valueChangeListener使用注意事项

运行健康的示例。页面代码

<f:view>
    <h:form id="event1Form">
       <h3>请选择课程类别</h3>
       <table>
           <tr>
              <td>课程类别：</td>
              <td><h:selectOneMenu value="#{course.category}"
                  onchange="submit()" valueChangeListener="#{course.categoryChanged}"
                  immediate="true">
                  <f:selectItem itemValue="null" itemLabel="--choose--" />
                  <f:selectItems value="#{course.categoies}" />
              </h:selectOneMenu></td>
           </tr>
           <tr>
              <td>课程：</td>
              <td><h:selectOneMenu value="#{course.course}">
                  <f:selectItems value="#{course.courses}" />
              </h:selectOneMenu></td>
           </tr>
       </table>
    </h:form>
    <h:messages globalOnly="true"></h:messages>
</f:view>

categoryChanged方法源码为

public void categoryChanged(ValueChangeEvent event) {
       FacesContext context = FacesContext.getCurrentInstance();
       if ("language".equals((String) event.getNewValue())) {
           courses.clear();
           for (int i = 0; i < courseNames[0].length; ++i) {
              courses.add(new SelectItem(courseNames[0][i]));
           }
       } else if ("datebase".equals((String) event.getNewValue())) {
           courses.clear();
           for (int i = 0; i < courseNames[1].length; ++i) {
              courses.add(new SelectItem(courseNames[1][i]));
           }
       } else {
           courses.clear();
       }
       context.renderResponse();
    }

这是一个valueChangeListener可以正常运行的情况，其中有两点要注意：

1、<h:selectOneMenu /> 标签中 immediate="true" 不可少。如果缺少该属性则会出现当第一次触发该事件时程序可以正常使用，但当第二次触发时会报

严重: JSF1054: (Phase ID: PROCESS_VALIDATIONS 3, View ID: /actionEvent/event1.jsp) Exception thrown during phase execution: javax.faces.event.PhaseEvent[source=com.sun.faces.lifecycle.LifecycleImpl@32bd65]
2009-6-28 13:47:42 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet Faces Servlet threw exception
java.util.NoSuchElementException
    at javax.faces.component.SelectItemsIterator.next(SelectItemsIterator.java:130)
    at javax.faces.component.SelectItemsIterator.next(SelectItemsIterator.java:157)
    at javax.faces.component.SelectItemsIterator.next(SelectItemsIterator.java:61) 

2、在监听方法 categoryChanged 中 context.renderResponse(); 不能少，如果少了该方法，虽然可以成功触发监听时间，但程序后台会有如下警告

信息: WARNING: FacesMessage(s) have been enqueued, but may not have been displayed.
sourceId=event1Form:j_id_jsp_1462394538_5[severity=(ERROR 2), summary=(event1Form:j_id_jsp_1462394538_5: Validation Error: Value is not valid), detail=(event1Form:j_id_jsp_1462394538_5: Validation Error: Value is not valid)]

将<h:messages globalOnly="true"></h:messages>的globalOnly属性更改为false上述警告消失，但在页面上会打印event1Form:j_id_jsp_1462394538_5: Validation Error: Value is not valid