Summer 2022 software innovation laboratory training JDBC

Use JDBC Preparation

• The database connection driver corresponding to the database version jar package , Download link , For example, my database version 8.0.25, So I should download 8.0.25 Connect packets

• MySQL database , It is recommended to install the database MySQL 8, because MySQL 5 There seemed to be some problems when I was a sophomore

• Create a database according to your preferences , You can operate it manually during class , The database I created is like this

  

JDBC Brief introduction

JDBC（Java DataBase Connectivity） yes Java Language used to regulate how the client program to access the database application program interface , Provides such as queries 、 to update （ Additions and deletions ） Method of data in database .JDBC, yes Sun The company provides a set of Interface ,java Programmers only need Programming for this set of interfaces . Different database vendors , We need to focus on this set of interfaces , Provide different implementations . A collection of different implementations , It is the driver of different databases . Make an inappropriate analogy , You should use different methods to use , Refer to the figure below , The following figure is from the blog

How to use JDBC？

got it JDBC What is it? , Why is there JDBC after , We need to learn to use JDBC.

1. Download the driver package

At the beginning of this blog , Let's download the driver package corresponding to the database , Because my database is MySQL 8.0.25, So the data package I downloaded is MySQL 8.0.25 Driver package , As shown in the figure

2. establish lib Catalog

After preparing the database driver package , We are going to create an ordinary Java project , What I use here is IDEA Enterprise Edition , Don't put the screenshot after the creation . Then we need to create one under the project folder lib Catalog , there lib The directory is dedicated to Java A series of bags , Like this time we went to JDBC, So here we are lib The directory stores MySQL Drive pack .

3. Unzip the driver package , Import driver jar package

After creation lib After the catalog , We need to unzip the driver package , After decompression, we can see the following content （ Only aim at MySQL 8.0.25 Drive pack ）

The only thing we need is inside mysql-connector-java-8.0.25.jar, Copy and paste this package into the newly created lib Catalog , Right click under the directory jar package ——> Add as library , As shown in the figure

Then directly determine

4. Use Java The code is right MySQL Database operation

• load MySQL drive

  Class.forName("com.mysql.cj.jdbc.Driver");
  

• Get database connection

  //  obtain mysql Connection address 
  String strURL = "jdbc:mysql://localhost:3306/ Database name ?&useSSL=false&serverTimezone=UTC";
  //  User name 
  String username = "root";
  //  User password 
  String password = "123456";
  Connection con = DriverManager.getConnection(strURL, username, password);
  

• establish statement object , In order to prevent SQL Statement Injection , We'll use it PreparedStatement To preprocess

  //  establish statement Class object , Used to perform SQL sentence ！
  Statement stm = con.createStatement();
  

  When we get stm Object can then be used to operate mysql sentence , Add, delete, change and query database

• understand Statement Object to perform SQL Method of statement

  //  adopt SQL Inquire about , Return the query result to Result The result set 
  ResultSet rs = st.executeQuery(sql);
  //  adopt SQL Update database content , Return the number of affected rows 
  int i = st.executeUpdate(sql);
  

• After the database operation is finished , Need to be closed ResultSet、Statement、Connection

  We need to know after operating the database ResultSet、Statement、Connection It takes up memory in the program , So we need to close , And we need to pay attention to the closing sequence , It's from left to right .

  The first is the simplest shutdown method , Close directly after the operation

  rs.close();
  st.close();
  con.close();
  

  But we can also change our thinking , Encapsulate the database shutdown operation directly into a method, like the following .

  //  Close database connection 
  public static void release(Connection con, Statement st, ResultSet rs) {
        
      boolean flag = true;
  
      if (rs != null) {
        
          try {
        
              rs.close();
              rs = null;
          } catch (SQLException e) {
        
              e.printStackTrace();
              flag = false;
          }
      }
  
      if (st != null) {
        
          try {
        
              st.close();
              st = null;
          } catch (SQLException e) {
        
              e.printStackTrace();
              flag = false;
          }
      }
  
      if (con != null) {
        
          try {
        
              con.close();
              con = null;
          } catch (SQLException e) {
        
              e.printStackTrace();
              flag = false;
          }
      }
  }
  

  In the following , We will make a database operation class by ourselves , To encapsulate some simple operations of the database , For example, database connection 、 Database shutdown or other database operations （ Database query and update )

Simple practice

Query operation

//  Defining variables 
Connection con = null;
Statement st = null;
ResultSet rs = null;

try {
    
    //  Load database driver com.mysql.cj.jdbc.Driver
    String strDriver = "com.mysql.cj.jdbc.Driver";
    //  obtain mysql Connection address 
    String strURL = "jdbc:mysql://localhost:3306/market?&useSSL=false&serverTimezone=UTC";
    //  Database user name 
    String username = "root";
    //  Database user password 
    String password = "123456";
    //  Load database driver 
    Class.forName(strDriver);
    System.out.println(" Load database driver ");
    //  Get database connection 
    con = DriverManager.getConnection(strURL, username, password);
    System.out.println(" Database connection successful ");
    //  obtain Statement object , To operate on database statements 
    st = con.createStatement();

    String sql = "SELECT * FROM book";
    //  adopt SQL Inquire about , Put the query results into Result The results are concentrated 
    rs = st.executeQuery(sql);
	//  Use next To traverse the data result set , And output 
    while (rs.next()) {
    
        int id = rs.getInt(1);
        String name = rs.getString(2);
        double price = rs.getDouble(3);
        System.out.println(id + " " + name + " " + price);
    }
    
} catch (SQLException | ClassNotFoundException e) {
    
    e.printStackTrace();
} finally {
    
    //  Close result set ResultSet
    if (rs != null) {
    
        try {
    
            rs.close();
            System.out.println("ResultSet Successfully closed ");
        } catch (SQLException e) {
    
            e.printStackTrace();
        }
    }
    //  Close the operation object Statement
    if (st != null) {
    
        try {
    
            st.close();
            System.out.println("Statement Successfully closed ");
        } catch (SQLException e) {
    
            e.printStackTrace();
        }
    }
    //  Close the database connection object Connection
    if (con != null) {
    
        try {
    
            con.close();
            System.out.println("Connection Successfully closed ");
        } catch (SQLException e) {
    
            e.printStackTrace();
        }
    }
}

The above example is only a query operation for the database , In many cases, we need more than just query , It is the update operation of the database , For example, add, delete, and modify .

1. Add operation

Here I want to insert a new database data , Number 6, Book name Criminal suspect X The dedication of , Price 43, In the database SQL The statement is as follows ：

INSERT INTO book VALUES(6, " Criminal suspect X The dedication of ", 43);

therefore , We just need to simply modify the... In the example 24~33 A line of code will do

String sql = "INSERT INTO book VALUES(6, ' Criminal suspect X The dedication of ', 43)";
int i = st.executeUpdate(sql);
//  Judge whether the number of affected rows is 0, if 0, Then the update operation just now did not succeed , If it is not 0, be SQL Statement operation succeeded 
if (i != 0) {
    
    System.out.println(" Insert the success ！");
} else {
    
    System.out.println(" Insert the failure ！");
}

As a result, this data was successfully inserted

view the database

2. Delete operation

Here we assume that we need to delete the data just now , database SQL The statement is

DELETE FROM book WHERE bid = 6;

Because the delete operation belongs to the database update operation , So just change one of them String Variables

String sql = "DELETE FROM book WHERE bid = 6";

The final result was successfully deleted , As shown in the figure

3. Change operation

Here, if I want to modify the 5 Record price , database SQL The sentence of is

UPDATE book SET bprice = 20 WHERE bid = 5;

Here we only need to modify String Variables

String sql = "UPDATE book SET bprice = 20 WHERE bid = 5";

The final result is modified successfully , As shown in the figure

PreparedStatement Use

Actually in PreparedStatement Use , Follow Statement The use of is almost the same , It's just PreparedStatement A little more steps , Here's the code , You can compare it with Statement The difference in the use of

//  Defining variables 
Connection con = null;
PreparedStatement pst = null;
ResultSet rs = null;

try {
    
    //  Load database driver com.mysql.cj.jdbc.Driver
    String strDriver = "com.mysql.cj.jdbc.Driver";
    //  obtain mysql Connection address 
    String strURL = "jdbc:mysql://localhost:3306/market?&useSSL=false&serverTimezone=UTC";
    //  Database user name 
    String username = "root";
    //  Database user password 
    String password = "123456";
    //  Load database driver 
    Class.forName(strDriver);
    System.out.println(" Load database driver ");
    //  Get database connection 
    con = DriverManager.getConnection(strURL, username, password);
    System.out.println(" Database connection successful ");
    
    
	//  Database operation statement , The question mark here is the parameter we need to set ourselves 
    String sql = "SELECT * FROM book WHERE bid = ?";
    // SQL Statement preprocessing , obtain PreparedStatement object 
    pst = con.prepareStatement(sql);
    //  by SQL Statement setting parameters , here setObject() The first parameter in the method represents SQL The question marks in the statement , The second parameter represents the setting variable , For example, the number we need to query here is 1 Book information 
    pst.setObject(1, 1);
    rs = pst.executeQuery();

    
    while (rs.next()) {
    
        int id = rs.getInt(1);
        String name = rs.getString(2);
        double price = rs.getDouble(3);
        System.out.println(id + " " + name + " " + price);
    }

} catch (SQLException | ClassNotFoundException e) {
    
    e.printStackTrace();
} finally {
    
    //  Close result set ResultSet
    if (rs != null) {
    
        try {
    
            rs.close();
            System.out.println("ResultSet Successfully closed ");
        } catch (SQLException e) {
    
            e.printStackTrace();
        }
    }
    //  Close the operation object Statement
    if (pst != null) {
    
        try {
    
            pst.close();
            System.out.println("Statement Successfully closed ");
        } catch (SQLException e) {
    
            e.printStackTrace();
        }
    }
    //  Close the database connection object Connection
    if (con != null) {
    
        try {
    
            con.close();
            System.out.println("Connection Successfully closed ");
        } catch (SQLException e) {
    
            e.printStackTrace();
        }
    }
}

It should be noted here that I used the lazy method when setting parameters , That is to use pst.setObject(1, 1) The way , But in fact, we should use pst.setInt(1, 1) In a more accurate way , Because we need to see according to the specific database fields . In the future, we will use the general class of database operation , To connect data 、 close 、 update operation , So in fact, I will still use pst.setObject() Method to set parameters

Then according to the above code , Whether it can be analogized , How to use Preparedment Perform database operations ？ The following is an example of deleting a data record , Other additions and modifications , Please also operate by yourselves

//  Database operation statement , The question mark here is the parameter we need to set ourselves 
String sql = "DELETE FROM book WHERE bid = ? AND bprice = ?";
// SQL Statement preprocessing , obtain PreparedStatement object 
pst = con.prepareStatement(sql);
//  by SQL Statement setting parameters , here setObject() The first parameter in the method represents SQL The question marks in the statement , The second parameter represents the setting variable , For example, the number we need to query here is 1 Book information 
pst.setInt(1, 1);
//  Because the book price field of the database I created is Double
pst.setDouble(2, 12);
int i = pst.executeUpdate();
if (i != 0) {
    
    System.out.println(" success ");
} else {
    
    System.out.println(" Failure ");
}

I don't know if you have any doubts here , Why Statement It also needs to be PreparedStatement？

Statement and PreparedStatement difference

Then we need to know the difference between the two , Reference blog

1. Performance aspect

   Statement sentence

   //  Get database connection 
   con = DriverManager.getConnection(strURL, username, password);
   //  obtain Statement object , To operate on database statements 
   st = con.createStatement();
   String sql = "SELECT * FROM book";
   //  adopt SQL Inquire about , Put the query results into Result The results are concentrated 
   rs = st.executeQuery(sql);
   

   PreparedStatement sentence

   //  Get database connection 
   con = DriverManager.getConnection(strURL, username, password);
   //  Database operation statement , The question mark here is the parameter we need to set ourselves 
   String sql = "SELECT * FROM book WHERE bid = ?";
   // SQL Statement preprocessing , obtain PreparedStatement object 
   pst = con.prepareStatement(sql);
   //  by SQL Statement setting parameters , here setObject() The first parameter in the method represents SQL The question marks in the statement , The second parameter represents the setting variable , For example, the number we need to query here is 1 Book information 
   pst.setObject(1, 1);
   rs = pst.executeQuery();
   

   From the above comparison , We can see PreparedStatement There is a precompile process , And it's bound to sql sentence , No matter how many times , Will not compile again . and Statement Is different ,sql How many times , How many times does it need to be compiled , therefore PreparedStatement More efficient than Statement high

2. Convenient for later changes , Improve code reading and maintainability

   We can notice that Statement The database operation statement of is dead , It cannot be changed when the program is running , however PreparedStatement stay SQL The user-defined parameters required in the statement are all carried ? Of , We need to pass setInt()、setObject()、setString() Wait for methods to set parameters , Easy to read and maintain the code

3. prevent SQL Inject

   If you're using Statement In words , Want to query a ID book , The statement reserved by the program is "SELECT * FROM book WHERE bid = " + bid, But if someone wants to know all the data at this time , Then he will bid = 2 OR 1 = 1, Finally formed SQL The statement is SELECT * FROM book WHERE bid = 2 OR 1 = 1, We know 1 = 1 Is absolutely true , So the final presentation is all the database data . This is the time PreparedStatement It becomes the key , stay PreparedStatement in SQL Statements are precompiled ,SQL The structure of the statement is also designed at the beginning , Adopt settings ? To fill in parameters , So it can't be realized by some strange characters SQL Inject .

Database tool class

We know that the database connection is written repeatedly 、 Closing this code is a very boring thing , We can't write this same piece of code many times in a project , This will greatly reduce the efficiency of writing code , So we came up with a way to package these operations into a Java class , And the basic information of database connection is stored by configuring the resource configuration file .

public class JDBCUtil {
    
    private static final String driver;
    private static final String url;
    private static final String userName;
    private static final String password;

    static {
    
        InputStream is = JDBCUtil.class.getClassLoader().getResourceAsStream("db.properties");
        Properties properties = new Properties();
        try {
    

            properties.load(is);
        } catch (IOException e) {
    
            e.printStackTrace();
        }
        driver = properties.getProperty("driver");
        url = properties.getProperty("url");
        userName = properties.getProperty("username");
        password = properties.getProperty("password");
    }

    //  Get database connection 
    public static Connection getConnection() {
    
        Connection con = null;
        try {
    
            Class.forName(driver);
            con = DriverManager.getConnection(url, userName, password);
        } catch (SQLException | ClassNotFoundException e) {
    
            e.printStackTrace();
        }
        return con;
    }

    //  Database query , Return result set 
    public static ResultSet query(Connection con, PreparedStatement st, ResultSet rs, String sql
            , Object[] params) throws SQLException {
    
        st = con.prepareStatement(sql, ResultSet.TYPE_SCROLL_SENSITIVE,
                ResultSet.CONCUR_READ_ONLY);
        if (params != null) {
    
            for (int i = 0; i < params.length; i++) {
    
                st.setObject(i + 1, params[i]);
            }
        }
        rs = st.executeQuery();
        return rs;
    }

    //  Database addition, deletion and modification 
    public static int update(Connection con, String sql
            , Object[] params, ResultSet rs, PreparedStatement st) throws SQLException {
    
        st = con.prepareStatement(sql);
        for (int i = 0; i < params.length; i++) {
    
            st.setObject(i + 1, params[i]);
        }
        return st.executeUpdate();
    }

    //  Close database connection 
    public static void release(Connection con, Statement st, ResultSet rs) {
    
        boolean flag = true;

        if (rs != null) {
    
            try {
    
                rs.close();
                rs = null;
            } catch (SQLException e) {
    
                e.printStackTrace();
                flag = false;
            }
        }

        if (st != null) {
    
            try {
    
                st.close();
                st = null;
            } catch (SQLException e) {
    
                e.printStackTrace();
                flag = false;
            }
        }

        if (con != null) {
    
            try {
    
                con.close();
                con = null;
            } catch (SQLException e) {
    
                e.printStackTrace();
                flag = false;
            }
        }
    }
}

db.properties file

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/XXX?&useSSL=false&serverTimezone=UTC
username=root
password=123456

about db.properties We need to create this resource file in the project , First we need to create a folder resources

Create a resource file under this folder db

Then enter the basic information of the configuration database , Last but not least JDBCUtil Class to operate on the database .

    } catch (SQLException e) {
            e.printStackTrace();
            flag = false;
        }
    }
}

}

`db.properties` file 

```properties
driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/XXX?&useSSL=false&serverTimezone=UTC
username=root
password=123456

about db.properties We need to create this resource file in the project , First we need to create a folder resources

Create a resource file under this folder db

Then enter the basic information of the configuration database , Last but not least JDBCUtil Class to operate on the database .