My Postgresql Salesdetails_DataService, and Java file , i export the jar file in /home/youtylity5/Downloads/LatestVersion/wso2dss-3.0.1/repository/components/lib and Schedule the task as per your blog, while i am restart the server executing the Schedule,it's not showing nothing in terminal side. Here i am sending my files.

Salesdetails_DataService.dbs

<data description="multiple services per each table" enableBatchRequests="false" enableBoxcarring="false" name="salesdetails_DataService" serviceNamespace="http://ws.wso2.org/dataservice" serviceStatus="active">
   <config id="default">
      <property name="carbon_datasource_name">Signingtest</property>
   </config>
   <query id="select_all_salesdetails_query" useConfig="default">
      <sql>SELECT productcode, productname, solditems, instock FROM public.salesdetails</sql>
      <result element="salesdetailsCollection" rowName="salesdetails">
         <element column="productcode" name="productcode" xsdType="xs:string"/>
         <element column="productname" name="productname" xsdType="xs:string"/>
         <element column="solditems" name="solditems" xsdType="xs:string"/>
         <element column="instock" name="instock" xsdType="xs:string"/>
      </result>
   </query>
   <query id="insert_salesdetails_query" useConfig="default">
      <sql>INSERT INTO public.salesdetails(productcode,productname,solditems,instock) VALUES(?,?,?,?)</sql>
      <param name="productcode" ordinal="1" paramType="SCALAR" sqlType="STRING" type="IN"/>
      <param name="productname" ordinal="2" paramType="SCALAR" sqlType="STRING" type="IN"/>
      <param name="solditems" ordinal="3" paramType="SCALAR" sqlType="STRING" type="IN"/>
      <param name="instock" ordinal="4" paramType="SCALAR" sqlType="STRING" type="IN"/>
   </query>
   <operation name="select_all_salesdetails_operation">
      <call-query href="select_all_salesdetails_query"/>
   </operation>
   
   <operation name="insert_salesdetails_operation">
      <call-query href="insert_salesdetails_query">
         <with-param name="productcode" query-param="productcode"/>
         <with-param name="instock" query-param="instock"/>
         <with-param name="solditems" query-param="solditems"/>
         <with-param name="productname" query-param="productname"/>
      </call-query>
   </operation>
   

ScheduledDataTask_abcCompany.java

package com.example.DssSchedlueTask;
 
import java.util.HashMap;
import java.util.Map;
import org.wso2.carbon.dataservices.core.DataServiceFault;
import org.wso2.carbon.dataservices.core.engine.ParamValue;
import org.wso2.carbon.dataservices.task.DataTask;
import org.wso2.carbon.dataservices.task.DataTaskContext;
import org.apache.axiom.om.OMElement;
 
public class ScheduledDataTask_abcCompany implements DataTask {
 
	public void execute(DataTaskContext ctx) {
		System.out.println("Data Task executing...");
		String solditems = getSoldItems();
		Map<String, ParamValue> params = new HashMap<String, ParamValue>();
		params.put("soldItems", new ParamValue(solditems));
		params.put("productcode", new ParamValue("0001a3"));
		try {
			ctx.invokeOperation("abcCompany", "UpdateSales", params);
			System.out.println("Successfully updated the database. \tSoldItems = "+solditems);
		} catch (DataServiceFault e) {
			System.out.println("Error...."+e.getMessage());
		}
    }
 
	private String getSoldItems(){
		int soldItems = (int )(Math.random() * 10000 + 1);
		return Integer.toString(soldItems);
	}
}

To get this work you must add a scheduled task in wso2 dss server, It is clearly explain in the step 6 of the following blog post,
http://itsmeisuru.wordpress.com/2013/11/29/wso2-dss-scheduled-task-example/
I mentioned this in the stackoverflow question also.