## Extraction of JDBC tool classes: JDBCUtils
* :: Purpose: to simplify writing
* :: Analysis.
	1. Register the driver and also extract
	2. Extract a method to get a connected object
* :: Requirements: do not want to pass parameters (cumbersome), but also need to ensure the generality of the tool class
* :: Resolution: configuration file
		jdbc.properties
			url=
			user=
			password= 
3. Extract a method to release resources

Create jDBc.properties in the SRC directory

url=jdbc:mysql:///db3
user=root
password=123456
driver=com.mysql.cj.jdbc.Driver

JDBCUtils.java

package cn.itcast.utils;

import jdk.swing.interop.SwingInterOpUtils;

import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.net.URI;
import java.net.URL;
import java.sql.*;
import java.util.Properties;

// JDBC Tool
public class JDBCUtils {

    private static String url;
    private static String user;
    private static String password;
    private static String driver;

    // The file is read only once to get these values. Using static code blocks
    static {
        // Read the resource file, get the value.
        try {
            // Create the Properties collection class.
            Properties pro = new Properties();
            // The way to get the files in the src path - >ClassLoader class loader
            ClassLoader classLoader = JDBCUtils.class.getClassLoader();
            URL res = classLoader.getResource("jdbc.properties");
            String path = res.getPath();
            System.out.println(path);
            // 2. Load the file.
            pro.load(new FileReader(path));

            // 3. Get the data and assign a value.
            url = pro.getProperty("url");
            user = pro.getProperty("user");
            password = pro.getProperty("password");
            driver = pro.getProperty("driver");

            // 4. Registration drive
            Class.forName(driver);
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    // Get connection, return connection object
    public static Connection getConnection() throws SQLException {
        return DriverManager.getConnection(url,user,password);
    }
    // Release of resources
    public static void close(Statement stmt, Connection conn){
        if(stmt != null){
            try {
                stmt.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }

        if(conn != null){
            try {
                conn.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
    }

    // Release of resources
    public static void close(ResultSet rs, Statement stmt, Connection conn){
        if(rs != null){
            try {
                rs.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }

        if(stmt != null){
            try {
                stmt.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }

        if(conn != null){
            try {
                conn.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
    }
}

jdbcdemo.java

    // Demo JDBC tool class
    // Query all emp objects.
    public List<Emp> findAll() {

        Connection conn = null;
        Statement stmt = null;
        ResultSet rs = null;
        List<Emp> list = null;

        try {
            conn = JDBCUtils.getConnection();
            // 3. Defining sql
            String sql = "select * from emp";
            // 4. Get the object to execute sql.
            stmt = conn.createStatement();
            // 5. execute sql
            rs = stmt.executeQuery(sql);
            // 6. traverse the result set, encapsulate the object, load the set
            Emp emp = null;
            list = new ArrayList<Emp>();
            while(rs.next()){
                // Access to data
                int id = rs.getInt("id");
                String ename = rs.getString("ename");
                int job_id = rs.getInt("job_id");
                int mgr = rs.getInt("mgr");
                Date joindate = rs.getDate("joindate");
                double salary = rs.getDouble("salary");
                double bounds = rs.getDouble("bounds");
                int dept_id = rs.getInt("dept_id");

                // Create an emp object and assign it a value.
                emp = new Emp();
                emp.setEname(ename);
                emp.setJob_id(job_id);
                emp.setMgr(mgr);
                emp.setJoindate(joindate);
                emp.setSalary(salary);
                emp.setBounds(bounds);
                emp.setDept_id(dept_id);

                // load a collection
                list.add(emp);
            }
        }catch (SQLException e){
            e.printStackTrace();
        }finally {
            JDBCUtils.close(rs,stmt,conn);
        }
        return list;
    }