Question

您好我使用h：selectOneMenu在jsf中为组合框创建了一个自定义转换器，

我的支持bean代码如下

@ManagedBean(name="studentMgBean")
public class StudentMBean {
..............
............
.....
      public StudentVO getMyStudent(Integer studentId) {
          return this.myStudents.get(studentId);
      }
      private List<SelectItem> studentList;
      // getter setter of studentList
      private Map<Integer,StudentVO> myStudents;
      private StudentVO selectedStudent;
      // getter setter of selectedStudent

     @PostConstruct
     public void loadStudents(){
         ..........
         ........
         if(this.getStudentList() == null){ 
             this.setStudentList(new ArrayList<SelectItem>());
         }else{
             this.getStudentList().clear();
         }
         this.myStudents = new HashMap<Integer, StudentVO>();
         while(rs.next()){
              vo = new StudentVO(String.valueOf(rs.getInt("studentId")),
                    rs.getString("studentName"), rs.getString("contactNo"));
              selectItem = new SelectItem(vo.getStudentId(), vo.getStudentName());
              this.getStudentList().add(selectItem);
              this.myStudents.put(Integer.parseInt(vo.getStudentId()),vo);
         }
     }
}

这是我的转换器，

@FacesConverter(value="studentComboConv")
public class StudentComboBoxConverter implements Converter{
     @Override
     public Object getAsObject(FacesContext context, UIComponent component, String value) {
     FacesContext ctx = null;
     ValueExpression vex = null;
     StudentMBean studentMgmtBean = null;
     StudentVO studentVO = null;
     .........
     ........
     ......
     vex = ctx.getApplication().getExpressionFactory()
                     .createValueExpression(ctx.getELContext(),"#{studentMgBean}", StudentMBean.class);
     studentMgmtBean = (StudentMBean) vex.getValue(ctx.getELContext());
     studentVO = studentMgmtBean.getMyStudent(Integer.parseInt(value)); 
     ...........
     ........
     .....
     return studentVO;
  }

这是我的jsp，我将我的转换器应用到组合框

 <td align="left">SELECT STUDENT</td>
 <td align="right">
     <h:selectOneMenu value="#{studentMgBean.selectedStudent}" id="cmbo" converter="studentComboConv">
        <f:selectItems value="#{studentMgBean.studentList}" />
     </h:selectOneMenu>
  .....
  ....
  ..

现在我的问题是这条线在我的转换器中做了什么

  vex = ctx.getApplication().getExpressionFactory() 
        .createValueExpression(ctx.getELContext(),"#{studentMgBean}", StudentMBean.class);
  studentMgmtBean = (StudentMBean) vex.getValue(ctx.getELContext());

ctx.getElContext（）做什么？

Answer 1

它获取ELContext（＆lt; - 单击链接以查看javadoc），以便您能够以编程方式在Java代码中评估EL表达式#{}。在您的特定情况下，您基本上以编程方式评估EL表达式#{studentMgBean}以获取StudentMBean的当前实例。

在JSF 2.0中，Application#evaluateExpressionGet()的快捷方式基本相同，隐藏了ELContext详细信息：

StudentMBean studentMgBean = (StudentMBean) ctx.getApplication().evaluateExpressionGet(ctx, "#{studentMgBean}", StudentMBean.class);

那就是说，你的方法非常笨拙。如果转换器紧密耦合到支持bean，你可能最好使它成为支持bean的属性：

converter="#{studentMgBean.studentConverter}"

将转换器作为内部类。

ELContext在我的JSF自定义Converter中做了什么

1 个答案: