WEB19_事务

傷城~ 2022-03-09 11:46 139阅读 0赞

## 一、事务概述 ##

### 1．什么是事务 ###

一件事情有n个组成单元 要不这n个组成单元同时成功 要不n个单元就同时失败  
就是将n个组成单元放到一个事务中

### 2．mysql的事务 ###

默认的事务：一条sql语句就是一个事务 默认就开启事务并提交事务  
手动事务：  
1）显示的开启一个事务：start transaction；  
2）事务提交：commit；代表从开启事务到事务提交 中间的所有的sql都认为有效 真正的更新数据库  
3）事务的回滚：rollback； 代表事务的回滚 从开启事务到事务回滚 中间的所有的sql操作都认为无效数据库没有被更新  
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小心这个commit必须开启事务并且提交事务后，实际上才是有效  
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## 二、JDBC事务操作 ##

默认是自动事务：  
执行sql语句：executeUpdate() ---- 每执行一次executeUpdate方法 代表 事务自动提交  
通过jdbc的API手动事务：  
开启事务：conn.setAutoComnmit(false);  
提交事务：conn.commit();  
回滚事务：conn.rollback();  
注意：控制事务的connnection必须是同一个  
执行sql的connection与开启事务的connnection必须是同一个才能对事务进行控制  
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package com.itheima.jdbc;
    
    import java.sql.Connection;
    import java.sql.DriverManager;
    import java.sql.Statement;
    
    public class JDBCDemo {
    	public static void main(String[] args) {
    		//通过jdbc控制事务
    		Connection conn = null;
    		try {
    			//1.注册驱动
    			Class.forName("com.mysql.jdbc.Driver");
    			//2.获得connection
    			conn = DriverManager.getConnection("jdbc:mysql:///web19", "root", "123");
    			
    			//手动开启事务
    			conn.setAutoCommit(false);
    			
    			//3.获得执行平台
    			Statement stmt = conn.createStatement();
    			
    			//4.操作sql
    			stmt.executeUpdate("insert into account values(null,'lucy',3000)");
    			
    			//提交事务
    			conn.commit();
    			
    			stmt.close();
    			conn.close();
    		} catch (Exception e) {
    			try {
    				conn.rollback();
    			} catch (Exception e2) {
    				System.out.println(e2);
    			}
    			System.out.println(e);
    		}
    	}
    }

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## 三、DBUtils事务操作 ##

### 1．QueryRunner ###

有参构造：QueryRunner runner = new QueryRunner(DataSource dataSource);  
有参构造将数据源(连接池)作为参数传入QueryRunner，QueryRunner会从连 接池中获得一个数据库连接资源操作数据库，所以直接使用无Connection参数 的update方法即可操作数据库

无参构造：QueryRunner runner = new QueryRunner();  
无参的构造没有将数据源（连接池）作为参数传入QueryRunner，**那么我们在使用QueryRunner对象操作数据库时要使用有Connection参数的方法**  
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package com.itheima.dbutils;
    
    import java.sql.Connection;
    
    import org.apache.commons.dbutils.QueryRunner;
    
    import com.itheima.utils.DataSourceUtils;
    
    public class DbutilsDemo {
    
    	public static void main(String[] args) {
    		Connection conn = null;
    		try {
    			QueryRunner runner = new QueryRunner();
    			//获得一个Connection
    			conn = DataSourceUtils.getConnection();
    			
    			//开启事务
    			conn.setAutoCommit(false);
    			String sql = "update account set money=15000 where name='tom'";
    			runner.update(conn, sql);
    			
    			//提交事务
    			conn.commit();
    		} catch (Exception e) {
    			try {
    				//回滚事务
    				conn.rollback();
    			} catch (Exception e2) {
    				System.out.println(e2);
    			}
    			System.out.println(e);
    		}
    	}
    
    }

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## 案例-完成转账 ##

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![在这里插入图片描述][20190313163441466.png]  
演示事务的重要性：  
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我们发现钱被转走了，但是没有转入相应账户，这凸显了事务的重要性  
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开启事务来处理  
注意：控制事务的connnection必须是同一个  
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开启事务：保证了执行sql的connection与开启事务的connnection必须是同一个才能对事务进行控制  
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这次事务就控制住了，数据没有更新到数据库  
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三层结构：  
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com.itheima.transfer.web.TransferServlet

package com.itheima.transfer.web;
    
    import java.io.IOException;
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    
    import com.itheima.transfer.service.TransferService;
    
    import sun.awt.RepaintArea;
    
    public class TransferServlet extends HttpServlet {
    
    	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		//接受转账的参数
    		String out = request.getParameter("out");
    		String in = request.getParameter("in");
    		String moneyStr = request.getParameter("money");
    		double money = Double.parseDouble(moneyStr);
    		
    		//调用业务层的转账方法
    		TransferService service = new TransferService();
    		boolean isTransferSuccess = service.transfer(out,in,money);
    		
    		response.setContentType("text/html;charset=UTF-8");
    		if(isTransferSuccess){
    			response.getWriter().write("转账成功!!!");
    		}else{
    			response.getWriter().write("转账失败！！！");
    		}
    	}
    
    	public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    		doGet(request, response);
    	}
    }

com.itheima.transfer.service.TransferService

package com.itheima.transfer.service;
    
    import java.sql.Connection;
    import java.sql.SQLException;
    
    import com.itheima.transfer.dao.TransferDao;
    import com.itheima.utils.DataSourceUtils;
    
    public class TransferService {
    
    	public boolean transfer(String out, String in, double money) {
    		TransferDao dao = new TransferDao();
    		boolean isTranferSuccess = true;
    		
    		Connection conn = null;
    		try {		
    			//开启事务
    			conn = DataSourceUtils.getConnection();
    			conn.setAutoCommit(false);
    			
    			//转出钱的方法
    			dao.out(conn,out,money);
    			
    			int i = 1/0;
    			
    			//转入钱的方法
    			dao.in(conn,in,money);
    		} catch (Exception e) {
    			isTranferSuccess = false;					
    			try {
    				//回滚事务
    				conn.rollback();
    			} catch (Exception e2) {
    				// TODO: handle exception
    			}
    			e.printStackTrace();
    		}finally {
    			try {
    				//提交事务
    				conn.commit();
    			} catch (SQLException e) {
    				e.printStackTrace();
    			}
    		}
    		return isTranferSuccess;
    	}
    
    }

com.itheima.transfer.dao.TransferDao

package com.itheima.transfer.dao;
    
    import java.sql.Connection;
    import java.sql.SQLException;
    
    import org.apache.commons.dbutils.QueryRunner;
    
    import com.itheima.utils.DataSourceUtils;
    
    
    public class TransferDao {
    
    	//执行sql的connection与开启事务的connnection必须是同一个才能对事务进行控制
    	public void out(Connection conn,String out, double money) throws SQLException {
    		QueryRunner runner = new QueryRunner();
    		//Connection conn = DataSourceUtils.getConnection();
    		String sql = "update account set money=money-? where name=?";
    		runner.update(conn,sql,money,out);
    	}
    
    	public void in(Connection conn,String in, double money) throws SQLException {
    		QueryRunner runner = new QueryRunner();
    		//Connection conn = DataSourceUtils.getConnection();
    		String sql = "update account set money=money+? where name=?";
    		runner.update(conn,sql,money,in);
    	}
    
    }

com.itheima.utils.DataSourceUtils

package com.itheima.utils;
    
    import java.sql.Connection;
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import java.sql.Statement;
    
    import javax.sql.DataSource;
    
    import com.mchange.v2.c3p0.ComboPooledDataSource;
    
    public class DataSourceUtils {
    
    	private static DataSource dataSource = new ComboPooledDataSource();
    
    	private static ThreadLocal<Connection> tl = new ThreadLocal<Connection>();
    
    	// 直接可以获取一个连接池
    	public static DataSource getDataSource() {
    		return dataSource;
    	}
    	
    	public static Connection getConnection() throws SQLException{
    		return dataSource.getConnection();
    	}
    
    	// 获取连接对象
    	public static Connection getCurrentConnection() throws SQLException {
    
    		Connection con = tl.get();
    		if (con == null) {
    			con = dataSource.getConnection();
    			tl.set(con);
    		}
    		return con;
    	}
    
    	// 开启事务
    	public static void startTransaction() throws SQLException {
    		Connection con = getCurrentConnection();
    		if (con != null) {
    			con.setAutoCommit(false);
    		}
    	}
    
    	// 事务回滚
    	public static void rollback() throws SQLException {
    		Connection con = getCurrentConnection();
    		if (con != null) {
    			con.rollback();
    		}
    	}
    
    	// 提交并且 关闭资源及从ThreadLocall中释放
    	public static void commitAndRelease() throws SQLException {
    		Connection con = getCurrentConnection();
    		if (con != null) {
    			con.commit(); // 事务提交
    			con.close();// 关闭资源
    			tl.remove();// 从线程绑定中移除
    		}
    	}
    
    	// 关闭资源方法
    	public static void closeConnection() throws SQLException {
    		Connection con = getCurrentConnection();
    		if (con != null) {
    			con.close();
    		}
    	}
    
    	public static void closeStatement(Statement st) throws SQLException {
    		if (st != null) {
    			st.close();
    		}
    	}
    
    	public static void closeResultSet(ResultSet rs) throws SQLException {
    		if (rs != null) {
    			rs.close();
    		}
    	}
    
    }

transfer.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
        pageEncoding="UTF-8"%>
    <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
    <html>
    <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>Insert title here</title>
    </head>
    <body>
    	<form action="${pageContext.request.contextPath }/transfer" method="post">
    		转出账户：<input type="text" name="out"><br>
    		转入账户：<input type="text" name="in"><br>
    		转账金额：<input type="text" name="money"><br>
    		<input type="submit" value="确认转账"><br>
    	</form>
    </body>
    </html>

## [四、使用ThreadLocal绑定连接资源][ThreadLocal] ##

[应用场景：][Link 1]  
当很多线程需要多次使用同一个对象，并且需要该对象具有相同初始化值的时候最适合使用ThreadLocal。

其实说再多也不如看一下源码来得清晰。如果要看源码，其中涉及到一个WeakReference和一个Map，这两个地方需要了解下，这两个东西分别是a.Java的弱引用，也就是GC的时候会销毁该引用所包裹(引用)的对象，这个threadLocal作为key可能被销毁，但是只要我们定义成他的类不卸载，tl这个强引用就始终引用着这个ThreadLocal的，永远不会被gc掉。b.和HashMap差不多。

事实上，从本质来讲，就是每个线程都维护了一个map，而这个map的key就是threadLocal，而值就是我们set的那个值，每次线程在get的时候，都从自己的变量中取值，既然从自己的变量中取值，那肯定就不存在线程安全问题，总体来讲，ThreadLocal这个变量的状态根本没有发生变化，他仅仅是充当一个key的角色，另外提供给每一个线程一个初始值。如果允许的话，我们自己就能实现一个这样的功能，只不过恰好JDK就已经帮我们做了这个事情。

package com.itheima.utils;
    
    import java.sql.Connection;
    import java.sql.SQLException;
    
    import com.mchange.v2.c3p0.ComboPooledDataSource;
    
    public class MyDataSourceUtils {
    	//获得Connection ---从连接池中获取
    	private static ComboPooledDataSource dataSource = new ComboPooledDataSource();
    	
    	//创建ThreadLocal
    	private static ThreadLocal<Connection> tl = new ThreadLocal<Connection>();
    	
    	//开启事务
    	public static void startTransaction() throws SQLException{
    		Connection conn = getCurrentConnection();
    		conn.setAutoCommit(false);
    	}
    	
    	//获得当前线程上的绑定的conn
    	public static Connection getCurrentConnection() throws SQLException{
    		//从ThreadLocal寻找,当前线程是否有对应的Connection
    		Connection conn = tl.get();
    		if(conn == null){
    			//获得新的connection
    			conn = getConnection();
    			//将conn资源绑定到ThreadLocal(map)上
    			tl.set(conn);
    		}
    		return conn;
    	}
    	
    	public static Connection getConnection() throws SQLException{
    		return dataSource.getConnection();
    	}
    	//回滚事务
    	public static void rollback() throws SQLException {
    		getCurrentConnection().rollback();
    	}
    	//提交事务
    	public static void commit() throws SQLException {
    		getCurrentConnection().commit();
    	}
    }

com.itheima.transfer.service.TransferService

package com.itheima.transfer.service;
    
    import java.sql.Connection;
    import java.sql.SQLException;
    
    import com.itheima.transfer.dao.TransferDao;
    import com.itheima.utils.DataSourceUtils;
    import com.itheima.utils.MyDataSourceUtils;
    
    public class TransferService {
    
    	public boolean transfer(String out, String in, double money) {
    		TransferDao dao = new TransferDao();
    		boolean isTranferSuccess = true;
    		
    		Connection conn = null;
    		try {		
    			//开启事务
    			//conn = DataSourceUtils.getConnection();
    			//conn.setAutoCommit(false);
    			MyDataSourceUtils.startTransaction();
    			
    			//转出钱的方法
    			//dao.out(conn,out,money);
    			dao.out(out,money);
    			
    			int i = 1/0;
    			
    			//转入钱的方法
    			//dao.in(conn,in,money);
    			dao.in(in,money);
    		} catch (Exception e) {
    			isTranferSuccess = false;					
    			try {
    				//回滚事务
    				//conn.rollback();
    				MyDataSourceUtils.rollback();
    			} catch (Exception e2) {
    				System.out.println(e2);
    			}
    			e.printStackTrace();
    		}finally {
    			try {
    				//提交事务
    				//conn.commit();
    				MyDataSourceUtils.commit();
    			} catch (SQLException e) {
    				e.printStackTrace();
    			}
    		}
    		return isTranferSuccess;
    	}
    
    }

com.itheima.transfer.dao.TransferDao

package com.itheima.transfer.dao;
    
    import java.sql.Connection;
    import java.sql.SQLException;
    
    import org.apache.commons.dbutils.QueryRunner;
    
    import com.itheima.utils.DataSourceUtils;
    import com.itheima.utils.MyDataSourceUtils;
    
    
    public class TransferDao {
    
    	//执行sql的connection与开启事务的connnection必须是同一个才能对事务进行控制
    	//public void out(Connection conn,String out, double money) throws SQLException {
    	public void out(String out, double money) throws SQLException {
    		QueryRunner runner = new QueryRunner();
    		//Connection conn = DataSourceUtils.getConnection();
    		Connection conn = MyDataSourceUtils.getCurrentConnection();
    		String sql = "update account set money=money-? where name=?";
    		runner.update(conn,sql,money,out);
    	}
    
    	//public void in(Connection conn,String in, double money) throws SQLException {
    	public void in(String in, double money) throws SQLException {
    		QueryRunner runner = new QueryRunner();
    		//Connection conn = DataSourceUtils.getConnection();
    		Connection conn = MyDataSourceUtils.getCurrentConnection();
    		String sql = "update account set money=money+? where name=?";
    		runner.update(conn,sql,money,in);
    	}
    
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 27]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 28]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 29]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 30]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 31]

## 五、事务的特性和隔离级别（概念性问题—面试） ##

### 1．事务的特性ACID ###

#### 1）原子性（Atomicity） ####

原子性是指事务是一个不可分割的工作单位，事务中的操作 要么都发生，要么都不发生。

#### 2）一致性（Consistency） ####

一个事务中，事务前后数据的完整性必须保持一致。

#### 3）隔离性（Isolation） ####

多个事务，事务的隔离性是指多个用户并发访问数据库时， 一个用户的 事务不能被其它用户的事务所干扰，多个并发事务之间数据要相互隔离。

#### 4）持久性（Durability） ####

持久性是指一个事务一旦被提交，它对数据库中数据的改变 就是永久性的，接下来即使数据库发生故障也不应该对其有任何影响。

### 2．并发访问问题----由隔离性引起 ###

如果不考虑隔离性，事务存在3中并发访问问题。

#### 1）脏读： ####

B事务读取到了A事务尚未提交的数据 ------ 要求B事务要读取A事 务提交的数据(就像转账例子中，事务未提交，但是读到了转账信息，但是没有更新到数据库，这是在rollback回来)

#### 2）不可重复读： ####

一个事务中 两次读取的数据的内容不一致 ----- 要求的是一个事 务中多次读取时数据是一致的 — unpdate

#### 3）幻读/虚读： ####

一个事务中 两次读取的数据的数量不一致 ----- 要求在一个事务多 次读取的数据的数量是一致的 --insert delete

### 3．事务的隔离级别 ###

#### 1）read uncommitted : ####

读取尚未提交的数据 ：哪个问题都不能解决

#### 2）read committed： ####

读取已经提交的数据 ：可以解决脏读 ---- oracle默认的

#### 3）repeatable read： ####

重读读取：可以解决脏读 和 不可重复读 —mysql默认的

#### 4）serializable： ####

串行化：可以解决 脏读 不可重复读 和 虚读—相当于锁表

注意：mysql数据库默认的隔离级别  
查看mysql数据库默认的隔离级别：select @@tx\_isolation  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 32]  
设置mysql的隔离级别：set session transaction isolation level 设置事务隔离级别  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 33]  
1）read uncommitted : 读取尚未提交的数据 ：哪个问题都不能解决  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 34]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 35]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 36]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 37]  
2）read committed：读取已经提交的数据 ：可以解决脏读 ---- oracle默认的  
不可重复读：一个事务中 两次读取的数据的内容不一致 ----- 要求的是一个事 务中多次读取时数据是一致的 — unpdate  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 38]  
![在这里插入图片描述][20190314094442446.png]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 39]  
3）repeatable read：重读读取：可以解决脏读 和 不可重复读 —mysql默认的  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 40]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 41]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 42]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MDgwNzI0Nw_size_16_color_FFFFFF_t_70 43]

## 总结： ##

mysql的事务控制：
    		开启事务：start transaction;
    		提交：commit；
    		回滚：rollback；
    JDBC事务控制：
    		开启事务：conn.setAutoCommit(false);
    		提交：conn.commit()；
    		回滚：conn.rollback()；
    	DBUtils的事务控制 也是通过jdbc
    		
    	ThreadLocal：实现的是通过线程绑定的方式传递参数
    
    	概念：
    		事务的特性ACID
    		并发问题：脏读、不可重读、虚读\幻读
    		解决并发：设置隔离级别
    			read uncommitted
    			read committed
    			repeatable read （mysql默认）
    			serialazable 
    
    		隔离级别的性能：
    			read uncommitted>read committed>repeatable read>serialazable 
    		安全性：
    			read uncommitted<read committed<repeatable read<serialazable

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[ThreadLocal]: https://blog.csdn.net/jiuqiyuliang/article/details/38417623
[Link 1]: http://www.cnblogs.com/dreamroute/p/5034726.html
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