用于selectOneMenu的JSF转换器

Question

我想使用selectOneMenu在hibernate实体中设置多对一关系。

<h:selectOneMenu value="#{taskBean.instance.currency}" class="form-control" id="currencyList" converter="#{currencyConverterBean}">
                <f:selectItems value="#{currencyBean.currencyList}" var="currency" itemLabel="#{currency.name}"/>
</h:selectOneMenu>

其中#{taskBean.instance}返回一个hibernate实体任务：

@Entity
@Table(name="task")
public class Task {
    //...

    public Currency currency;

    @ManyToOne
    @JoinColumn(name = "currencyID")
    public Currency getCurrency() {
        return currency;
    }

    //...
}

如果为selectOneMenu设置值#{taskBean.instance.currency.id}而为itemValue设置#{currency.id}"则不起作用。因此，我尝试使用转换器：

@ManagedBean(name="currencyConverterBean")
@FacesConverter(value = "currencyConverter")
@RequestScoped
public class CurrencyConverter implements Converter {

    CurrencyBO currencyBO;

    @Override
    public Currency getAsObject(FacesContext ctx, UIComponent component, String value) {
        return getCurrencyBO().getCurrencyById(new Integer(value));
    }

    @Override
    public String getAsString(FacesContext fc, UIComponent uic, Object o) {
        return String.valueOf(((Currency) o).getId());
    }

    public CurrencyBO getCurrencyBO() {
        return currencyBO;
    }

    public void setCurrencyBO(CurrencyBO currencyBO) {
        this.currencyBO = currencyBO;
    }
}

它不起作用。它返回以下错误：

PWC1406: Servlet.service() for servlet Faces Servlet threw exception
java.lang.NullPointerException  at com.otv.converter.CurrencyConverter.getAsObject(CurrencyConverter.java:33)

我做错了什么？