JDBC connection database

JDBC connection database operation

1. Several elements of JDBC database connection

• The standard of JDBC URL consists of three parts, each part is separated by a colon

• Format:

  jdbc：<子协议>：<子名称>
  

  • Protocol: The protocol in the JDBC URL is always jdbc
  • Subprotocol: The subprotocol is used to identify a database driver
  • Subname: A way to identify a database. The sub-name can be changed according to different sub-protocols, the purpose of using the sub-name is to provide enough information for locating the database
• JDBC URLs for several commonly used databases:
  
  • Oracle database:
    
    • jdbc:oracle:thin:@localhost:1521:orcl (database name, generally default to orcl)
  • SQL Server database:
    
    • jdbc:Microsoft:sqlserver//localhost:1433:DatabaseName=sid(database name)
  • MySQL database:
    
    • jdbc:mysql://localhost://3306/sid(database name)
    • : 1521, 1433, 3306 represent the listening port number of the database

2. Use the basic Driver to connect to the database

• Single connection fixed database:

  public void tes1t() {
      try {
          //1.创建Driver实现类的对象
          Driver driver = new com.mysql.jdbc.Driver();
          System.out.println(driver);
          //2.创建连接数据库的内容(用户名、密码)对象
          Properties properties = new Properties();
          //3.设置数据库地址
          String url = "jdbc:mysql://127.0.0.1:3306/company";
          //3.向对象中 插入数据库登陆的用户名与密码
          properties.put("user", "root");
          properties.put("password", "java123");
          //4.通过驱动获取数据库连接对象
          Connection coon = driver.connect(url, properties);
          System.out.println(coon);
      } catch (Exception e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
      }
  }
  

  • The running result is shown in the figure:

    

• Dynamically obtain database drivers through reflection mechanism

  • Configuration file: my.properties

    driverClass=com.mysql.jdbc.Driver
    jdbcUrl=jdbc:mysql://localhost:3306/java
    user=root
    password=java123
    //注意：在properties文件中,注释是以#号开头的，在这里使用了//
    //driverClass=oracle.jdbc.driver.OracleDriver
    //jdbcUrl=jdbc:oracle:thin:@localhost:1521:orcl
    //user=scott
    //password=java123
    

  • Test Case:

    public void test3() throws Exception{
        String driverName;
        String jdbcUrl;
        String user;
        String password;
    
        //1.获取当前类对象下的文件
        InputStream is = getClass().getClassLoader().getResourceAsStream("my.properties");
        Properties ps = new Properties();
        ps.load(is);
        //3.获取响应的文件内容
        driverName = ps.getProperty("driverClass");
        jdbcUrl = ps.getProperty("jdbcUrl");
        user = ps.getProperty("user");
        password = ps.getProperty("password");
        //调用方法返回连接数据库对象
        Connection coo = getConnection(driverName, jdbcUrl, user, password);
        System.out.println(coo);
        }
    
    //获取数据库连接方法
        public Connection getConnection(String driverName, String driverPath, String name, String pwd){
        Connection connection = null;
        try {
            //1.通过反射机制获取Driver实现类的对象
            @SuppressWarnings("static-access")
            Driver driver = (Driver)getClass().forName(driverName).newInstance();
            Properties info = new Properties();
            info.put("user", name);
            info.put("password", pwd);
            //2.获取数据库连接对象
            connection = driver.connect(driverPath, info);
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return connection;
    }
    

  • The results of connecting the two databases are shown in the following figures: just change the configuration file (.properties)

    • Connect to MySQL database

      

    • Connect to Oracle database

      

3. JDBC connects to the database and implements database operations through PreparedStatement (Statement) objects

• Using PreparedStatement can effectively avoid injection problems caused by string concatenation in database operations
• Using PreparedStatement can effectively optimize the code

• The following is a case of connecting to the MySQL database and operating the database, using the object-oriented (Java Bean) form to operate the database, such as:

  • Java Bean：customer.class

    package com.ajb.statement;
    /**
     * Java Bean 对象
     * @author duanjunhua
     *
     */
    public class Customer {
        private String name;
        private String password;
        private int id;
    
        public int getId() {
            return id;
        }
        public void setId(int id) {
            this.id = id;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public String getPassword() {
            return password;
        }
        public void setPassword(String password) {
            this.password = password;
        }
    }
    

  • 主类：TestPreparedStatement.class

    package com.ajb.statement;
    import java.io.InputStream;
    import java.sql.Connection;
    import java.sql.DriverManager;
    import java.sql.PreparedStatement;
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import java.sql.Statement;
    import java.util.Properties;
    import java.util.Scanner;
    import org.junit.Test;
    public class TestPreparedStatement {
        Connection coon = null;
        PreparedStatement pStatement = null;
        private boolean flag;
        private Customer customer;
        private String sql;
        private int Id;
    
        @Test
        public void test(){
            //1.获取Java Bean对象
            customer = getCustomer();
            //2.创建SQL语句
            if(customer == null)
                return;
            else{
                //PreparedStatement使用占位符的形式
                if(customer.getId()==1){
                    sql = "insert into customer values(?, ?)";
                }else if(customer.getId() == 2){
                    sql = "select * from customer where name=? and password=?";
                }
            }
            //3.获取数据库连接对象
            coon = getConnection();
            //表若不存在则创建表
            if(tableExists(coon) == false){
                createTable(coon);
            }
            //4.执行SQL语句操作
            executeSQL(coon, sql, customer);
            //5.释放连接(必须手动释放)
            release(coon, pStatement);
        }
    
        //判断数据库表是否存在
        public boolean tableExists(Connection connection){
            flag = false;
            String sqlquery = "select table_name from information_schema.tables";
            try {
                pStatement = connection.prepareStatement(sqlquery);
                ResultSet resultSet = pStatement.executeQuery();
                while (resultSet.next()) {
                    if(resultSet.getString(1).equals("customer")){
                        flag = true;
                        return flag;
                    }
                }
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return flag;
        }
        //创建表
        public void createTable(Connection connection){
            String sql = "create table customer("
                    + "name varchar(15),"
                    + "password varchar(15)"
                    + ")";
            try {
                //获取PreparedStatement对象
                pStatement = connection.prepareStatement(sql);
                pStatement.executeUpdate();
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        //从键盘输入获取Java Bean对象数据
        public Customer getCustomer(){
            Customer customer = new Customer();
            Scanner scanner = new Scanner(System.in);
            System.out.println("请选择模式：1、注册用户\n\t  2.用户登录");
            customer.setId(scanner.nextInt());
            //如果是两者之外的其他模式，直接退出并提示用户信息
            if(customer.getId()!= 1 && customer.getId() != 2){
                System.out.println("选择错误，请重新运行输入...");
                return null;
            }
            System.out.print("请输入用户名:");
            customer.setName(scanner.next());
            System.out.print("请输入登录密码:");
            customer.setPassword(scanner.next());
            return customer;
        }
        //获取数据库的配置文件
        //根据配置文件获取对应的数据库连接对象
        public Connection getConnection(){
            //
            Properties ps = new Properties();
            try {
                InputStream is = getClass().getClassLoader().getResourceAsStream("my.properties");
                ps.load(is);
                coon = DriverManager.getConnection(ps.getProperty("jdbcUrl"), ps.getProperty("user"), ps.getProperty("password"));
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return coon;
        }
        //执行数据库的更新操作
        public void executeSQL(Connection connection, String sql, Customer customer){
            try {
                //4.由连接对象获取PreparedStatement对象
                pStatement = connection.prepareStatement(sql);
                //5.加载数据
                if(customer.getId() == 1){
                    if(TestPreparedStatement.queryUser(connection,customer) == false){
                        TestPreparedStatement.update(pStatement, customer.getName(), customer.getPassword());
                        pStatement.executeUpdate();
                    }else {
                        System.out.println("用户名已存在,请重新输入用户名!!!");
                        return;
                    }
                }else {
                    TestPreparedStatement.update(pStatement, customer.getName(), customer.getPassword());
                    ResultSet resultSet = pStatement.executeQuery();
                    while(resultSet.next()){
                        System.out.println("登录成功 !!!");
                        return;
                    }
                    System.out.println("用户名或密码错误,请重新登录!!!");
                }
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        //使用setXxx()方法填充占位符
        public static void update(PreparedStatement pStatement, Object ... objects) throws SQLException{
            for(int i = 0; i < objects.length; i++){
                pStatement.setObject(i + 1, objects[i]);
            }
        }
        //6.关闭数据库连接
        public static void release(Connection connection, PreparedStatement pStatement){
            if(pStatement != null){
                try {
                    pStatement.close();
                } catch (SQLException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
    
            if(connection != null){
                try {
                    connection.close();
                } catch (SQLException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    
        //查询用户名是否存在
        public static boolean queryUser(Connection connection,Customer customer){
            //Statement使用字符串拼接的形式
            String sql = "select name from customer where name= '" + customer.getName() + "'";
            try {
                Statement statement = connection.createStatement();
                ResultSet resultSet = statement.executeQuery(sql);
                while (resultSet.next()) {
                    return true;
                }
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return false;
        }
    }
    

  • Create the database at the same time when registering for the first time

    

  • registration failed

    

  • login successful

    

  • Login failed

    

  • database

    

Note: The corresponding database driver is required when connecting to the database, as shown in the figure:

• Project directory

  

• Create a new lib folder in the project directory, and copy the corresponding driver .jar file to this directory
• Right-click Build Path -> Add to Build Path to add it to the project directory library file in the Referenced Libraries directory for calling