[Database] Four characteristics of transaction

数据库事务

1.概念

A transaction is a series of rigorous operations in an application,Either all operations are performed successfully,要么全部执行失败.

2.事务的四大特性

• 原子性（Atomicity)
  业务动作对应的SQL应该是一个整体,不可以再拆分,Modifications to data can either all succeed or all fail.
• 一致性(Consistency)
  The consistency of the data is reflected in two aspects：
  ①利用数据库的一些特性来保证部分一致性需求：比如声明某个列为NOT NULL 来拒绝NULL值得插入等.
  ②Most of them still need our programmers to guarantee when they write business code.
• 隔离性(Isolation)
  当有多个DBMS的用户,When adding, deleting, checking, and modifying data at the same time,Operations between users are relatively independent,One user's actions are invisible to other users.
• 持久性(Durability)
  一个事务一旦提交成功,Changes to data in the database are persistent.

3.如何使用事务

• 方式一 Databases use transactions

--开启事务
START TRANSACTION;
--执行sql的语句 
INSERT INTO records(rid,bid) VALUES(1,2);
UPDATE books SET count=count-1 WHERE bid=2;
--rollback; //手动回滚
--提交事务
COMMIT;		//代表一个事务的结束

注意点：
1.事务开启后,一旦执行SQL语句出现错误,All operations in the transaction will be rolled back to before the data operation;
2.Data is not submitted before,All operations on the database are not written to disk,Once some error occurs,The data will be restored to the state it was in before the operation;
3.When the database uses transactions,提交事务后,If something happensSQL的执行错误,The system will automatically roll back;

• 方式二 通过JDBC使用事务
  1.Create a tool class for connecting to the database

package com.qk.utils;

import com.mysql.jdbc.jdbc2.optional.MysqlDataSource;

import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.SQLException;

public class DbUtils {
    
    private static final DataSource DATA_SOURCE;
    static {
    
        MysqlDataSource db=new MysqlDataSource();
        String url="jdbc:mysql://localhost:3306/beta?useSSL=false&characterEncodiing=utf-8&severTimezone=Asia/Shanghai";
        db.setUrl(url);
        db.setUser("root");
        db.setPassword("787426");
        DATA_SOURCE =db;
    }
    public static Connection getConnection() throws SQLException {
    
        return DATA_SOURCE.getConnection();
    }
}

2.测试事务提交

package com.qk;

import com.qk.utils.DbUtils;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;

public class Demo1 {
    
    public static void main(String[] args) throws SQLException {
    
    String sql1="insert into records(rid,bid) values(1,2)";
    String sql2="update books set count=count-1 where bid =2";
    //同一个事务中,执行sql1和sql2,means must be in the same oneConnection中完成
        try(Connection connection = DbUtils.getConnection()) {
    
            //connection中有一个autocommit属性,默认情况下是开启（true）
            //开启状态下,意味着每一条sqlare treated as one transaction
            //要让sql1和sql2看作一个整体,Autocommit needs to be turned off,手动提交事务
            connection.setAutoCommit(false);
            try(PreparedStatement ps=connection.prepareStatement(sql1)) {
    
                ps.executeUpdate();
            }
            try(PreparedStatement ps=connection.prepareStatement(sql2)) {
    
                ps.executeUpdate();
            }
            //手动提交事务,The above data operations are actually executed,数据写入磁盘
            connection.commit();
        }
    }
}

3.JDBCFour scenarios for transaction use

• 有事务,commit成功
  demo1,事务成功提交.
• 没有事务,被动失败（重启服务器）
  demo2,没有执行事务,重启服务器后,sql2执行失败,Database data was not updated successfully

package com.qk;

import com.qk.utils.DbUtils;

import java.sql.Connection;
import java.sql.PreparedStatement;

import java.sql.SQLException;

public class Demo2 {
    
    public static void main(String[] args) throws SQLException {
    
        String sql1="insert into records(rid,bid) values(1,2)";
        String sql2="update books set count=count-1 where bid =2";

        try(Connection connection = DbUtils.getConnection()) {
    
            try(PreparedStatement ps=connection.prepareStatement(sql1)) {
    
                ps.executeUpdate();
            }
            //Finish the first daysql1后,第二天sql2执行失败
            try(PreparedStatement ps=connection.prepareStatement(sql2)) {
    
                ps.executeUpdate();
            }
        }
    }
}

• 有事务,被动失败（重启服务器）程序出错
  demo3,开启事务,After restarting the server or a program error,The data will be rolled back

package com.qk;

import com.qk.utils.DbUtils;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;

public class Demo3 {
    
    public static void main(String[] args) throws SQLException {
    
        String sql1="insert into records(rid,bid) values(1,2)";
        String sql2="update books set count=count-1 where bid =2";

        try(Connection connection = DbUtils.getConnection()) {
    
            //开启事务
            connection.setAutoCommit(false);
            try(PreparedStatement ps=connection.prepareStatement(sql1)) {
    
                ps.executeUpdate();
            }
            //Finish the first daysql1后,第二天sql2执行失败
            try(PreparedStatement ps=connection.prepareStatement(sql2)) {
    
                ps.executeUpdate();
            }
            connection.commit();
        }
    }
}

• 有事务,主动失败（rollback）
  demo4,开启事务,执行完sql语句后,Actively roll back the transaction,All data operations will be rolled back to before the operation

package com.qk;

import com.qk.utils.DbUtils;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;

public class Demo4 {
    
    public static void main(String[] args) throws SQLException {
    
        String sql1="insert into records(rid,bid) values(1,2)";
        String sql2="update books set count=count-1 where bid =2";

        try(Connection connection = DbUtils.getConnection()) {
    
            //开启事务
            connection.setAutoCommit(false);
            try(PreparedStatement ps=connection.prepareStatement(sql1)) {
    
                ps.executeUpdate();
            }
            try(PreparedStatement ps=connection.prepareStatement(sql2)) {
    
                ps.executeUpdate();
            }
            connection.rollback();      //主动回滚
        }
    }
}