.........
   <import value="java.util.*" />
   <import value="net.sf.jasperreports.engine.*" />
   <import value="net.sf.jasperreports.engine.data.*" />
   <import value="com.mycompany.myscriptlet.*" />

   <parameter name="MY_SCRIPTLET" isForPrompting="false" class="com.mycompany.myscriptlet"/>
........
                  <font size="12"/>
               </textElement>
            <textFieldExpression   class="java.lang.String"><![CDATA[$P{MY_SCRIPTLET}.getWeekNumber(""+$F{col3})]]></textFieldExpression>
            </textField>
.........

<parameter name="MY_SCRIPTLET" isForPrompting="false" class="com.mycompany.myscriptlet"/>

import java.sql.*;
import java.util.*;
import com.mycompany.myscriptlet.*;

public void fillReport(con, filter, result)
{
PreparedStatement ps=con.prepareStatement(result.sql("select contract.date1 as col3, "+
"contract_group.title as col1, "+
"count(contract_group.title) as col2 "+
"from contract "+
"join contract_group on contract.gr=contract_group.id "+
"group by col3",filter));
ResultSet rs=ps.executeQuery();
result.setDataSource(rs);
}

import java.sql.*;
import java.util.*;
import com.mycompany.myscriptlet.*;

public void fillReport(con, filter, result)
{
/*
*Ниже например как получить текущий год и минус месяц, этот код заменить на получение недели
*/
Calendar c = Calendar.getInstance();
currentYear = c.get(java.util.Calendar.YEAR);
c.add( Calendar.MONTH, -1 );
mounth = c.getDisplayName(Calendar.MONTH, Calendar.LONG, new Locale("ru"));

PreparedStatement ps=con.prepareStatement(result.sql("select contract.date1, "+
"contract_group.title, "+
"count(contract_group.title) as col2 "+
"from contract "+
"join contract_group on contract.gr=contract_group.id "+
"group by contract.date1",filter));
ResultSet rs=ps.executeQuery();
List res = new ArrayList();
  while(rs.next())
  {
    Map  map = new HashMap();
    map.put("col1", rs.getString(2));
    map.put("col2", rs.getString(3));
    map.put("col3", rs.getString(1));
    map.put("col4", mounth);



    res.add(map);
  }

  result.setDataSource( res );
result.setDataSource(rs);
}

package com.mycompany;
import net.sf.jasperreports.engine.JRDefaultScriptlet;
import net.sf.jasperreports.engine.JRScriptletException;

public class myscriptlet extends JRDefaultScriptlet {

 public String getWeekNumber(String mydate) {

int D = Integer.valueOf(mydate.substring(1,2));
int M = Integer.valueOf(mydate.substring(4,5));
int Y = Integer.valueOf(mydate.substring(7,10));


int a = (14 - M) / 12;
int y = Y + 4800 - a;
int m = M + 12 * a - 3;
int J = D + (153 * m + 2) / 5 + 365 * y + y / 4 - y / 100 + y / 400 - 32045;
int d4 = (J + 31741 - (J % 7)) % 146097 % 365224 % 1461;
int L = d4 / 1460;
int d1 = ((d4 - L) % 365) + L;
int WN = d1 / 7 + 1;
String str = String.valueOf(WN);
return str;
 }
}