There is a database (northwind) on my machine and I have to write a code in java so as to extract the data from the table (Customers) stored in the database.
If this was only specific to Customers table then I would have done it but I want to make my code generic so that I can extract data from other tables also by simply giving the name of the table in a string variable.
Please have a look to my code.
Main class
package main;
import java.io.File;
import java.sql.Connection;
import java.sql.SQLException;
import java.util.List;
import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.ResultSetHandler;
import org.apache.commons.dbutils.handlers.BeanListHandler;
import model.TableModel;
import service.DBConnection;
import service.WriteExcel;
public class Main {
public static void main(String[] args) throws SQLException, ClassNotFoundException {
double start = System.nanoTime();
String tableName = "Customers";
Class<?> c = Class.forName(tableName);
Connection conn = new DBConnection().getConnection();
System.out.println("Connection Established");
QueryRunner run = new QueryRunner();
ResultSetHandler<List<TableModel>> resultHandler = new BeanListHandler<TableModel>(c.getClass())
List<TableModel> data = run.query(conn, "SELECT * FROM `" + tableName + "`;",
resultHandler);
WriteExcel we = new WriteExcel(tableName+"_sheet", new File(tableName+".xlsx"));
we.writeMultipleRows(data);
we.writeWorkbookToFile();
System.out.println("File Written Succesfully");
conn.close();
System.out.println("Time Taken: " + (System.nanoTime()-start)/1000000+" ms");
}
}
In the above code, at line 27, If the statement would have been as follows
ResultSetHandler<List<TableModel>> resultHandler = new BeanListHandler<TableModel>(Customers.class);
This is running perfectly, as I said I want this statement to be independent of the table name, making my code more general.
TableModel
package model;
import java.util.List;
public interface TableModel {
public List<String> getObjectAsList();
}
Customers
package model;
import java.util.ArrayList;
import java.util.List;
public class Customers implements TableModel {
private String customerId;
private String companyName;
private String contactName;
private String contactTitle;
private String address;
private String city;
private String region;
private String postalCode;
private String country;
private String phone;
private String fax;
public String getCustomerId() {
return customerId;
}
public void setCustomerId(String customerId) {
this.customerId = customerId;
}
public String getCompanyName() {
return companyName;
}
public void setCompanyName(String companyName) {
this.companyName = companyName;
}
public String getContactName() {
return contactName;
}
public void setContactName(String contactName) {
this.contactName = contactName;
}
public String getContactTitle() {
return contactTitle;
}
public void setContactTitle(String contactTitle) {
this.contactTitle = contactTitle;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getRegion() {
return region;
}
public void setRegion(String region) {
this.region = region;
}
public String getPostalCode() {
return postalCode;
}
public void setPostalCode(String postalCode) {
this.postalCode = postalCode;
}
public String getCountry() {
return country;
}
public void setCountry(String country) {
this.country = country;
}
public String getPhone() {
return phone;
}
public void setPhone(String phone) {
this.phone = phone;
}
public String getFax() {
return fax;
}
public void setFax(String fax) {
this.fax = fax;
}
public List<String> getObjectAsList(){
List<String> fields = new ArrayList<>();
fields.add(customerId);
fields.add(companyName);
fields.add(contactName);
fields.add(contactTitle);
fields.add(address);
fields.add(city);
fields.add(region);
fields.add(postalCode);
fields.add(country);
fields.add(phone);
fields.add(fax);
return fields;
}
@Override
public String toString() {
return "{ CustomerID = "+getCustomerId()+","
+ " CompanyName = "+getCompanyName()+","
+ " ContactName = "+getContactName()+","
+ " ContactTitle = "+getContactTitle()+","
+ " Address = "+getAddress()+","
+ " City = "+getCity()+","
+ " Region = "+getRegion()+","
+ " PostalCode = "+getPostalCode()+","
+ " Country = "+getCountry()+","
+ " Phone = "+getPhone()+","
+ " Fax = "+getFax()+"}";
}
}
I have used DbUtils library for extracting database. Any further suggestion for enhancing my code is welcomed.
If I understand your question right, you could try something like below.
To query the table, you can use run.query(SQL, ResultHandler).
ResultSetHandler<List<Map<String, Object>>> resultHandler = genericResultHandler();
List<Map<String, Object>> result = null;
// Execute the SQL statement and return the results in a List of
// T objects generated by the BeanListHandler.
try
{
result = run.query(sqlQuery, resultHandler, varargs);
}
catch (SQLException e)
{
e.printStackTrace();
}
result.stream().forEach(System.out::println);
The interesting part here is the private method genericResultHandler. For demonstration purposes, I used a HashMap to store the values and the corresponding cloumn names.
private ResultSetHandler<List<Map<String, Object>>> genericResultHandler()
{
return new ResultSetHandler<List<Map<String, Object>>>()
{
@Override
public List<Map<String, Object>> handle(java.sql.ResultSet rs) throws SQLException
{
List<Map<String, Object>> result = new ArrayList<>();
// Query all rows of the table.
while (rs.next())
{
// Get metadata of the table.
java.sql.ResultSetMetaData meta = rs.getMetaData();
int cols = meta.getColumnCount();
Map<String, Object> data = new HashMap<>();
// For each column store column name and value of the cell into the hashmap.
for (int i = 1; i < cols; i++)
{
String colName = meta.getColumnName(i);
Object value = rs.getObject(colName);
data.put(colName, value);
}
// Add the row to the result list.
result.add(data);
}
return result;
}
};
}
Afterwards some imports I have used:
import org.apache.commons.dbcp.BasicDataSource;
import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.ResultSetHandler;
Output would be something like this (for my test table):
{month=JANUARY, temperature=1.6, globalradiation=0.0, monthid=1}
{month=FEBRUARY, temperature=-0.9, globalradiation=0.0, monthid=2}
{month=MARCH, temperature=0.9, globalradiation=0.0, monthid=3}
{month=APRIL, temperature=7.2, globalradiation=0.0, monthid=4}
{month=MAY, temperature=14.1, globalradiation=0.0, monthid=5}