JDBC details

Recommended reading ：Druid Database connection pool

Quick start

Java The process of operating the database
1. To write Java Code ;
2.Java Code will SQL Send to MySQL Server side ;
3.MySQL The server receives SQL Statement and execute the SQL sentence ;
4. take SQL The result of statement execution is returned to Java Code ;

JDBC Specific implementation process
1. Create a project , Import driver jar package ;

2. Registration drive ：Class.forName("com.mysql.jdbc.Driver");
3. Get the connection : Connection conn = DriverManager.getConnection(url, username, password);Java The code needs to be sent SQL to MySQL Server side , You need to establish a connection first ;
4. Definition SQL sentence ：String sql = “update…” ;;
5. Access to perform SQL object ： perform SQL Sentence needs SQL Perform object , And the execution object is Statement object ,Statement stmt = conn.createStatement();
6. perform SQL：stmt.executeUpdate(sql);
7. Processing return results ;
8. Release resources ;

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API Detailed explanation

DriverManager Get the connection

Registration drive

DriverManager（ Driver management ） effect ： Registration drive ;

stay Java In the document static void registerDriver(Driver driver) Method is used to register drivers .

Inquire about MySQL Provided Driver class , You can see that this method is implemented in static code blocks . The code is as follows ：

In the static code block in this class DriverManager Object's registerDriver() Method to register the driver , Then we just need to load Driver class , The static code block will execute . and Class.forName("com.mysql.jdbc.Driver"); It can be loaded Driver class .

So before connecting to the database , We just need to execute the following code .

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

Be careful
MySQL 5 Later driver package , You can omit the steps of registering the driver , It will automatically load jar In bag META-INF/services/java.sql.Driver Driver class in file .

Get database connection

static Connection getConnection(String url, String user, String password)

Parameter description

url： Connection path

 grammar ：jdbc:mysql://ip Address ( domain name ): Port number / Database name ? Parameter key value pair 1& Parameter key value pair 2…
 Example ：jdbc:mysql://127.0.0.1:3306/db1

*  If the connection is local mysql The server , also mysql The default port for the service is 3306, be url I could just write it as ：jdbc:mysql:/// Database name ? Parameter key value pair 
*  To configure  useSSL=false  Parameters , Disable secure connection mode , Resolve warning tips 

user： user name
poassword ： password

Code example

public class connect {
    
    public static void main(String[] args) throws Exception {
    
// MySQL 5 Later driver package , You can omit the steps of registering the driver 
// Class.forName("com.mysql.jdbc.Driver");

        String url="jdbc:mysql://127.0.0.1:3306/sd?useSSL=false";
        String user="root";
        String password="root";

        try {
    
            Connection conn= DriverManager.getConnection(url,user,password);
            System.out.println(" Successful connection ！");
        } catch (SQLException e) {
    
            throw new RuntimeException(e);
        }
    }
}

Connection Get objects

Access to perform SQL The object of

1. Common execution SQL object

Statement createStatement()

2. precompile SQL Implementation SQL object ： prevent SQL Inject

PreparedStatement prepareStatement(sql)

This method is commonly used , Can prevent SQL Inject , It will be explained in detail later in this article .

3. The object that executes the stored procedure

CallableStatement prepareCall(sql)

Obtained in this way CallableStatement Execution objects are used to execute stored procedures , But stored procedures are MySQL It's not often used in English .

Code example

public class connect {
    
    public static void main(String[] args) throws Exception {
    
// MySQL 5 Later driver package , You can omit the steps of registering the driver 
// Class.forName("com.mysql.jdbc.Driver");

        String url = "jdbc:mysql://127.0.0.1:3306/sd?useSSL=false";
        String user = "root";
        String password = "root";
        
        Connection conn = DriverManager.getConnection(url, user, password);

        String sql="update course set cname='C++' where cno=1";
        
        Statement stmt=conn.createStatement();

        int count=stmt.executeUpdate(sql);
        System.out.println(count);
    }
}

Business management

Open transaction ： BEGIN; perhaps START TRANSACTION;
Commit transaction ： COMMIT;
Roll back the transaction ：ROLLBACK;

MySQL The default is to automatically commit transactions

Open transaction

void setAutoCommit(boolean autoCommit)

Participate in autoCommit Indicates whether the transaction is automatically committed ,true Represents an automatic commit transaction ,false Indicates that the transaction is committed manually . To start a transaction, you need to set this parameter to false.

Commit transaction

void commit()

Roll back the transaction

void rollback()

Code example

public class connect {
    
    public static void main(String[] args) throws Exception {
    
// MySQL 5 Later driver package , You can omit the steps of registering the driver 
// Class.forName("com.mysql.jdbc.Driver");

        String url = "jdbc:mysql://127.0.0.1:3306/sd?useSSL=false";
        String user = "root";
        String password = "root";

        Connection conn = DriverManager.getConnection(url, user, password);

        String sql1="update course set cname='Java' where cno=1";
        String sql2="update course set cname='Java' where cno=2";

        Statement stmt=conn.createStatement();

        try {
    
            int count1=stmt.executeUpdate(sql1);
            System.out.println(count1);

            int i=1/0; //  Man made anomalies , To observe the return roll operation 

            int count2=stmt.executeUpdate(sql2);
            System.out.println(count2);

            // If there is no abnormality  , Then submit all changes , Database update 
            conn.commit();
        } catch (SQLException e) {
    
            // If there are anomalies , Then undo all previous operations (sql1 Update operation for )
            conn.rollback();
        }

    }
}

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Statement Execute statement

Statement Object is used to execute SQL sentence . And for different types of
SQL Statements use different methods .

perform DDL、DML sentence ：executeUpdate

int executeUpdate(String sql)

perform DQL sentence ：executeQuery

ResultSet executeQuery(String sql)

This method involves ResultSet object , It will be explained in detail later in this article .

Code example

public class connect {
    
    public static void main(String[] args) throws Exception {
    
// MySQL 5 Later driver package , You can omit the steps of registering the driver 
// Class.forName("com.mysql.jdbc.Driver");

        String url = "jdbc:mysql://127.0.0.1:3306/sd?useSSL=false";
        String user = "root";
        String password = "root";

        Connection conn = DriverManager.getConnection(url, user, password);

        String sql1 = "update st set age=15 where name='Tom'";
        String sql2 = "select * from st";

        Statement stmt = conn.createStatement();

        // perform DDL、DML sentence 
        int c = stmt.executeUpdate(sql1);
        System.out.println(c);

        System.out.println("------------------");

        // perform DQL sentence ： Inquire about 
        ResultSet rs = stmt.executeQuery(sql2);
        while (rs.next()) {
    
            int id = rs.getInt("id");
            String name = rs.getString("name");
            int age = rs.getInt("age");
            System.out.println(id + " " + name + " " + age);
        }

    }
}

Running results

ResultSet Processing return results

summary

ResultSet Encapsulates the SQL The result of the query statement , Yes DQL Statement will return the object .

ResultSet executeQuery(String sql)

ResultSet Object provides methods to manipulate query result data , as follows ：

Method	explain
boolean next()	Move the cursor down one line and judge whether the new line is valid ,true It works 、false Invalid

get data
Xxx getXxx( Parameters );
Xxx： The data type to query , Such as getInt( Parameters );
Parameters ：
If you query with the name of the column, it is passed to String type , Such as getString("name");
If you use the index of the column to query , Pass on int type , The index of the first column of the table is 1, Such as getString(1);

Code example

public class connect {
    
    public static void main(String[] args) throws Exception {
    
// MySQL 5 Later driver package , You can omit the steps of registering the driver 
// Class.forName("com.mysql.jdbc.Driver");

        String url = "jdbc:mysql://127.0.0.1:3306/sd?useSSL=false";
        String user = "root";
        String password = "root";

        Connection conn = DriverManager.getConnection(url, user, password);

        String sql = "select * from st";

        Statement stmt = conn.createStatement();

        System.out.println("--------- Query with the name of the column ----------");
        ResultSet rs = stmt.executeQuery(sql);
        while (rs.next()) {
    
            int id = rs.getInt("id");
            String name = rs.getString("name");
            int age = rs.getInt("age");
            System.out.println(id + " " + name + " " + age);
        }

        System.out.println("--------- Use the index of the column to query ----------");

        ResultSet rs1 = stmt.executeQuery(sql);
        while (rs1.next()) {
    
            int id = rs1.getInt(1);
            String name = rs1.getString(2);
            int age = rs1.getInt(3);
            System.out.println(id + " " + name + " " + age);
        }

    }
}

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PreparedStatement Anti Injection

PreparedStatement effect ： precompile SQL Statement and execute , The prevention of SQL Injection problem . It is achieved by escaping special characters .

obtain PreparedStatement object

PreparedStatement prepareStatement(String sql)

// SQL Parameter value in statement , Use ？ Placeholder replacement 
String sql = "select * from user where username = ? and password = ?";
//  adopt Connection Object acquisition , And pass in the corresponding sql sentence 
PreparedStatement pstmt = conn.prepareStatement(sql);

pstmt.setString(1,"user");
pstmt.setInt(2,123456);

Set the parameter value
PreparedStatement object ：setXxx( Parameters , value )： to ? assignment
Xxx： data type ; Such as setInt(1,10);
Parameters ：? Location number of , from 1 Start ;
** value ：** The value assigned to the parameter

perform SQL sentence

call executeUpdate() and executeQuery() These two methods do not need to be passed SQL sentence , Because get SQL Statement has been executed on SQL The sentence is precompiled .

public void testPreparedStatement() throws  Exception {
    
    //2.  Get the connection ： If the connection is local mysql And the port is the default  3306  Can simplify writing 
    String url = "jdbc:mysql:///db1?useSSL=false";
    String username = "root";
    String password = "1234";
    Connection conn = DriverManager.getConnection(url, username, password);

    //  Receive user input   User name and password 
    String name = "zhangsan";
    String pwd = "' or '1' = '1";

    //  Definition sql
    String sql = "select * from tb_user where username = ? and password = ?";
    //  obtain pstmt object 
    PreparedStatement pstmt = conn.prepareStatement(sql);
    //  Set up ？ Value 
    pstmt.setString(1,name);
    pstmt.setString(2,pwd);
    //  perform sql
    ResultSet rs = pstmt.executeQuery();
    //  Judge whether the login is successful 
    if(rs.next()){
    
        System.out.println(" Login successful ~");
    }else{
    
        System.out.println(" Login failed ~");
    }
    //7.  Release resources 
    rs.close();
    pstmt.close();
    conn.close();
}