Question

Hi i have created a custom converter in jsf for combo box using h:selectOneMenu,

my backing bean code in as follows

@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);
         }
     }
}

this is my converter,

@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;
  }

and this is my jsp where i am applying my converter to the combo box

 <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>
  .....
  ....
  .. 

Now my question is what does this line do in my converter

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

What does ctx.getElContext() do ?

Answer 1

It obtains the ELContext (<– click the link to see the javadoc), so that you’re able to evaluate an EL expression #{} programmatically in Java code. In your particular case, you’re basically programmatically evaluating the EL expression #{studentMgBean} to get the current instance of StudentMBean.

In JSF 2.0 there’s by the way a shortcut by Application#evaluateExpressionGet() which does basically the same and hides the ELContext details under the hoods:

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

That said, your approach is pretty clumsy. If the converter is that tight coupled to a backing bean, you’d probably better make it a property of the backing bean instead:

converter="#{studentMgBean.studentConverter}"

with the converter as an inner class.