Question

I’m implementing a JPA-like process: all classes representing a table must subclass a DBTable abstract class which contains a HashMap linking column names (String) to java.lang.reflect.Field I got from calling getClass().getDeclaredFields() in the super-class constructor, called from the sub-class constructor.
This is working (I get the members I want from the sub-class).

I want to dynamically set these members of the subclass from a ResultSet which would call the Field.set(this, ResultSet.getXXX(column)) in a method originally implemented in the super-class as a helper, but I get a java.lang.IllegalAccessException: Class DBTable can not access a member of class MyTable with modifiers "protected".

So it turns out the method is executed in the super-class context (the abstract class!): if I copy/paste the method in my sub-class it’s working fine.

Here is a snippet:

public abstract class DBTable {
    public DBTable() {
        hm = new HashMap<String,Field>();
        for (Field f : getClass().getDeclaredFields()) {
            hm.put(f.getAnnotation(Annot.class).value(), f); // 'annot' gives the column name
        }
    }

    protected boolean setFieldClass(Field f, ResultSet rs, int iCol) {
        Class<?> type = f.getType();
        if (type == String.class) {
            f.set(this, rs.getString(iCol)); // IllegalAccessException
        }
    }

    public boolean read(ResultSet rs) {
        ResultSetMetaData meta = rs.getMetaData();
        for (int i = 1; i < meta.getColumnCount(); i++) {
            Field f = hm.get(meta.getColumnName(i));
            setFieldClass(f, rs, i); // IllegalAccessException
        }
    }

    public abstract someAbstractOtherMethod();
}

public class MyTable extends DBTable {

    @Annot("DataCol") // Set the column name to "DataCol"
    protected String dataStr;

    public MyTable() {
        super();
    }

    // 'dataStr' getter/setter
    // 'someAbstractOtherMethod' implementation
}

public static void main(String[] args) {
    MyTable mt = new MyTable();
    ResultSet rs = ...; // Execute SELECT query returning a single row with a "DataCol" column containing a varchar(32)
    mt.read(rs); // IllegalAccessException
}

The read method called on a MyTable object is inherited from the DBTable class which, in turns, calls the setFieldClass method which throws the error.
If I copy/paste the setFieldClass method from DBTable to MyTable it works, as well as changing the dataStr member to public.

I could call a setter instead of directly setting the member value, but would like to avoid doing that (I’m not sure sub-classes will implement it nor will they all give the same names).

Thank you for sharing your insights with me! 🙂

Answer 1

call Field.setAccessible(true) before calling Field.set.