h:selectOneMenu>
自定义转换器：
@FacesConverter(value = "userConverter")
public class UserConverter implements Converter {
@Override
public Object getAsObject(FacesContext arg0, UIComponent arg1, String arg2) {
if (arg2 != null && !arg2.equals("null"))
return Long.valueOf(arg2);
return null;
}
@Override
public String getAsString(FacesContext arg0, UIComponent arg1, Object arg2) {
return String.valueOf(arg2);
}
}
该bean（非常简短的版本）：
The bean (very short version):
@Named(value = "userBean")
public class UserController implements Serializable {
@Getter
@Setter
private UserModel selectedModel;
@Getter
@Setter
private List<UserModel> items;
public void saveModel() {
selectedModel = userService.saveModel(selectedModel);
}
}
问题是， fk_id 是 Long 0 ，但应该是 null 。如果（selectedModel.getFk_id（）== 0）
selectedModel.setFk_id（），我可以通过编写
The problem is that the fk_id is Long 0, but should be null. I can solve the problem by writing
if (selectedModel.getFk_id() == 0)
selectedModel.setFk_id(null);
，但这不是我很满意的解决方案。
, but thats not a solution I am comfortable with.
我的问题：为什么转换器返回的值 null ， saveModel（）方法中的 Long 0
My Question: Why is the value null, which the converter returns, the Long 0 inside the saveModel() method?
推荐答案
此转换器是不必要的，没有任何意义（我希望 UserConverter 在用户和之间转换，而不在 Long 和 String ）。去掉它。
This converter is unnecessary and makes no sense (I'd expect an UserConverter to convert between User and String, not between Long and String). Remove it.
现在，您需要告诉JSF将空字符串提交的值解释为 null 。将以下内容添加到您的 web.xml 中：
Now, you need to tell JSF to interpret empty string submitted values as null. Add the following to your web.xml:
<context-param>
<param-name>javax.faces.INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL</param-name>
<param-value>true</param-value>
</context-param>
这应该可以解决大多数容器的问题。但是如果您使用Tomcat或其叉/克隆（JBoss AS，WebSphere AS等）和/或容器使用Apache EL解析器实现，那么还需要将以下参数添加到其VM启动参数中：
This should solve the problem on most containers. But if you're using Tomcat or a fork/clone of it (JBoss AS, WebSphere AS, etc) and/or the container uses the Apache EL parser implementation, then you also need to add the following argument to its VM startup arguments:
-Dorg.apache.el.parser.COERCE_TO_ZERO=false
这将阻止Apache EL解析器将提交的值强制为原语的默认值，即使您使用原始包装器类型作为属性。
This will prevent the Apache EL parser from coercing the submitted value to the primitive's default, even though you're using a primitive wrapper type as property.
- Communication in JSF 2.0
不相关的具体问题，你没有一个组合框。这只是一个简单的下拉列表。组合框是可编辑的下拉列表。获取您的条款正确;）
Unrelated to the concrete problem, you don't have a combobox there. That's just a simple dropdownlist. A combobox is an editable dropdownlist. Get your terms right ;)
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