Question

I have a requirement to concatenate a list of Strings using Java EL. Please note that this is not in a JSP/Web context, but in a standalone Java client (using juel-2.2.4.jar).
The expression is of the form:

${for(int i=1; i<Employees.length; i++){Employee[i].Name + "--" }}

I already have a custom ELResolver which understands Employee objects.
The output I am expecting is a String with the names of all employees with a separator in between.

Is this even possible in a standalone Java EL application?

Answer 1

You probably have to make a helper function, afaik, there is no way java can handle this with an expression out of the box.

${Employee.joinEmployeesByName(Employees, "--")}

Code in the Employee class:

public static String joinEmployeesByName( Employee[] arr, String separator ) {
    //implement
}

Here’s dynamic example using reflection:

public static String getFieldByName(Employee obj, String fieldName) {
    String ret = null;
    Class<?> c = obj.getClass();
    try {
        Field field = c.getDeclaredField(fieldName);
        ret = field.get(obj).toString();
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
    return ret;
}

public static String joinEmployeesByField(Employee[] arr, String fieldName, String separator) {
    if(arr.length < 1) {
        return "";
    }
    StringBuilder ret = new StringBuilder();
    ret.append(getFieldByName(arr[0], fieldName));
    for( int i = 1; i < arr.length; ++i ) {
        ret.append( separator );
        ret.append( getFieldByName( arr[i], fieldName) );
    }
    return ret.toString();
}

Usage would be:

${Employee.joinEmployeesByField(Employees, "Name", "--")}
${Employee.joinEmployeesByField(Employees, "Designation", "--")}