MySQL数据库——-事务的隔离级别（三）Read committed

程序员文章站 2022-05-09 15:58:09

...

文章目录

2、Read committed（读提交）

脏读
不可重复读
幻读

2、Read committed（读提交）

可以避免脏读，但可能出现不可重复读和幻读。

设置MySQL的事务隔离级别为Read uncommitted
在my.ini配置文件最后加上如下配置，然后在服务中重启MySQL服务
```
[mysqld]
transaction-isolation = READ-COMMITTED
```
查询当前的事务隔离级别

脏读

场景：公司发工资了，领导把5000元打到Tom的账号上，但是该事务并未提交，而Tom正好去查看账户，发现工资已经到账，账户多了5000元，非常高兴，可是不幸的是，领导发现发给Tom的工资金额不对，是2000元，于是迅速回滚了事务，修改金额后，将事务提交，Tom再次查看账户时发现账户只多了2000元。

create table account(
	id int(36) primary key comment '主键',
	card_id varchar(16) unique comment '卡号',
	name varchar(8) not null comment '姓名',
	balance float(10,2) default 0 comment '余额'
)engine=innodb;
# 插入数据
insert into account (id,card_id,name,balance) values (1,'6226090219290000','Tom',1000);

表中数据
MySQL数据库——-事务的隔离级别（三）Read committed

老板类

import java.sql.*;
public class Boss {
	public static void main(String[] args) {
		Connection connection = null;
		Statement statement = null;
		try {
			Class.forName("com.mysql.jdbc.Driver");
			String url = "jdbc:mysql://127.0.0.1:3306/test";
			connection = DriverManager.getConnection(url, "root", "root");
			connection.setAutoCommit(false);
			statement = connection.createStatement();
			String sql = "update account set balance=balance+5000 where card_id='6226090219290000'";
			statement.executeUpdate(sql);
			Thread.sleep(30000);//30秒后发现工资发错了
			connection.rollback();
			sql = "update account set balance=balance+2000 where card_id='6226090219290000'";
			statement.executeUpdate(sql);
			connection.commit();
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			//释放资源
				try {
					if(statement != null) { 
						statement.close();
					}
				} catch (SQLException e) {
					e.printStackTrace();
				}
				try {
					if(connection != null) {
						connection.close();
					}
				} catch (SQLException e) {
					e.printStackTrace();
				}
		}
	}
}

员工类

import java.sql.*;
public class Employee {
	public static void main(String[] args) {
		Connection connection = null;
		Statement statement = null;
		ResultSet resultSet = null;
		try {
			Class.forName("com.mysql.jdbc.Driver");
			String url = "jdbc:mysql://127.0.0.1:3306/test";
			connection = DriverManager.getConnection(url, "root", "root");
			statement = connection.createStatement();
			String sql = "select balance from account where card_id='6226090219290000'";
			resultSet = statement.executeQuery(sql);
			if(resultSet.next()) {
				System.out.println(resultSet.getDouble("balance"));
			}
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			//释放资源
		try {
				if(resultSet != null) { 
				resultSet.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
			
			try {
				if(statement != null) { 
					statement.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
			try {
				if(connection != null) {
					connection.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}
	}
}

执行Boss类主方法之后，再执行Employee主方法
当Boss类主方法执行完之后，再一次执行Employee主方法

并没有出现脏读的情况，所以当事务的隔离级别为READ-COMMITTED的时候可以避免脏读

不可重复读

场景：Tom拿着工资卡去消费，酒足饭饱后在收银台买单，服务员告诉他本次消费1000元，Tom将银行卡给服务员，服务员将银行卡插入POS机，POS机读到卡里余额为3000元，就在Tom磨磨蹭蹭输入密码时，他老婆以把Tom工资卡的3000元转到自己账户并提交了事务，当Tom输完密码并点击“确认”按钮后，POS机检查到Tom的工资卡已经没有钱，扣款失败。

准备工作
创建表

# 建表
create table account(
	id int(36) primary key comment '主键',
  card_id varchar(16) unique comment '卡号',
  name varchar(8) not null comment '姓名',
  balance float(10,2) default 0 comment '余额'
)engine=innodb;
# 插入数据
insert into account (id,card_id,name,balance) values (1,'6226090219290000','Tom',3000);
insert into account (id,card_id,name,balance) values (2,'6226090219299999','LilY',0);

MySQL数据库——-事务的隔离级别（三）Read committed

机器类

import java.sql.*;
public class Machine {
	public static void main(String[] args) {
		Connection connection = null;
		Statement statement = null;
		ResultSet resultSet = null;
		try {
			double sum=1000;//消费金额
			Class.forName("com.mysql.jdbc.Driver");
			String url = "jdbc:mysql://127.0.0.1:3306/test";
			connection = DriverManager.getConnection(url, "root", "root");
			connection.setAutoCommit(false);
			statement = connection.createStatement();
			String sql = "select balance from account where card_id='6226090219290000'";
			resultSet = statement.executeQuery(sql);
			if(resultSet.next()) {
				System.out.println("余额："+resultSet.getDouble("balance"));
			}
			System.out.println("请输入支付密码：");
			Thread.sleep(30000);//30秒后密码输入成功
			resultSet = statement.executeQuery(sql);
			if(resultSet.next()) {
				double balance = resultSet.getDouble("balance");
				System.out.println("余额："+balance);
				if(balance<sum) {
					System.out.println("余额不足，扣款失败！");
					return;
				}
			}
			sql = "update account set balance=balance-"+sum+" where card_id='6226090219290000'";
			statement.executeUpdate(sql);
			connection.commit();
			System.out.println("扣款成功！");
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			//释放资源
			try {
				if(resultSet != null) { 
				resultSet.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
			try {
				if(statement != null) { 
					statement.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
			try {
				if(connection != null) {
					connection.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}
	}
}

妻子类

import java.sql.*;
public class Wife {
	public static void main(String[] args) {
		Connection connection = null;
		Statement statement = null;
		try {
			double money=3000;//转账金额
			Class.forName("com.mysql.jdbc.Driver");
			String url = "jdbc:mysql://127.0.0.1:3306/test";
			connection = DriverManager.getConnection(url, "root", "root");
			connection.setAutoCommit(false);
			statement = connection.createStatement();
			String sql = "update account set balance=balance-"+money+" where card_id='6226090219290000'";
			statement.executeUpdate(sql);
			sql = "update account set balance=balance+"+money+" where card_id='6226090219299999'";
			statement.executeUpdate(sql);
			connection.commit();
			System.out.println("转账成功");
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			//释放资源
			try {
				if(statement != null) { 
					statement.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
			try {
				if(connection != null) {
					connection.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}
	}
}

先运行Machine类的主函数，
再运行Wife类的主函数
30秒之后Machine类主函数显示余额不足

分析：
“事务A：POS机扣款”、“事务B：Tom的老婆网上转账”，事务A事先读取了数据，事务B紧接了更新数据并提交了事务，而事务A再次读取该数据扣款时，数据已经发生了改变。

幻读

场景：Tom的老婆工作在银行部门，她时常通过银行内部系统查看Tom的工资卡消费记录。她查询到Tom当月工资卡的总消费额（select sum(amount) from record where card_id=‘6226090219290000’ and date_format(create_time,’%Y-%m’)=‘2019-05’）为80元，而Tom此时正好在外面收银台买单，消费1000元，即新增了一条1000元的消费记录并提交了事务，老婆又查询了Tom当月工资卡消费明细（select amount from record where card_id=‘6226090219290000’ and date_format(create_time,’%Y-%m’)=‘2019-05’），可查出的结果竟然发现有一笔1000元的消费。

准备工作
建表

create table account(
	id int(36) primary key comment '主键',
  	card_id varchar(16) unique comment '卡号',
  	name varchar(8) not null comment '姓名',
  	balance float(10,2) default 0 comment '余额'
)engine=innodb;
insert into account (id,card_id,name,balance) values (1,'6226090219290000','Tom',3000);

create table record(
	id int(36) primary key comment '主键',
    card_id varchar(16) comment '卡号',
    amount float(10,2) comment '金额',
    create_time date comment '消费时间'
)engine=innodb;
insert into record (id,card_id,amount,create_time) values (1,'6226090219290000',37,'2019-05-01');
insert into record (id,card_id,amount,create_time) values (2,'6226090219290000',43,'2019-05-07');

MySQL数据库——-事务的隔离级别（三）Read committed
银行类

import java.sql.*;
public class Bank {
	public static void main(String[] args) {
		Connection connection = null;
		Statement statement = null;
		ResultSet resultSet = null;
		try {
			Class.forName("com.mysql.jdbc.Driver");
			String url = "jdbc:mysql://127.0.0.1:3306/test";
			connection = DriverManager.getConnection(url, "root", "root");
			connection.setAutoCommit(false);
			statement = connection.createStatement();
			String sql = "select sum(amount) total from record where card_id='6226090219290000' and date_format(create_time,'%Y-%m')='2019-05'";
			resultSet = statement.executeQuery(sql);
			if(resultSet.next()) {
				System.out.println("总额："+resultSet.getDouble("total"));
			}
			Thread.sleep(30000);//30秒后查询2019年5月消费明细
			sql="select amount from record where card_id='6226090219290000' and date_format(create_time,'%Y-%m')='2019-05'";
			resultSet = statement.executeQuery(sql);
			System.out.println("消费明细：");
			while(resultSet.next()) {
				double amount = resultSet.getDouble("amount");
				System.out.println(amount);
			}
			connection.commit();
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			//释放资源
			try {
				if(resultSet != null) { 
				resultSet.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
			
			try {
				if(statement != null) { 
					statement.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
			try {
				if(connection != null) {
					connection.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}
	}
}

丈夫类

import java.sql.*;
public class Husband {
	public static void main(String[] args) {
		Connection connection = null;
		Statement statement = null;
		try {
			double sum=1000;//消费金额
			Class.forName("com.mysql.jdbc.Driver");
			String url = "jdbc:mysql://127.0.0.1:3306/test";
			connection = DriverManager.getConnection(url, "root", "root");
			connection.setAutoCommit(false);
			statement = connection.createStatement();
			String sql = "update account set balance=balance-"+sum+" where card_id='6226090219290000'";
			statement.executeUpdate(sql);
			sql = "insert into record (id,card_id,amount,create_time) values (3,'6226090219290000',"+sum+",'2019-05-19');";
			statement.executeUpdate(sql);
			connection.commit();
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			//释放资源	
			try {
				if(statement != null) { 
					statement.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
			try {
				if(connection != null) {
					connection.close();
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}
	}
}

执行Bank类的主函数，查询结果为80
执行Husband类的主函数
Bank类的主函数结束，本月消费明细多出了一条一千元的消费记录

分析：上述情况即为幻读，两个并发的事务，“事务A：获取事务B消费记录”、“事务B：添加了新的消费记录”，事务A获取事务B消费记录时数据多出了一条。

相关标签： MySQL 事务事务隔离级别读提交

上一篇： Spring的声明式事务

下一篇： sql join

MySQL数据库——-事务的隔离级别（三）Read committed

文章目录

2、Read committed（读提交）

脏读

不可重复读

幻读

MySQL数据库事务隔离级别详解

高性能MySQL--innodb中事务的隔离级别与锁的关系

通过实例分析MySQL中的四种事务隔离级别

一文搞定MySQL的事务和隔离级别

MySQL数据库事务隔离级别详解

深入解析Mysql中事务的四大隔离级别及其所解决的读现象

数据库事务的四大特性以及事务的隔离级别

数据库事务的概念、特性、隔离级别等示例讲解

MySQL数据库的隔离级别之可重复读为什么能够有效防止幻读现象的出现

一文读懂mysql事务隔离级别和锁的关系