public class MyAction extends ActionSupport {
  private MyParameter param;

  public String execute() { ..... }
  // getter and setters
}

public class MyParameter {
  private List<GridData> gridDataList;
  private String subParam1;
  private String subParam2;
  // getters and setters here
}

public class GridData {
  private String title;
  private String arg1;
  // etc... getters and setters...
}

super.addFieldError("param.subParam1", "subParam1 error message");
super.addFieldError("param.subParam2", "subParam2 error message");

for(int intRow=0; intRow < param.griDataList.size(); intRow++) {
  super.addFieldError("param.gridDataList[" + intRow + "].arg1", "error message");
}

<s:textfield
name="param.gridDataList[%{#outerStat}].data"
cssClass="grid_text_field"
cssErrorClass="error_field" maxlength="7"/>

<input
type="text"
name="param.gridDataList[0].data"
class="grid_text_field error_field"
maxlength="7"/>

<s:fielderror fieldName="param.gridDataList[%{#outerStat}].data"/>