JDBC中的事务-Transaction

Posted 2020-08-08 kuillldan

事务-Transaction

某些情况下我们希望对数据库的某一操作要么整体成功，要么整体失败，经典的例子就是支付宝提现。例如我们发起了支付宝到银行卡的100元提现申请，我们希望的结果是支付宝余额减少100元，银行卡余额增加100元，而不是支付宝的100元被扣除，而银行卡的100元却没收到。也就是说，要么100元从支付宝扣除的同时银行卡也会多出一百元，要么这次提现失败支付宝的100元还在，银行卡也没有收到钱。支付宝扣钱和银行卡收钱，这两件事要么都成功要么都失败。

 

 

事物的ACID特性：

满足ACID特性的操作，我们可以说它是一个事物。

1. 原子性：该操作是最小逻辑单元整体，已经不可分隔。
2. 一致性：要么所有都执行，要么所有都不执行。
3. 隔离性：多个事务相互隔离，互不影响。
4. 持久性：事物的执行结果永久生效。

 

 

对事物的控制：

在JDBC中可以调用Connection对象的setAutoCommit(false)这个接口，将commit()之前的所有操作都看成是一个事物。同时，如果事务执行过程中发生异常，可以调用rollback()接口进行回滚到事务开始之前的状态。

 

 

示例代码:

下面代码演示了将cjk的100元转账到ly的账户上:

package org.lyk.main;   import java.sql.Connection; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.sql.SQLException; import org.apache.commons.dbcp2.BasicDataSource;       public class Main {        public static String DBDRIVER = "com.mysql.jdbc.Driver";        public static String DB_URL = "jdbc:mysql://localhost:3306/mldn";        public static String USERNAME = "root";        public static String PASSWORD = "admin";        public static BasicDataSource bds = null;          public static void main(String[] args)        {               dbPoolInit();               transferAmount();               System.out.println("///Done~~~");        }          public static void dbPoolInit()        {               bds = new BasicDataSource();               bds.setDriverClassName(DBDRIVER);               bds.setUrl(DB_URL);               bds.setUsername(USERNAME);               bds.setPassword(PASSWORD);        }               public static boolean transferAmount()        {               boolean retVal = true;                             String sql = "UPDATE user SET amount=? WHERE userid=?";               Connection conn = null;               PreparedStatement stmt = null;               ResultSet rs = null;                                           try               {                      conn = bds.getConnection();                      conn.setAutoCommit(false);                      stmt = conn.prepareStatement(sql);                      stmt.setInt(1, 0);                      stmt.setString(2, "cjk");                      stmt.execute();                      stmt.setInt(1, 100);                      stmt.setString(2, "ly");                      stmt.execute();                      conn.commit();               } catch (SQLException e)               {                      // TODO Auto-generated catch block                      e.printStackTrace();                      try                      {                            conn.rollback();                      } catch (SQLException e1)                      {                            // TODO Auto-generated catch block                            e1.printStackTrace();                      }               }               finally               {                      try                      {                            if(conn != null)                                   conn.close();                            if(stmt != null)                                   stmt.close();                            if(rs != null)                                   rs.close();                      }                      catch(Exception e)                      {                            //ignore all exceptions when closing...                      }               }                                           return retVal;        } }

 

事务断点(Savepoint):

某些时候，我们对一个事物操作失败，我们并不像回滚到最初状态，而是回滚到事务开始后的某一个地方，这时我们可以使用断点的方式让事物回滚到指定的断点(Savepoint)上.

示例代码:

下面的代码演示了如果cjk的100元转账到ly失败的话，我们将这100元转到cyx的账户上。（其中用手动跑出异常的方式模拟cjk到ly的转账失败）

package org.lyk.main;   import java.sql.Connection; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.sql.SQLException; import java.sql.Savepoint;   import org.apache.commons.dbcp2.BasicDataSource;       public class Main {       public static String DBDRIVER = "com.mysql.jdbc.Driver";       public static String DB_URL = "jdbc:mysql://localhost:3306/mldn";       public static String USERNAME = "root";       public static String PASSWORD = "admin";       public static BasicDataSource bds = null;         public static void main(String[] args)       {             dbPoolInit();             transferAmount();             System.out.println("///Done~~~");       }         public static void dbPoolInit()       {             bds = new BasicDataSource();             bds.setDriverClassName(DBDRIVER);             bds.setUrl(DB_URL);             bds.setUsername(USERNAME);             bds.setPassword(PASSWORD);       }             public static boolean transferAmount()       {             boolean retVal = true;                         String sql = "UPDATE user SET amount=? WHERE userid=?";             Connection conn = null;             PreparedStatement stmt = null;             ResultSet rs = null;             Savepoint sp = null;                         try             {                   conn = bds.getConnection();                                     conn.setAutoCommit(false);                   stmt = conn.prepareStatement(sql);                   stmt.setInt(1, 0);                   stmt.setString(2, "cjk");                   stmt.execute();                   sp = conn.setSavepoint();                                     stmt.setInt(1, 100);                   stmt.setString(2, "ly");                   stmt.execute();                   throw new Exception();             }             catch (Exception e)             {                   e.printStackTrace();                   try                   {                         conn.rollback(sp);                         stmt.setInt(1, 100);                         stmt.setString(2, "cyx");                         stmt.execute();                         conn.commit();                   } catch (SQLException e1)                   {                         // TODO Auto-generated catch block                         e1.printStackTrace();                   }             }             finally             {                   try                   {                         if(conn != null)                               conn.close();                         if(stmt != null)                               stmt.close();                         if(rs != null)                               rs.close();                   }                   catch(Exception e)                   {                         //ignore all exceptions when closing...                   }             }                                     return retVal;       } }